Not really stompboxes but...

[DWBH]

... how do I measure the direct current a certain transformer outputs?
My dad has some 12v transformers, and he'd like to know how much current is there on the secondary tap. Is there any way to measure it?
Just use Ohm's Law?
(measure resistance on the secondary, measure voltage on the secundary and simply do the math?)

[Zben3129]

Do you have a multimeter?

Zach

[DWBH]

Do you have a multimeter?

Zach

Yes, I do.
But is it as simple as that? I thought I couldn't measure current like that. Guess I was misinformed.
I feel so stupid right now.

[teletroy]

Don't feel stupid! 

instead of typing one up - Here's a good explanation about using a multimeter to measure current.  Just make sure you set your meter correctly or you could blow up (well at least the meter) 

http://www.allaboutcircuits.com/vol_6/chpt_2/4.html

[R.G.]

Hang on. Potentially dangerous and misunderstood situation coming up.

While it's true you can simply measure the current with a multimeter, it is not true that you will then know how much current you can safely run through the transformer. Except in very small transformers where there is so much wire resistance that it limits the amount of power through the transformer, a transformer can be made to self destruct by shorting the secondary and connecting power to the primary to measure the secondary current.

The thing that limits transformer power transfer is either regulation (that is, how much does the secondary voltage sag?) or temperature rise. With small transformers, the amount of thin wire needed to keep it from self-destructing on the primary side often causes a high resistance and the secondary voltage sags a lot under load. It can sag so far that it's not usable. So the makers specify it at something like "12Vac at 200ma". What that really means is "if you pull 200ma through it, it will sag down no lower than 12Vac; it may be (and is!) 14, 16, maybe 18V under no load to offset the sagging". This is a classical issue with small transformers that mystifies each new year's crop of effects builders.

But if the wire resistance is not so big, the transformer's ability to supply current will be temperature limited. A transformer dies when either a wire burns open or when the insulation between wires burns and the copper wires conduct between turns. This is a smoke-comes-out condition. The iron and copper by themselves will work up to the temperature where they are beginning to glow a dull red. Insulation inside transformers is rated by the temperature it will withstand before it burns through and lets the wires short. Class A is rated at 105C, and is what most transformers are done with. For sure, if it does not say something about Class B, E, F, H, etc, then it's no better than class A.

Class A has the quirk that the external temperature of a transformer with a 105C hot spot inside is about 130F, which happens to be the temperature where you will not willingly leave your finger on the transformer iron. If it's too hot to touch, it's too hot.

The reason I'm going through all this is that normal small transformers reach 130F external temperatures long before they either glow red or emit smoke. And certain long before they pass currents equal to the secondary voltage divided by the secondary resistance. Short the secondary  of most transformers (which is what measuring the current with a multimeter does, in essence) and it will have a very high current for a very short time before the AC power breaker blows (good!) or the transformer begins to burn (bad!).

The way you tell how much current a transformer will put out is to load it down and let it get hot. When the temperature is 130F externally, I would not go any higher unless I knew it was rated for more.

Transformers do not put out DC, they put out AC. So if you want to know how much DC it will put out, you must test it by putting it through rectifiers and filters and loading down the DC until it gets to temperature.

A transformer of about 3-5 lb may have a thermal time constant of 10-15minutes, so testing temperature rise will require seeing how hot it gets after five time constants - an hour to an hour and a half. The test is: guess at an output current; now load it to there and measure temperature as it rises. If it is rising faster than would give 130F in an hour, back off until it stabilizes. If it's below 130F then, inch the current up and let it stabilize again.

Ugly and complicated? Yep, it is. Unfortunately. this aspect of transformers is not simple.

[teletroy]

While it's true you can simply measure the current with a multimeter, it is not true that you will then know how much current you can safely run through the transformer.

Excellent point- I was only thinking about that first part when I typed that up- not the last.

[drewl]

I like that..."smoke comes out condition".....can I use that?
Excellent explanation RG.