I have a 120v <-> 9v ac transformer rated at 3.34 amps, but my multimeter suggests that (after running it through a diode bridge), I can get 7+ amps out of it @ 9.4 volts ac.

Can I?

I have a device that draws 4 amps DC at 6-10 volts,a nd I'd like to power it from this device.

  • 1
    \$\begingroup\$ How does your multimeter suggest you that? \$\endgroup\$
    – miceuz
    Feb 18, 2013 at 19:24
  • \$\begingroup\$ There is an 'unfused' 10A setting -- I tried it at the outputs of my diode bridge. Not sure I was doing it right. \$\endgroup\$
    – gbronner
    Feb 18, 2013 at 19:25
  • 2
    \$\begingroup\$ I'm pretty sure you didn't do it right if you had no load in series with the Amp meter. Always make sure there is a load when measuring ampère. \$\endgroup\$
    – jippie
    Feb 18, 2013 at 19:40
  • 1
    \$\begingroup\$ what he did was practically use the Multimeter's ( should i say ammeter's ) resistance as the series resistance for measuring Current. NOT RECOMMENDED UNLESS you know what you are doing. \$\endgroup\$
    – D34dman
    Feb 19, 2013 at 1:39

3 Answers 3


If it's rated at 3.3A and you draw much more than that from it for an extended time, it will exceed its rated temperature and eventually its insulation is likely to break down. At which point, smoke ensues.

Whether 20% overload (4A) is too much more I can't say. I would guess short periods of operation (up to an hour, then cooling down for an hour) would be OK for my own use, but I wouldn't sell anything made that way.

7 amps is definitely too much.

  • \$\begingroup\$ Reading his question again, i have a feeling that @gbronner had made mistake in assessing his observation. He measured voltage and Current at two different load condition. So practically his claim that he gets 7+ amps out of it @ 9.4 volts ac is invalid. Nonetheless your answer is very helpful and shows true experience :). \$\endgroup\$
    – D34dman
    Feb 19, 2013 at 1:45
  • \$\begingroup\$ It is possible that he read 7A at 9VDC after the bridge. Now 9V * 1.4 - 1.4V(bridge) = 11.2V so 9.4V DC (not AC) at 7A would be plausible, if he measured both with the same load. I suspect a transformer rated for 3A would have greatly exceeded 7A short circuit directly into a multimeter! But in any case, 7A is not recommended for operation. \$\endgroup\$ Feb 19, 2013 at 8:58

You have to measure voltage while you are measuring current to get power rating. The transformer will not be giving out the same voltage when shorted out thru a shunt resistor in a multimeter.

Take another multimeter, set it up as voltmeter and measure the voltage while measuring current. Then power delivered would be P = I * V and you can roughly expect I = P/V of current in normal operating conditions.

Note that the voltage measured won't be accurate, rather some approximation your multimeter comes out with as you will not be getting constant voltage, rather something like that:

enter image description here

Try putting the biggest electrolytic capacitor you can find accross the bridge output while measuring (watch polarity and voltage rating).

BTW, you are lucky you didn't pop the fuse in your multimeter, never measure current while shorting the power source with multimeter, put some load in series, like 1ohm power resistor.


After reading about how you measured current i am pretty sure you made a slight mistake in assessing your observation.

You measured current by connecting your multimeter directly across the Bridge rectifier without another load in series. Which means the only resistance was your Multimeter's resistance in ammeter mode.

The Transformer through bridge rectifier must have spit out 7 Ampere, but at a much lower voltage. You could confirm this by measuring voltage across your bridge rectifier while measuring current.

It is something called load regulation, if you increase the load ( reduce resistance ) the voltage starts dropping when its demanding more than the power rating of the Source.

So to answer you question,

VA rating of your tarnsformer = 9 x 3.34 ~= 30VA Extreme demand for your load is 10V @ 4A = 40W. Which is not possible (well you can get it in practice see @Brian Drummond's answer) since it requires more voltage and more power and more current than your transformer can supply.

Minimum demand by your load is 6V @ 4A = 24W. Here we have hope since its demanding less voltage and less power. You would have to use extra components for regulation thought.


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