# How I can test the power supply across the load?

I want to test my power supply at the 12 V 10 A specification. I had tried with connect a 100-watt 0.7-ohm load resistor across it, but by that time the voltage went down to 3 to 4 volt. How does it happen? And how can I test this power supply?

## 4 Answers

how can i test this power supply?

Start by using the right load. The supply is rated for 10 A maximum at 12 V. By Ohm's law, that means the smallest valid load resistance is (12 V)/(10 A) = 1.2 Ω.

By connecting a 700 mΩ resistor to the supply, you violated it's current spec. Again by Ohm's law, (12 V)/(700 mΩ) = 17 A. The supply dropping its output voltage when you attempt to draw more than rated current from it is a totally reasonable thing for it to do.

You really need to look up Ohms's Law and understand what it means.

Also consider the power the load resistor must dissipate. If you manage to load the supply to its maximum rating, then the power into the resistor will be (12 V)(10 A) = 120 W. Even if your resistor were the right resistance, it doesn't have enough power handling capability.

You could get a second 700 mΩ 100 W resistor and put them in series. That effectively makes a 1.4 Ω 200 W resistor. That is within what the supply can drive and the combined resistance can handle. By Ohm's law again (yes this comes up a lot and is really useful), (12 V)/(1.4 Ω) = 8.6 A. That's what the current will be thru the combined resistor. It doesn't test the supply to the limit, but it's a good start.

• What happens if you use a resistor without the ability to dissipate enough power Commented Jun 9, 2016 at 20:16
• @imm: It will get hotter than it was designed to get, and any specs from the manufacturer no longer apply. It could just go out of tolerance, or it could melt, catch fire, or vanish into a small mushroom cloud. It could also cause other things it is in contact with to catch fire. Commented Jun 9, 2016 at 21:20
• @OlinLathrop Did you miss that that was not a question, but a link to a video of someone (stupidly) burning themselves on a resistor and then using pliers to hold the resistor as it gets red hot? Commented Jun 9, 2016 at 21:49

Is that 12V @ 10A ? Your 0.7 Ohm resistor will try to draw 17A which will overload the power supply. Most power supplies have a current limit of some sort which will reduce the output voltage until the current is at a safe level. Try loading your power supply with 1.2 Ohm. That should give you 10A.

If your power supply were going to be able to drive full output voltage of 12V to a 0.7 ohm resistor it would have to source over 17A. Since you said your design was for 10A it was unable to source the 17A and something inside the supply is causing the output to be drug down to the 3 to 4V. May be caused by current limiting or source impedance.

To test the supply you need to think about realistic loads. Use multiple of the 0.7 ohm resistors in series or find resistors with a higher ohms value.

For testing power supplies I have built myself an electronic load which is basically controllable current sink. The load is powered from a separate low current 12V wall wart. The circuit consists of a mongo power MOSFET in series with a small value sense resistor to GND. An opamp compares the voltage drop across the sense resistor to the setting of a multi-turn pot and then drives the gate of the mongo FET to keep the load current through the FET constant as per the pot setting. Separate small digital panel meters monitor the load current and load voltage. When testing at high currents and higher load voltages the mongo FET can generate an appreciable amount of heat so the FET is mounted to a good size heatsink which is then cooled by two 12V fans which are powered off the wall wart supply.

This is a picture of the type of N-MOSFET used for the electronic load.

A device like that can handle 10A to 20A easily and support a power level well over 100W. FETs of this type are very expensive but I got lucky and happened upon a box of them at a surplus store for $4 to$5 each. At mail order retail a part of that type could be over 10X that price.

If we consider that your power supply is ideal you get 17.12 A, and 12v between the terminal if you connect a 0.7 Ω across your power source

But in reality you have just a physical power source with internal resistance about 2 Ω, and in this case you got a 3v at the terminal of your power supply