Saturday, January 31, 2009

Lightning Bolt damages Components

A ThunderBolt Zap can damage electronic parts. The Earth Leakage Circuit Breaker ELCB or Overload Breaker may Trip.

Surge Protection and Lightning Arrestors

Mains Strip Board should have protection, The EMI RFI filter, Varistors can help. Earthing and Lightning Arrestors should be good in the building.

The Blue ones are MOV Metal Oxide Varistors, sometimes they fail as a short circuit, so better to use a fast blow fuse. Then you see Gas Tube Arrestors that are even used in a Landline Terminal Box.

Surge Arrestors

A Surge Arrester is a device to protect electrical equipment from over-voltage transients caused by external (lightning) or internal (switching) events. Also called a surge protection device (SPD) or transient voltage surge suppressor (TVSS),

Line-Filter - A special class of capacitor is used in power-line (mains) filters.  Line filtering is a very tough application for a capacitor.

Achieving EMC for Dc-Dc Convertors - DC-DC converters, whether they be an off the shelf brick design or a discrete equivalent, are a source of EMI (Electromagnetic Interference). EMI is unwanted electromagnetic energy that propagates by radiation and conduction over system signal and power lines.

SMPS switch mode power supply design, PSpice simulation - These definitions should be considered relevant to a SMPS, and may not apply identically to other technical areas.

Wednesday, January 21, 2009

Design Notes - Product Development - 07

Product Design includes Schematic and Firmware Design. The Mechanical Engineering that goes into making the enclosure and internal construction is also important. The Electromechanical design which is made of electrical circuit, pcb, panel-parts and cable management is sometimes so involved that it is developed with the help of vendors or suppliers.

Product design engineering also needs to take care of the costing and related issues. The costing includes the one time investment on design and tools, even custom Mold-Die, Jigs-Fixtures etc. The engineering costs are amortized over the quantity produced, in a product type life-cycle. Manufacturing small quantities is expensive, large quantities depends on marketing and customer base. Hence custom product manufacturing is a specialized technology all-together, this needs to be modular, programmable and configurable.

  • Just like in ICs, product design is of three types commercial, industrial and military or medical.
  • Quality: cost effective design, quality components, quality workmanship with good testing can result in a quality product. The packaging and production engineering too is important.
  • Whatever be the specs, design or simulation nothing like a prototype being tested by the customer in his environment. So do not go into volume production till such a test is done.
  • Design as per inventory : when you design a new product, we can design it with new types of parts, but it is also important to use parts that are standard to the company's inventory, Also you need to use up the parts which have been accumulated due to a discontinued product line or a failed idea. This way the design becomes more frugal and efficient. It also saves the company to build a new part type stock.
  • A Temperature Controller was fixed in a PCB manufacturer's Works, The readings was fluctuating and soon stopped working properly, the service engineer went and found that there was a strong smell of ammonia around the unit. When opened the tracks were eaten away by rapid corrosion and some of the component leads had disappeared. So another controller was made with extra coats of lacquer-varnish all over and RTV compound in many places. That solved the problem. For large numbers vacuum impregnate modules in epoxy resin.
  • Reworks or fixes will add rapidly to the BOM bill of Materials Cost. Then cost of servicing. So customers are of five types home-user, industrial, hazardous, military and medical. So Components are to be selected and product engineered and costed accordingly in that order.
  • If a product gives three years life with minimum service support it can save itself from building a bad reputation. six years will establish the brand on a long term basis, twelve years of product life you will have the user of the product selling for you. if you want to improve sales by giving a new model every three years, you will need a lot of innovation on the product and you need to buy back the old ones for a big discount, you can bear some cost for brand loyalty and hence build a reputation for more sales in the future. This implies proper and sensible investments in product design and development, engineering, reliability, standards and quality will help a company survive long term.
  • Theory and simulation can give you only part of the picture, you have to breadboard to test properly.
  • In schematic indicate pin numbers of every IC or map the correct part if manual place and route is used.
  • Keep functional blocks of circuits as modules or separate areas in a big PCB, this helps in testing and troubleshooting.
  • When you design an equipment keep the controls minimum, too many options and too many knobs and dials are not good ergonomics . For your equipment to be user friendly use simple navigation and not menus nested 6 deep.
  • Power circuits, digital circuits and analog circuits should have separate supplies and ground when possible.

Digital and Embedded Systems Design

An Embedded Microcontroller or DSP system is made of Chips, Circuits and Firmware. The digital voltage levels, speed, bus width, fan out, power consumption are some factors that a designer has to keep in mind. As portable and wireless gadgets are becoming more popular, RF, Ethernet and Energy Efficient Design aspects should be studied. Power electronics and Analog Circuits knowledge is used around the system, all this is integrated to make an instrument, equipment or gadget.

Digital and Embedded Systems Design
  • 80C51 ports can sink more current but source very less, hence use a 10k pull up at all the ports or outputs.
  • Firmware must be developed in increments, tested in increments, backed up in increments, must be modular (include) reuse.
  • Tristate output, High Impedance and Floating all mean the same when it comes to IC Inputs-Outputs. It means the pin is insulated from rest of circuit in the IC. That means it will not influence the node or bus it is connected to. A DMM terminals are floating means that the hand held plastic DMM has no electrical conductive link to earth or ground.
  • You can use hyper terminal to upload code to single board computers 80C51 like in BINARY or ASCII. 
  • ASICs are for large volume production, or for products which have a long product life cycle.
  • Low volume production use FPGA or CPLD, or even flash based microcontrollers, so that all your inventory can be reused and recycled.
  • Cell phones or a LAN card ASIC is ideal as volumes are good in cell phones and for LAN cards the technology is matured. For either FPGA or ASIC's you get IP Modules or Code Libraries for many functions and applications.
  • Whatever the method keep design flexible and modular for reuse and to save cost. remember the hardware is difficult to alter, software can be altered even at customer site, flash has made this possible
  • In the future chips may be both analog and digital programmable with flash.
  • Some FPGA, CPLD, ASIC links, WinCUPL, Design and Reuse, fpga4fun.
  • Unused CMOS inputs should have a pull up or pull down resistor, it should not float, or it oscillates.
  • Have a decoupling capacitor 104 that is 0.1uF or 100nF across the supply of every IC very near the IC supply pins.
  • A watchdog timer should be used in every microcomputer circuit like 8051 so that the system resets on hanging.
  • The reset on a microcomputer should be applied till the supply to it is stabilized, this will enable a clean start.
  • Analog ground (opamps), digital ground (CMOS) and power ground (relays and LED) should be separate, (linked at root)
  • Pull up or pull down resistors in TTL can be 10K and in CMOS 100K and in battery operated systems 1M.
  • CMOS gates and Opamps have a output drive capability of ~ 10-20mA, so when you drive a load say an LED use a series resistor to limit the current to 5mA to 10mA.
  • When the number of digital chips you use in a project goes above 20 or 30 then it is better to use PLD or CPLD types from Altera, Xilinx or Lattice etc.
  • Try to use same family ICs in a circuit, like only LS or only HCT, if you mix up then you have to do a design review.
  • In a industrial environment many motors, DC drives and AC drives will be running, this will produce EMI, RFI, kickback spikes which cause microcontroller based equipment to hang. Use a watchdog timer for uC.
  • More EMI immunity by using opto couplers for all input and outputs, 4-20mA current signals for input and output and an isolated wide range SMPS.

Instrumentation and Automation - 02

In Test and Measurement and Industrial Process Control a common factor is instrumentation. The measuring aspect is common to both. T&M instruments are used in Testing, Research,Automated Testing, Troubleshooting and Servicing.

Instrumentation and Automation - 02

Process control Instruments are used in Production Lines made of Machines in a Factory. It could be Counters and Timers used for Textile or Steel Industry. It can also be Temperature and Pressure Controllers used in Oil Refining or Food Processing.

So you see a difference. We have handheld Test Instruments like DMM and Scope and We have Handheld Instruments for Gas Monitoring or Humidity too. In automated testings we have rack mount test instruments. In Control panels we have Rail Mount carrying DIN standard Instruments.
  • On-Off controls or level trip relays can oscillate if hysteresis dead band is very small or absent.
  • Hysteresis is like on at 300 off at 304 like in heaters or air conditioners. If you on-off at sharp 300 there is a chatter or oscillation. This is called dead-band in temperature control. In above example 304-300=4 is the dead-band.
  • LF356, TLO71 and CA3140 are FET input Op-Amps, the bias is in pA, The caps for sample hold with these opamps are low leakage ones.10uF tantalum for long time, low resolution data. Long hold times with 12 bit data (analog) use 1uF plastic multilayer and clean-IPA + coat PCB with epoxy.
  • Design the product in such a way that neither the pot terminals or pot switch is used for mains voltage, instead use the pot switch or variable resistance for signaling and the mains can be switched with Relay or Thyristor.
  • If you need a 10.00K resistor 0.1% and you only have 1% in the market, you can trim lower values to higher values by removing the paint of the resistor and scratching the black metal film till you get the desired value. (this is only for testing or prototype development, R&D, not for manufacturing).
  • 7107 and 7135 of Intersil can be tested in-circuit, check Vref for 100mV or 1V reference, then short Vin and Vref the reading should be 1000 counts for 7107 and 10000 counts for 7135. ground plane should be good, no ground loops, PCB only glass epoxy and low leakage caps to be used, then reading will be stable, use some coating on soldered PCB.
  • An electrically Noisy environment can be simulated with a hair drier, or hot air blower. Another way Looping the NC (normally closed) contacts of a 230V Electrical Contacter-Relay can make it oscillate, this makes a lot of Sound and more important is the Electrical noise rich in EMI-RFI. This helps basic testing of timers, counters, uP, uC and embedded systems as they may malfunction in noisy environment.
  • A Single Pole 12V 100mA relay in a feedback loop, with its contacts, can make a Electrical or Electromechanical Oscillator. This can test your ground loops and digital circuit decoupling caps. It simulates power spikes which may reset some counters.
  • When a Machine with many stages or even an entire production line has to be automated, a PLC system is easiest to configure. It has digital inputs for limit switches and proximity sensors, it has analog inputs for any other parameter sensor. The PLC output modules can drive Electrical Contactor circuits with 230V Signal or 4-20 mA for an AC drive. The tacho-generator voltage feedback or pulses feedback from a motor can be fed to PLC inputs too.
  • Automation is used to enhance the quality and quantity of the product, also where there is danger to operators or workers. This moves the worker up, into the maintenance and management of materials, machines and systems.
  • Industrial Automation is essential to enhance the manufacturing worker to deliver consistent, repeatable and timely delivery of products. Absence of automation makes quality and deliveries unpredictable and stresses worker on monotonous jobs. When the worker is creatively employed with his machine on automation, it is the best scenario.

Solderman Talks 1702 AD
"Over-Automation is an utopia which is incorrect. This is because if you have no workers in any factory, there are no consumers in that city. No consumers means no sales of goods, no building of houses, no buying of automobiles. All strata of People correctly employed, reasonably paid, seeds consumerism, promotes the economy and markets. Automate everything, economy grinds to an halt."

- Solderman Talks 1702 AD

Sunday, January 18, 2009

Design Notes - Analog Basics - 06

Building discrete semiconductor and passive designs, using Opamps and Mixed Design Circuits are the first step in electronics. Most consumer Electronics have a high analog circuit content. Even in Embedded Systems the interface design that matters for real time systems is an analog circuit.

Here are some more analog points in my old notes.

  • The Contact resistance of connectors, thermocouple effects in connectors and solder joints and thermal gradients over the PCB can cause errors when you measure in high resolution.
  • High impedance points of circuit like 500 kilo ohm and above can pick up AC noise and DC leakage currents. this will affect the performance of circuit, so for DC you have to put a 'gaurd ring'of the signal ground around that point in PCB. For RF you have to shield with things related to iron and mu metal, for low signals even a copper shield will do.
  • Unused high impedance inputs of opamps or gates or any IC should be pulled-up or down.
  • High Impedance inputs which have to float can be made noise immune with a small cap to ground. Like a 102 CD.
  • If an opamp circuit with feedback oscillates, then a cap at the right place can stop it, it will dampen the oscillations, just like eddy current dampening in moving coil meters. One way is a cap across the feedback resistor another way a cap between inv- and non-inv+ inputs. A closed loop system may oscillate at border points. The value of cap depends on how fast (response time) the system has to be and the type of oscillations seen.
  • Inputs and Outputs are the points of electric abuse, opto isolate, use zener barriers with overrated devices.
  • If one layer of PCB is a ground plane and second layer has wide tracks they may form a capacitance (say 1pF) with the PCB glass epoxy as dielectric, more so in multilayer PCB as dielectric is thinner.
  • In a 3-1/2 digit meter circuit you may need 0.1% Resistors 10ppm or less, so in a 4-1/2 meter we need 0.01% Resistors 1ppm or less or temperature compensation circuits and trimpots.
  • Tolerance of resistors in a precision circuit and thermal drift of both opamps and resistors; can show movements in high resolution measurements. So the whole circuit may have to be built into a sealed 40 deg C Instrumentation Oven sealed in glass wool. This will work both in cold and hot climates.
  • Humidity, chemical fumes, dust and grime should not reach the analog circuit areas. Depending on application, a coating may have to be given or the entire unit hermetically sealed.
  • Use MFR (metal film resistor) 1% in all analog designs and if possible use only MFR for better reliability.
  • Glass epoxy PCB have high insulation resistance, above 10 tera ohms, and are not hygroscopic which means they do not drink water vapor, this makes them very suitable for precision instrumentation and sensitive circuits.

Design Notes - Components Selection - 05

Selecting the parts for your product design, choosing the components intelligently is a part of Product Design Engineering.

Budget, Availability, Size, Alternate Vendors, Product Life and Quantity to be manufactured are some factors that determine the selection of a prudent and experienced design professional.

To choose the right type of part, an exhuastive knowledge of the behaviour of passives and discretes is essential. Some points i remember among the things that i learnt the hard way are listed below.
  • Over rate components at least double, if you need a 1A-100V-100uS diode, use a 2A-200V-50uS diode.
  • Electrolytic capacitors have a shelf life, if you need to store them you have to charge them every month.
  • Gold plating is used in connectors because it has low contact resistance and does not corrode or react
  • Use a 100uF and 0.1uF CD in parallel to filter because the inductance of 100uF is a lot (the electrodes are wound as in a coil) and high frequency passes over and is filtered by the 0.1uF which has negligible inductance.
  • Ceramic capacitors leak a bit, electrolytics leak a lot but plastic or mica capacitors do not leak at all, near perfect. An oscillator with a RC time setting needs a plastic cap for stability.
  • In industrial electronics equipment, connectors are a source of many problems, hence avoid connectors. vibrations, corrosion and frequent usage will result in strange problems.
  • Small signal diode 1N4148 switches at 4nS, current of 70mA and 70V withstand capacity.
  • 2N2222 and 2N2907 form a fast switching NPN-PNP pair and have been around for decades.
  • LED displays are best indoors and are not so good for sunlit outdoors, LCD is good for that.
  • All electromechanical parts like switches, relays, connectors and pots have limited number of operations.
  • Failure of parts are when parts are stressed beyond limit, bad environment, misuse, infant mortality and normal aging.
  • Most pots have a dielectric insulation between metal pot shaft and terminals of 1KV or more. Some commercial pots may have lesser breakdown voltage and that will be a product safety issue. So either way use Plastic Knobs or pots with plastic shafts.

Saturday, January 17, 2009

Production Notes - Prototype Fabrication - 03

Prototype Fabrication

Sometimes a component can be defective, may not be what it is labeled or may be of a wrong value. In manufacturing 100% incoming inspection of components can catch these errors, Before Assembly of prototypes just test all passive and active components with a DMM and put all ICs in bases, then troubleshooting is easy.

  • When you make your prototype check diode and el-cap polarity, check pin 1 of chips and connectors, resistor values, dry solder and loose contacts, hairline cuts or shorts, e-b-c etc. of Transistors, FETs and thyristors.
  • Observe color code in wires. positive is red and negative is black and green is earth and See Wire Color Code.
  • When you are soldering or cutting leads, room should have cross ventilation, protective glasses for eyes and a good stand.
  • When using a sharp blade the cutting stroke must move away from you and make sure nobody is close, Wear protective or even plain glasses, If you use a blade with the stroke moving towards your body it will cut badly.
  • Power tools and machinery must be used only after exhaustive training and with safety precautions. The power tools used carelessly and wrongly can cause an injury which will last a lifetime or may even handicap you.
  • Have a lathe lamp or table lamp with a 40W edison-filament lamp (ordinary bulb) on your workbench.
  • Have an antistatic mat on the work or test table, if you cannot afford or get it then get a large aluminum sheet, laminate it with an insulating polycarbonate sheet and earth the metal plate with two 1M resistors in series.
  • Use thin multi strand teflon wires for your jumpers within a PCB, these are not damaged by your iron too.
  • Silicone shrink sleeves can withstand high temperature and use this in your work to make it safer and neat.
  • When you make a connection with a wire you need mechanical strain relief or on use connection will open.

Soldering and Desoldering Tips

Use the soldering iron as a heat transfer tool, heat the junction of the lead of the component and the copper pad on pcb then touch the hot junction with a flux cored soldering lead wire, it should melt and form a concave shining joint. If air bubbles are formed or you see dull convex joints, then it means you have a dry solder and bad contact. Leads of old components due to exposure to moisture, brine or sunshine will corrode, store them well cool-dry-clean-air. Anyway scratch them with a blade and solder them with extra flux.

When you are desoldering a double sided pcb, use a desoldering pump or desoldering wick. Heat a joint to be desoldered and then quickly tap it hard on the table, the molten lead will fall off.

Design Notes - Schematic and PCB - 04

PCB Thermal Design Care for Analog Circuit Stability

If you have a hot power chip near a DC analog circuit, the values will drift, take time to stabilize, this happens even if you use 1% MFR 100ppm. The impact is more if the heatsink is not used and vents and coolers are absent, So in a PCB keep distance between heating power circuits and sensitive analog circuits, some times PCB tracks will carry heat to the Analog area, Hence keep sensitive analog circuits as plugin modules.

When you make a PCB try to orient all ICs in one direction, so also all diode and cap polarities, then it is easy to populate the parts, easy to verify and troubleshoot for production and testing.

High Frequency and RF Design of PCB

  • Sharp bends in PCB tracks or wires will radiate away the RF energy, so do not have hairpin bends.
  • RF or near RF multilayer PCB have 2 layers as ground plane and tracks in the layer in between, Faraday cage.
  • Practical RF PCB Design - IETTraining Event - "How to Design RF Circuits"

Schematic Design Basics

  • When we draw circuits we have to keep in mind the readability, the circuit should be laid out, according to the signal flow, the input output ports shown in the periphery of circuit.
  • Drawing a neat circuit diagram will ensure a better design, lesser parts means greater reliability, simple solutions.
  • How To Draw Schematic Diagrams

Product Design Mockup and Prototypes

  • Make a 3D mockup of the final product, both the box and the PCB with components before PCB done.
  • The product with the PCbs, transformer and other panel components should fit in the box it was intended for, or the cabinet that was designed for the Product. To make sure make a Mock-up Product - 3D and display. Even if you have all documents and drawings in place.
  • Make sure of IC pin numbers, size and packages and indicate in circuit like 40 pin dip before going to PCB. Also check every EDA symbol in your software with the company datasheet to make double sure.
  • Do not mix mechanical and electrical connections, these leads to failures and frequent need of repairs and service.
  • When using a double or multi layer pcb make one layer a ground plane, all decoupling caps ground linked to it.
  • Reinforce copper tracks on PCB with a copper wire when it needs to carry more current, tinning may improve a bit.

Friday, January 16, 2009

Design Notes - Power Electronics - 03

SMPS designs should have clear line or patch of isolation for product safety and long term reliability. If you use opto-couplers they need to have 5kV or better isolation and 10mm between pins. The Live circuitry and the output circuitry should not overlap on PCB, tracks should not stray to the opposite side, The Transformers can be split bobbin if possible. A 10 to 20 mm desert (means no tracks) running from end to end of PCB under Transformer and under optos etc., Even ground tracks and ground planes should not stray.

The high voltage side must have layer to layer (mylar) insulation, means wind a layer and then put the yellow mylar tape, make tape concave so no strands should slip over to next layer, use split bobbin if possible. have terminations far away and enclosed for safety. vacuum impregnate with natural resin or epoxy depending on voltage and environment. product safety is very important.
  • If in your power supply you have a varistor, then you should have a fast acting fuse in series, as varistor fail as a short.
  • Wire and crimping of wires for supporting the high currents and high voltages must be carefully chosen. The copper cross section area gives its ability to carry current, the quality, thickness and flexibility of the insulation gives its voltage capability. The Tightness of the crimped contact will ensure long term reliability. Mechanical Stress prevention for all wiring is very important and guarded termination for user-operator safety.
  • BTA16600 and triacs of this series from ST have the metal TO220 tab electrically insulated from the device.
  • MOC3041 and others, switch triacs at zero crossover which reduces EMI-RFI and spikes. It means when the sine wave is close to zero volts the triac is turned on.
  • You have to ensure proper air circulation and fix proper heat-sinks with thermally conductive heat-sink compound or silicone grease with alumina. Anodized Aluminum heat-sinks with large surface areas and small fans are used in power electronic products for this purpose. If a component works very hot the specs will get derated, precision is lost and undesirable and unpredictable results will happen.
  • A thermistor must be used in series with huge power electrolytic capacitors to limit the enormous current inrush on start, or spikes may stress components, cap or EMI-RFI.
  • A freewheeling diode should be used across a relay, solenoid coil or motor because inductive kickback will damage transistor or mosfet. High current loads like relays can reset logic circuits if ground is not connected well, It also produces more ripple in supply, so it is better to have a separate supply for such parts.
  • EMI-RFI causes-Switching Loads Simultaneously (inductive loads). Power factor correction capacitors and devices. Lightning strikes (enhanced by earth faults). Line Inductance (inductive kickback, resonance). DC and AC Drives for Motors. Rectifiers with large filters and stray inductance.
  • Small battery operated gadgets when turned on after a very long time may not work properly due to a thin oxide layer at the battery connector, replace cells if required, scratch and clean battery and connector surface, the reason is some gadgets consume so low power that the current cannot break a micronic oxide layer. Keep using regularly.
  • Loose Contacts may have a resistance like 100mE (milliohm). A bit of corrosion and 10A of current will make that dissipate 100 * 102 = 10,000mW = 10W , if the contact area is 1 sqmm the heat will cause the resistance to increase, then the dissipation becomes more, sparking and welding may happen, it can even cause fire.

Design Notes - Power Electronics - 02

Power Supplies and SMPS, Transformers, Drive circuits for Motors, Heaters and Solenoids are all Power Electronics. Thyristors, Mosfets and High Power Transistors are important components used in these circuits.

Tek Power Supply Facia - Observe the user friendly interface design.

Tek Power supply

Power Design Points to note - delabs
  • Foldback circuits in the form of constant current or voltage can protect a Power Circuit from overload. eg. Charger.
  • Air gap in inductor core prevents saturation of the Magnetics, it must be made like that when required. eg. Choke.
  • High power equipment should have a soft start, so that fuses dont blow on start, inrush can cause damage to parts.
  • Caps can be put in series to double voltage withstand capability, when we put el-caps in series they have to be identical in value. put caps in parallel to increase the cap uF value and it also lowers ESR which is equivalent series resistance, this is good when filtering in SMPS. Two el-caps with the negatives of both connected and the positive terminals serving as the two terminals will give a non-polar cap, used in crossover networks.
  • When smps has to work at 100Khz or more, the primary of transformer must have multiple strands, and secondary can be a copper flat ribbon. this is due to skin effect, as high frequency current flows on surface of conductor.
  • MOSFET needs a turn on pulse and a turn off pulse as gate capacitance is huge.
  • Do not club Heatsinks unless the heatsink is very big or the clubbed components form a part of a current sharing set. If you club say two TO220 devices one dissipating 10W with a 2 Watt heating device, you may reduce reliability of the 2W device. Then clubbing parallel current sharing devices on one heatsink may be a good idea as it may reduce thermal runaway, also parallel power transistors should each have 0.1E in emitter path, this also can force load sharing on lazy devices.
  • Test a mosfet, charge the gate with the DMM in diode mode, then there is a short between source and drain.
  • Snubbers consisting of R-C should be used across switches, relay contacts, MOSFETS when switching inductive loads, this will absorb the spike and save the device.
  • Transformers used in SMPS could be split bobbin for product safety, and transformers should be resin impregnated, hazard may arise when using a badly made product.
  • In a SMPS which is not earthed the floating (unconnected) earth terminal will give a slight shock due to the two Y caps connected to earth from phase and neutral.
  • Power mosfets, transistors or IGBT's should be electrically isolated very well from heatsink with alumina, mica or silpad. It could withstand 2KV -5KV DC, related to product safety. Also these heatsinks should not be earthed but floating, do not fix to the metal chassis for maximum safety.
delabs design notes from the 90s

Production Notes - Electrical - 02

All Electronic Products have interfaces to external electrical circuits. The power supply is also derived using converters or power supplies. Motors, Heaters and Transformers are examples of components in the electric power circuits.

  • 12V lead acid batteries should be charged with a constant current CC and constant voltage charger the CV being 13.6V. Deep discharge and over charging will reduce the cycles.
  • Lead acid batteries need a monthly distilled water update to ensure long life, sealed lead acid dont need attention. Keep lead acid batteries in well ventilated area due to acid fumes.
  • Electric Shock can kill hence take great care, Electrolytic capacitors burst and Transformers catch fire. turn off equipment after use. turn off all electricity when going out of town.
  • Inductive kickback starts a tubelight or automobile as high voltages are produced when current in inductor is turned off. This also is the cause of failure in power semiconductors.
  • Earthing is important, ensure it's perfect, the neutral to earth could be 5V AC max., also a circuit breaker and fuse a must. Use a electrical earth leakage circuit breaker to protect both man and machine.
  • Extra care should be taken to prevent injury to eyes while working. Also 230V/110V shock can be fatal. working on high energy electrical circuits and power electronics should be with all precautions.
  • You need a Fuse, Circuit breaker and earth leakage circuit breaker for safety against shock and fire hazard.
  • High voltage or current stress, mechanical vibration, user misuse, High Energy stress (freq and voltage), aging (cycles) cause failure.
  • Good earthing, servo motor regulated mains supply, earth leakage circuit breaker and over current circuit breaker, HRC fuse are some safety steps to take.
  • When working with HV or HE circuits wear shoes, keep yourself dry, the supply board should have above protections in red.
  • Use silicone shrink sleeves to insulate or close exposed high voltage wire ends or joints.
  • Sparking at the high voltage mains supply power contacts like a plug can cause electronic gadgets to fail.

Production Notes - Electromechanical - 01

Production, especially prototype testing and Pilot Production exposes lot of engineering and serviceability issues. When you mass produce, it is more streamlined, yet the scope for improvement never ends. The process should always be open to feedback and improvements.

  • The tracks or components on a PCB should not be near the mounting holes, the tracks may form electrical connections with the cabinet earth if they touch the pillar, washer or nut-bolt.
  • When you tighten the screws-nuts-bolt, limit torque by feeling or with torque limit screwdrivers, or thread will slip or PCB-Cabinet break, or CRT-Glass-Plastic will crack-shatter.
  • After assembly and testing is over on a board, do not drill-file it, it should have been done before. Else components crack or tracks break or small metal pieces will form shorts.
  • The Mechanical and electro-mechanical must be streamlined before production, for example when fitting the board the screwdriver may slip and break a zener. The product should be engineered in such a way that it fits easily and the testing and quality checks need not be repeated.
  • Some mechanical failures may not be visible but may show up as a field failure after even 3 months. this will make you lose further orders and ruin the reputation. Hence engineering is vital.
  • Alumina-Aluminum oxide powder mixed in epoxy resin (araldite) can make thermally conductive insulating filler, heatsinks. The Alumina must be free of moisture and very dry.
  • When you drive nails into walls ensure that that part of the wall is free of mains, telephone or any wiring.
  • Some pcbs can have hairline cuts or hairline shorts which will make troubleshooting difficult.
  • Never put or remove ICs, cards or plug-in modules when the power to that system is on, power off system wait for some time for caps to discharge, then insert ICs or cards, applies to PCs too, USB and Firewire ports are exceptions as they are hotplug, anyway even in this, take a bit of care.
  • Use RTV (room temperature vulcanizing) silicone rubber compound or Varnish on PCB-devices to avoid corrosion-dust. more so in industrial environment, ships and chemical industries.
  • Whenever you use nuts and bolts to fasten a PCB or parts to a cabinet use a spring washer, double nut or adhesive to prevent slip. When a loose nut falls on the pcb or transformer sparks will fly.
  • Do not mix mechanical and electrical connections, these leads to failures and frequent need of repairs and service.

Thursday, January 15, 2009

Design Notes - Analog and Opamps - 01

LM335 and LM336 -

LM335 is a temp sensor, LM335 . use it for temp for CJC. Temperature Sensors - RTDs and Thermocouples in  Temperature Measurement.

LM336-2.5 is an voltage Reference. precision 2.5V shunt regulator diodes, Applied as a precision 2.5V low voltage reference for digital voltmeters, power supplies or in opamp signal conditioning.

Current Loop -

4-20 ma will drive DC drives, motors and steam valves. It is a current loop, for long distance transmission, current loop is not prone to noise and hum, EMI RFI. As it is a small power transmission no loss of data, also many equipments can be in one data loop. A single transmitter with 4-20mA out can drive a strip-chart recorder, motor, controller and SCADA input. all in a series loop. Current loop is like 4-20 mA

Opamp Notes and Types
  • TL062, LF353, TL072, fet input. 1 tera ohm input imp.
  • OP07 higher price, 1 giga ohm. 75uV offset.
  • LF353 pin to pin replacement of TL062-72 and 82.
  • LM358 works on single supply too, low price, inp imp 1 meg.
  • LM324 Quad, works of batteries well.
  • All work best with dual +/- 5 V or more. LM7805 -LM 7905.
Analog Electronics: Basic Circuits of Operational Amplifiers

Examine the operation of the following circuits of operational amplifiers: inverting amplifier, summing amplifier, difference amplifier, differentiator, and integrator.

Remembering Bob Pease

Remembering Bob Pease

Points to Note -
  • Analog ground (opamps), digital ground (CMOS) and power ground (relays and LED) should be separate, (linked at root)
  • Glass epoxy PCB have high insulation resistance, above 10 tera ohms, and are not hygroscopic which means they do not drink water vapor, this makes them very suitable for precision instrumentation and sensitive circuits.
  • When you measure DC levels in 16bit accuracy or more you need resistors which have temp. coeff. of 10ppm, or you may have to put the entire circuit in a stable 45 deg oven. Thermoelectric effects, EMI, RFI, pA Leakage currents, ground loops, contact resistance all can make the readings drift and unusable.
  • High impedance points of circuit like 500 kilo ohm and above can pick up AC noise and DC leakage currents. this will affect the performance of circuit, so for DC you have to put a guard ring of the signal ground around that point in PCB. For RF you have to shield with things related to iron and mu metal, for low signals even a copper shield will do.
  • Percentage and ppm: when percentage becomes like 0.001% it is difficult to manage, so we use ppm-parts per million.5% means 5 parts in 100 parts. 23 ppm means 23 parts per million parts. 0.001% - shift the dp-decimal point four places to right it is 10ppm. 0.01% is 100ppm which is the variation of value of mfr resistors on temperature change.
Learn this at my Industrial Process Control Circuits

Instrumentation and Automation - 01

  • If the waveform on the scope droops then you may need to use a 10X attenuating probe or mode. this attenuator is at the tip of the probe hence better measurement possible, also very high frequency signals may be best transmitted over a 50 ohm impedance matched line to the Oscilloscope. The History of Oscilloscope, Learn more about Oscilloscope from the Innovator of the Scope as a Measuring Instrument here is XYZs of Oscilloscopes.
  • Karl Ferdinand Braun - In 1897 he built the first cathode-ray tube...oscilloscope. CRT technology is to this day used by most television sets and computer monitors. The CRT is still called the "Braun tube".
  • Test a NPN transistor, DMM in 200K range. emitter negative probe, collector positive probe, it should show open. while still holding that way press wet finger on base and collector, it may show some reading, then it is ok. for PNP swap probes polarity and do the same thing. the wet finger when pressed applies a base current in uA.
  • Testing a Mosfet - Transistor Tester - Testing Transistors - Basic Testing of Semiconductor Devices
  • The most common reason of DMM failure is when a person tries to measure 230V AC with a DMM in current or ohms mode, with the probes in the wrong sockets. Use external attenuators and shunts.
  • A DMM in diode mode or resistance mode can be used to test IR diodes, take it under 100W lamp and measure, cover the IR diode and measure you will see a response in proper polarity.
  • If you put the DMM in 2V mode and measure the voltage across a ordinary LED near a 40W lamp you will see around 200mV. Turn off 40W lamp the mV will fall near 0. Even metal-can transistors with their top ground away will become photo sensors.
  • When you measure low voltages or high resistance like 1M with a DMM your body must not be in contact with the probe tips. Body resistance is low in comparison so error in reading or loading of mV from sensor, in sensitive and high impedance ranges.
  • Do not use an oscilloscope to measure 230V directly, use a transformer, differential probe or attenuator.
  • An oscilloscope 1M probe or a DMM 10M impedance can load circuits with nano-pico amps of bias.
  • Red socket and probe for positive and black socket and probe for negative
  • Is it oscillating ?.... We cannot say, it could also be DC, you need to see if a Clock is present. use a signal tracer or a DMM freq mode, or AC measurement. or build this When you do not have a scope you can build one for the PC.
  • Polarity and Zero Cross see the last circuit it is a polarity indicator, which will tell you to swap the leads. VRS Zero Crossing detector circuit, Zero Crossing Detectors, Comparators, Discover Circuits - Zero Cross.

Testing Tools Resources

Wednesday, January 07, 2009

Theory and Tutors in Electronics

Learning electronics in theory by studying books is the conventional way, this method may help in History and Maths. Maths needs a paper and pencil for practicals and history needs a well equipped Library. In these modern times, the web has made access to wide range of learning information easy, which is true Knowledge Empowerment. The greatest gift a Child can get from the Parents or School, is Knowledge.

Electronics Theory and Tutorials  

As an Electronics Engineer i had always needed data sheets and application notes. The web makes these documents not only accessible to all, but searchable too.

Process Control Instruments

These are some instruments i manufactured when i used to run my own industrial automation business.

Theory and Tutors in Electronics

Now many Virtual Electronics Workbench Software and Interactive Tutors are available, these aid learning. We have to design, build and apply some electronic ideas in 3D Hardware, for complimenting the Book, Academic or Web learning. Practical and Theoretical study in electronics has to be in Tandem, this is possible only when Industry works with Universities to create real talent and skills in students. The applied technology is evolving, hence the academic education must be relevant when the student gets into his first job.
Solderman Talks 1702 AD
"when you solder the windows should be opened, cross ventilation must be good, because the fumes are not very good for health. After you finish wash your hands with soap, as lead is poisonous or toxic"

- Solderman Talks 1702 AD

Tuesday, January 06, 2009

Thermocouple Temperature using DPM or DMM

Description -

If wires of two dissimilar metals are joined at both the ends and the junction formed at one of the ends, is heated more than the other junction, a current flows in the circuit due to Seebeck thermal emf. This effect is used in thermocouple temperature sensors.

The Peltier effect is the converse of above Seebeck effect, which means that if a current is forced through junctions of dissimilar metals, the junction will generate heat or absorb heat (cooling) depending on direction of the applied emf. This effect is used to make portable and small refrigerators.

Going to practical temperature measurement, we know that one of the junctions is the sensing or hot junction (Tmes) and the other junction is the terminating or cold junction (Tref), the voltage between terminals 'a' and 'b' is proportional to Tmes - Tref (and given in the Table 1) . The formula being Vab = alpha x (Tmes - Tref), where 'alpha' is the Seebeck coefficient of the thermocouple.

Thermocouple Junction

Table 1

MV Thermocouple Temperature in Deg C As cold junction is not zero but is at room temperature (RT) add RT to temperature.
0 0
2.585 50
5.268 100
10.777 200 Example -

Feed 10.777 mV to the TC+ and TC- terminal if RT then is 30 Deg C reading on 2V DPM Will be 230 counts - 230mV.

16.325 300
21.846 400
27.338 500
33.096 600
Reference junction or cold junction at 0 deg C.

In the circuit, use only metal film resistors (MFR) of 1 per cent tolerance, as this is an instrumentation application. Power supply should be a stable +5V, -5V supply, for which one can use 7805 and 7905 regulators.

The inputs TC+ and TC- terminals should go to a 4-way barrier terminal block, the 2 extra terminals are used to mount TH1 Cu thermistor. This forms an isothermal block, which is good enough.

A simple way to make a TH1 Cu thermistor, is to take a 1 Meg-ohm 2W resistor as a former and wind 2 meters of 46 SWG enameled copper (Cu) wire (5.91 ohm/meter) over it. This gives a 12-ohm value. Terminate wire ends on resistor leads.

Circuit Diagram -

Thermocouple Temperature using DPM or DMM

Thermocouple Amplifier Circuit - PDF version of above, more details and easily printable.

Test and Calibration -

For calibration, you will need a DMM-DPM and a milli-volt source (as shown in the Fig.). First connect source to terminals TC+ and TC-, then set source to 0.00 mV (verify with DMM for zero). The output across +out and -out (use DMM) terminals must be mV representing the room temperature (RT). For example, if RT is 30° C (use a glass thermometer) then +out should be 30mV at 0mV input. Adjust VR1 till 30mV is read at +out terminal. This is 'zero cal'.
Millivolt Source
Now increase mV input to 21.85 (corresponding to 400° C). Now vary VR2 till +out terminal is at 430mV (temp. +RT). This is 'gain cal'. Now as VR1 and VR2 are interdependent you may have to repeat 'zero cal' and 'gain cal' a few times till you get the above values.

Properties of J thermocouple and design aspects of gain block used in the temperature measurement instrument are summarized below:

J Thermocouple Ansi Symbol 'J' -

  1. J is a thermocouple made of iron + VE and constantan -VE.
  2. Constantan is an alloy of copper and nickel.
  3. Full range of use is from -200° to +700°C
  4. Practical to use only from 0°C to 400°C.
  5. Useful in reducing and Alkaline atmosphere.
  6. Corrodes-rusts in acidic and oxidizing atmosphere
  7. Color code of wires negative-red and positive-white.
  8. J type is popular because of Low price and high mV output.
  9. J type TC used in rubber-plastic forming and general purpose use.

Design of Gain Block -

  1. Minimum input from thermocouple is very low like 1-2 mV. Hence ultra low offset (100uV opamp is required - OP07 used).
  2. Inputs may be subjected to wrong connections or high voltage. Use of R1 limits current and Zener ZD1 clamps voltage to safe level. (low leakage zener or use diode).
  3. Gain required is 400mV - 21.8mV that is approx 18 at 400° C. Gain Av = ( Rf + Ri ) / Ri here Rf is R7 and Ri = R5 + R6 + VR2 (in circuit value).
Design of TH1 cold junction compensation copper thermistor -
J Type TC output changes by 0.052mV per deg C as per Table 1. Copper has a temperature coefficient of 0.0042 ohm per ohm/deg C. eg. for a copper wire of 12 ohms, it is 12 x 0.0042 = 0.05 ohm/deg C.

For R1 of 5K current Thru TH1 =5V / 5K = 1mA. Change of voltage across TH1 with temperature is
0.05 x 1mA = 0.05mV / deg. This rate is same as J type TC hence it simulates cold junction

delabs Technologies

Monday, January 05, 2009

Blind Dial Analog Temperature Controller

This is a Low cost controller, Analog Dial Temperature Controller. It is also called Blind Controller. This essentially means Open Loop, just control the fuel or energy input to the system to regulate heat. This is not a Blind Controller that way, it only cannot display the temperature value, that could be another reason it is called blind.

Dial cyclic timers were used to control heat, these were purely mechanical clockwork devices. They could regulate well, when the material flow (liquid) is constant and mains power is regulated. But when the job to be heated, varies in quantity, control temperature is close to ambient or when a precise control is required; closed loop controllers are used. Even a thermostat is like closed loop, as the bimetallic sensor is temperature dependent. But not good enough.

This controller is closed loop, precision controller, only the digital display of temperature is absent. Fine one deg variations may not be easy in this.

Blind Temperature Controller

PCB Boards for Blind Controller -

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