[sdiy] Power supply testing

[Tim Parkhurst]

On 10/19/06, mrmike <mrmike at clickbang.com> wrote:
> Hey all-
>
> I have acquired a linear power supply of unknown provenance. It's
> unmarked, but from the internals appears to be a linear 24V  3A supply.
> I'd like to test if this is true; can I simply plug it in and use a dvm
> across the open terminals, or do I have to have some kind of load? It
> does have a fuse on the input side of the transformer, which I am sure
> will be of great consolation to my widow if I screw up.
>
> Thanks,
> mrmike
>
Well, your ISP of "clickbang.com" could be very appropriate then!

But seriously, use extreme caution around power supplies. For most
linear supplies, you don't need a load to measure the VOLTAGE. Simply
connect the DVM leads across the output terminals and turn the PS on.

You WILL however, need a load to measure CURRENT. Others may have
better suggestions here, but I would get something like a 100 Ohm /
100 Watt resistor. Hook up the resistor and DVM in series between the
PS terminals, make SURE you set the DVM to measure Current (probably
use the 10A input and setting if your meter is so equipped). PLEASE
USE ALLIGATOR CLIPS WHEN DOING STUFF LIKE THIS. DON'T HOLD THE LEADS
WITH YOUR HANDS. SEE THE NEXT PARAGRAPH! Why use a 100W resistor?
Well, if this supply really is capable of 24V/3A, then it will produce
up to 72 Watts of power!

A little safety info: When doing any measurements on a power supply
(either voltage or current) I would use alligator clips to secure the
DVM leads first and THEN turn on the PS. This way, you're not touching
test leads to the GND and V+ (or V-) terminals with your bare hands.
If you need to move the positive DVM lead around to measure various
voltages, then still clip the GND lead on with an alligator clip and
CAREFULLY use your hand with the other lead. Why? Because if you use
your hands to press both leads and you mess up or slip, the voltage
has a nice path from V+, through your body, and out to GND.
Congratulations, you've just completed a circuit, and hopefully all
you've done is given yourself a nasty jolt. Techs working with high
voltages have a habit of keeping one hand in their pocket while
measuring voltages precisely to avoid this!

Finally, if you're still unsure about this, definitely take the time
to ask a friend / acquaintance and have them show you how to do this
safely.


Tim (have I scared you enough yet?) Servo
-- 
"Imagination is more important than knowledge." - Albert Einstein