Let's say for example that you have a power supply without a label. You know it outputs, say 12V. How do you determine it's max Amperage output?

Ohm's law says that it depends on my resistance, but really, can a 12V wall wart output 12A at 1 ohm resistance?

Thanks in advance for my silly question.

 

For this u will need a good voltmeter and an ammeter together.

Connect the meters and just apply a variable load like a Rheostat.

There are lot of ways to identify the current capacity of any supply.
To tell u how, I need to know more from ur side,

By the way. What are u up to?

 

I was just curious cause I am converting an ATX PSU to a bench PSU and wanted to know how much amperage I can draw from each voltage.

Unfortunately all I've got is a cheap multimeter. Guess I will stick to small loads for a while.

I'm doing good, learning a lot at work. More than I could have imagined. Got some customers with some funky issues with transfer switches.

 

Try reading the spec written on the ATX PSU.

By the way, what is ur line of work?

 

I sell generators, transfer switches and parts for these items. I also help support the products by providing service to our customers.

The electronic knowledge necessary is really not much when compared to everything you guys talk about here. But, we get a little in depth when it comes to troubleshooting failures/issues.

 

Do u like to make a bench PSU? from an ATX

 

That is what I want to try to do, yes. I will probably have time this weekend. I will post up pictures if I survive...

>_>

 

Survive??? Are trying to blow up something..

Pictures wud be nice.

 

From the 12V rail, you can usually draw 8A or more, but don't rely on this. Relying on this will have your supply dead very soon because manufacturers often overspecify how much the supply can provide. 12V at 8A is almost 100W anyway, testing it to this level will be difficult and expensive.

 

I think this is normally done by increasing the load until the regulation starts to fail without exceeding the specs for internal circuitry (which of course might fail otherwise).

That is once the regulation starts to go bad you can safely say that the power supply can not regulate the voltage at that load

 

You don't have to have them loaded for a long period. Calculate a resistance value that would give you 5 amps and then with your meter connected to the supply, watch as you connect the resistor. If the voltage falls out of range then the current draw is excessive and you can try again with a lower amperage level.

No need to have these low ohm resistors hooked up for more than one or two seconds at the most. A 12 volt line will drop just a fraction or may even rise a few millivolts when loaded to something near its max. If it goes down to 11.6 or 11.7 then the current draw is excessive. A 1% drop would be .12 volts, or 11.88 from 12.00 unloaded.

If you have an old toaster that doesn;t work. Cut out the element wire(nichrome) and use it for a variable resistor. Wrap it around a paper towel roll and then remove the paper roll. by connecting clips at variousdistances from the end you can make several different low value ohm resistors, with a respectable power handling capability.

 

Actually, if it is prebuilt look for a fuse. Voltage and current tell a lot.

 

Many supplies have a current limiter ( and sometimes temperature limiter ) that will kick in at high current. This can be a tip off as to the max current. However some supplies may overheat ( and possibly destroy components ) even as they struggle to maintain the voltage. Almost all computer power supplies are marked as to max current. It is usually more than we need for a bench supply unless working on high power, low impedance circuits.

 

I just use a high-wattage (50W or so) wirewound resistor in a glass baking dish of clean water. For short-term testing, it works fine and is cheap.

John

 

I just confirmed that the PSU will output the following:

+5V = 28A (Red)
+12V1 = 14A (Yellow)
+3.3V=30A (Orange)
PG = Gray
+12V2 = 15A (Yellow/Black)
-12V = 0.3A (Blue)
+5VSB = 2.5A (Purple)

Output 400W, +5V, +3.3V : 120W

Stupid question...

I can obviously limit the amperage output by putting in say a 5A fuse in line with each output, correct?

 

Yes, or you can actually build a current limiter. Fuses are much simpler though, and current limiters are cheaper in the long run (depending on use, maybe not). When I build a project I like to tape fuses in a plastic bag inside the case, just in case.

 

Can you elaborate a bit on this current limiter?

First time I hear about this.

Thanks everybody!

 

You have two basic types of regulators, voltage and current. The current limiter is one kind. With power supplies there is another limiter, current fold back. Don't see them much anymore, but if the current exceeds a value (measure by a resistor) the voltage on the supply drops dramatically. Some voltage is maintained to sense when the short is removed, if it is the voltage pops back up. The old LM724 voltage regulator has this feature.

Current regulation can still smoke whatever it is powering, but the power supply survives. With commercial electronics the power supply can be the cheap end of the electronics.

Which brings me to the last protection, a combination of fuse and a circuit called a crowbar. A crowbar throws a SCR full across the power supply if it exceeds a voltage, shorting it and forcing the fuse to blow. Power supplies subjected to this ususally need major repair, but again, it could be better then blowing a whole rack.

 

Power input equals power output minus some power lost.

 

The tutorial that I am following also says that I can use a 317 IC to make this a variable voltage power supply, but I am not sure how I would wire this up or which IC this is supposed to be.

Anybody have any clues?

Thanks.