EPE Mag - Free projects downloads

[Yorkie]

Over the years of subscribing to a number of magazines I have noticed many glaring errors, one classic one which has been repeated many times, is fitting an LED as a power on indicator on a piece of battery powered equipment. You may say what is wrong with that. The designer has obviously looked at the LED data sheet and seen figures for the forward current of 10 to 20mA therefore the dropper resistor for the LED is set for that current in the LED resulting in a piece of equipment where the power on indicator takes significantly more current than the rest of the circuit.

Oddly enough, for bits of test gear which is only used occasionally and for a few minutes at a time, I usually do include an LED, because although as Keith says, it will most likely use more current than the circuit itself, without a warning LED it's so easy to leave things switched on, so next time I come to use it, the battery is duff anyway. Really, for such projects it would make much more sense if I fitted a power socket and plugged in a wall-wart to power them as and when needed, rather than using Poundland PP3s!