Design and Development Notes 2004-1

• 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.
• Reinforce copper tracks on PCB with a copper wire when it needs to carry more current, tinning will improve a bit.
• 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.
• The Lead acid battery breathes a lot, so keep them in a well ventilated open space but rain proof. Use sealed maintenance free battery when possible, these are safe. they are costly and do not have a long life.
• 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.
• Electronics assistant by simon carter on electronics2000 will help you learn some basics of electronic components, it is a windows-win32 application, it is a freeware.
• emu8086 will help you learn the x86 opcode set, assembly language, mnemonics which is the foundation code on which even the pentium runs, first try the examples then tweak the examples and learn.

Product Design Mockup and Prototypes

• 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.
• 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.
• You need a Fuse, Circuit breaker and earth leakage circuit breaker for safety against shock and fire hazard.
• 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.
• 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.
• 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.

Acronyms -

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Techniques of Circuit-PCB Design and Component Selection

• 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.
• All elektro-mechanical parts like switches, relays, connectors and pots have limited number of operations.
• Reinforce copper tracks on PCB with a copper wire when it needs to carry more current, tinning may improve a bit.
• Drawing a neat circuit diagram will ensure a better design, lesser parts means greater reliability, simple solutions.
• Observe color code in wires. positive is red and negative is black and green is earth and See Wire Color Code.
• 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.
• 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.
• Failure of parts are when parts are stressed beyond limit, bad environment, misuse, infant mortality and normal aging.
• 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.
• 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. Read a Tutorial on PCB Design.

Hands on Electronics from Solderman

• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
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What is Hands-On Electronics ?

Radio-Electronics (Gernsback Publications Inc.) started a quarterly Special Projects magazine in 1980. After 10 issues it was renamed Hands-On Electronics in the summer of 1984.

"More than 600 pages of assembly instructions, schematics, program listings, catalogs, photos and much, much more!" swtpc

(I used to read the Radio Electronics in The Indian Institute of World Culture which has a unique magazine section with magazines from all over the world - ananth)