How to measure an output current of an AC adapter

[syjuvekar]

I am measuring output current of an AC adapter, the output ratings of the AC adapter is 12 V dc, 2 A. I connected the AC adapter to an adapter socket and connected the load resistor, rated 10 ohms, 25 watts on the pins of the adapter socket in order to measure the output current with a multimeter. The resistor is connected in series.
I am measuring 6.3 A, instead of the output current 2 A. I dont know why is the output current measured is 6.3 A & not 2 A? Please provide an answer as soon as possible. Thanks.

- Sneha

 

[KerimF]

If you are sure that your load resistor is 10 Ohm and you trust your ampermeter that gave you 6.3 A, then the output of your AC adaptor is about 63V!
You are right to be confused.

But since your load is known (it can be measured by an ohmmeter), you can connected it directly to the output and measure the voltage on it instead.
I = V / R

 

[Audioguru]

Why do you think the multimeter should show 2A? Ohm's Law says that 12V/10 ohms is 1.2A, not 2A.

For 2A then the resistor must be (Ohm's Law again) 12V/2A= 6 ohms.

 

[Tahmid]

The output current depends on the output voltage and the load resistance. The 2A rating does not mean that you will get 2A output but that you can expect a maximum of 2A to be safely provided by the adaptor. You will get this when the load resistance is 6 ohms. Since you get 6.3A, then either your adaptor is supplying 63V or the load resistance is much lower than 10 ohms or the ammeter is giving a false reading. Measure the output voltage with a voltmeter. Measure the load resistance with an ammeter. Then you will know where the problem lies.

Hope this helps.
Tahmid.