I have a small PSU board out of a defunct DVD player that I'd like to use as
a supply for my breadboards. It has two rails, +12 and +5 with two GND
lines, but there's no current indication. My guess is that it's in the
500mA-1A range, but I really have no clue.
What's the best way to test it and determine a safe limit?
Also, if it requires a load in order to operate, should I just wire in a big
resistor across +12 to ground?
Electronic – Power supply amperage
power supply
Related Solutions
If the power supply outputs can be put in series (i.e. their 0V ends can be "lifted" from ground) then you get 48V @5A and you can probably get a cheap off-the-shelf buck regulator that will convert down to 24V @9A. If you were designing from scratch I'd consider this part: -
The above circuit is shown generating a 12V output from inputs ranging from 15V to 60V. If both power supplies are connectable to produce one at 48V then the above circuit, with ratio modification to the two resistors in the red box will easily produce 24V. However, I'm sure you could pick something up from a chines source or ebay for ~$10.
If they both have to be grounded then you could use a H-bridge driver chip where one half of the H bridge is powered from one supply whilst the other half is powered from the other 24 v supply. Maybe something like this.
Or you could have both supplies individually driving their own 12V flyback converters and wire the outputs in series (transformer outputs are used in flyback converters). This will work too.
As I mentioned in my comment, your question is too broad for this format. However, in the interest of moving you forward in your project, I propose the question of: "What kind of power supply should I be thinking of building for this use case?"
A lab power supply is generally of the linear variety (as opposed to a switching power supply). I think the first decision here is whether a linear supply will be feasible for you.
That question comes down immediately to the question of heat dissipation. A linear power supply reduces its output voltage by dissipating the excess energy as heat. Worst case for the specs you mention would be 50A at short circuit, although 50A at 1V would be almost as severe. If your unregulated supply (transformer + rectifier + bulk capacitor) output say 15V, the worst case heat dissipation would be \$15V \times 50A = 750W\$. That's one heck of a big heatsink, even with a noisy set of cooling fans.
So I'd be inclined in your position to strongly consider a switching power supply which would vastly reduce the amount of heat dissipation required.
For that I think you'll need to elaborate your statement of your requirements and perhaps then ask a more specific question addressing the next step for you.
Best Answer
The case for home appliances usually has the power consumption written on it. That will give you an upper bound if you assume say 80% efficiency.
You could hook a fat power resistor up chosen to dissipate say 1A and see if the output voltage drops indicating overload.
Really, you could do better with a dual-rail PSU from some obsolete computer equipment (eg Zip drive), available really cheaply at flea markets etc. If you only want 5v, old mobile phone chargers are good; prefer switchmode (light, efficient, well regulated) over transformer (heavy, inefficient, poor regulation).