PSU test load (was: My PDP11/23 arrived...)

I start by knowing/estimating a suitable load current. If the power supply ratings are given (e.g in a service manual),m then I ill pick somethign that's around 10%-50% of the maximum load (if there's a minimum load figure given, I'll make sure the current I use is above that, of course). If I don;t have nay details on the PSU, I have to estimate the current. In the case of a lienar PSU, that's quite easy -- eitehr from the types of 3-terminal regualtors used, or from the design of the over-current trip (if it uses a 723, say). For a switcher it gets harder, but you may be able to make some sourt of guess from the circuitry it powers. Then calcualte the resistance in the obvious way (R=V/I where V = the voltage of that PSU output, I = the current you'be just guestimated). And the power dissipated in the load is of cource V*I (It's DC, so there's no power factor to worry about). Often car bulbs amke a suitable loard. It's worth rememebriogn that a filament lamp has an approxximately constant-current (not constant-resistance) characteristic. So a 6V 24W bulb will draw 4A from a 6V supply. I'tll also draw close to 4A from a 5V supply. -tony