Design and Development Notes 2004-6

• Over rate components at least double, if you need a 1A-100V-100uS diode, use a 2A-200V-50uS diode.
• 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.
• Use MFR (metal film resistor) 1% in all analog designs and if possible use only MFR for better reliability.
• Electrolytic capacitors have a shelf life, if you need to store them you have to charge them every month.
• 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.
• The current thru a 3mm or 5mm LED (light emitting diode) should not exceed 20mA, 15mA is quite good.
• 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.

Design and Development Notes 2004-5

• Keep functional blocks of circuits as modules or separate areas in a big PCB, this helps in testing and troubleshooting.
• Power circuits, digital circuits and analog circuits should have separate supplies and ground when possible.
• Inputs and Outputs are the points of electric abuse, opto isolate, use zener barriers with overrated devices.
• When a microwave oven is turned on, a big spike is caused in the power line due to heavy inrush current on start up, Even pumps and motors cause such short time surges. This causes tripping of electrical protection circuits.
• When a tree branch falls on the high voltage overhead electrical lines, they spark violently and supply trips.
• Big or tall trucks go in small roads they may snap all low level cable or telephone wires on their way.
• Do you need to turn on many equipments all at once? do it one after an other sequentially or use a timer for delays. This is because at peak power demand time, turning on all can cause your fuse to blow or worse.
• 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.
• 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.

Design and Development Notes 2004-4

• In a circuit swap all PNP and NPN, reverse all diodes and el-caps, reverse the supplies and input polarities, most of the circuits may still work like they did before.
• 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.
• A sharp bend in a PCB track is a huge inductance (say 1nH ) when it comes to RF. it may radiate or reflect, it may even act like an antenna and pick up RF from the SMPS, Motor or Cellphones nearby.
• 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.
• 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.
• 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. looping the NC (normally closed) contacts of a 230V Electrical Contacter-Relay can make it oscillate which makes a lot of electrical noise rich in EMI-RFI. This may be required for basic testing of timers, counters, uP, uC and embedded systems as they may malfunction in noisy environment.
• 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.
• 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.
• 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.

Design and Development Notes 2004-3

• 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.
• 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, opto couplers for all input and outputs, 4-20mA current signals for input and output, and a isolated wide range SMPS as power supply. more care is needed now as mobile phones and transmitters are filling the air with a lot of RF. so design circuits with care.
• 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.
• 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.. But when high speed is not required and for simplicity use microcontrollers.
• A battery that can be charged is best charged in CC constant current and CV constant voltage modes, that means both current and voltage have to be limited. When a battery is deep discharged many times it will age fast, and an empty battery will take huge currents so limit current. overcharging a battery will kill it so after a voltage level is reached it should only trickle charge in mA and a battery has to be cut off on overload or before deep discharge
• Look at this page on electromagnetic spectrum which will tell you about light and frequencies see at wikipedia.