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.