Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

If you can design the board for a 10 degree C rise, that would be great, but you probably can live with 20 degrees C. I would keep the leads fairly short and mount the FET's as close to the terminals as practical, given that you may want to have a heat sink for the FET's.I would also add a few ceramic capacitors between DC power and ground as close to the FET as possible. It is probably a good idea to use a 0.1 uF in parallel with a 1.0 uF to take care of fast current spikes and limit noise. Be sure to use one pair for each FET. The reason for this is that all conductors have some resistance and more importantly a significant amount of inductance. When a fast pulse of current occurs the inductance allows the voltage to drop due to the quickly changing current, and the capacitors tend to keep the voltage more constant.

The subject of trace width to handle current is often overlooked. For example in the last two years I had a Bosch dishwasher that failed to heat the water, and a refrigerator that failed to turn on a compressor motor, both due to traces overheating to the point that sometimes a solder connection melted causing failure. Both were repaired by adding a 14 AWG copper wire to parallel the narrow trace. If you search online, there are hundreds of people who had similar failures with the Bosch dishwashers. Bosch even changed a relay to a higher current rating trying to fix it, but did not replace the wimpy circuit board.





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