DIY power supply load tester ideas

Thread Starter

zirconx

Joined Mar 10, 2010
48
I'd like to have a way to "test" (place a load on) 12v DC power supplies by drawing 0-40 amps from them.

I have this 40A PWM module, and a power meter to put in front of it. I've used this against a home made bank of power resistors that could draw 10A and it worked well.

What would happen if I hooked this 40A pwm module up to a 100A car battery tester? Would I be able to dump the full 40A into it, or would that .12 ohm load destroy the PWM module?

I've been trying to find pictures of what the inside of the car battery tester looks like, I wonder if I might be able to easily rewire it into a 50A load. Of course I could buy two of them and connect them in series. :) But if the 40A pwm controller can't handle a load that tries to source more than 40A then even that wouldn't work.

Thanks
 

DNA Robotics

Joined Jun 13, 2014
506
I use headlight bulbs and heating elements for load testing.
Stove burners, water heating elements, preheat glow plugs for diesel engines.
You can put a variable friction load on DC motors.
If you have the resources, the best battery testers use a carbon pile. Many carbon disks that conduct more as you compress them.
 

AnalogKid

Joined Aug 1, 2013
8,153
if the 40A pwm controller can't handle a load that tries to source more than 40A then even that wouldn't work.
Your description is very confusing. A load does not source current, it sinks it from whatever the power source is. In you case I guess it is a 12 V battery. The PWM module you link to is not a 40 A load - it is a variable power supply that can supply up to 40 A to its load.

If you are looking for an electronic load, such as a controllable bank of power transistors that can provide a constant current load to a power source, that is a different type of circuit. Search for 'electronic load schematic' (without quotes) to get dozens of examples. Here is a ***concept*** schematic. Most of the components are wrong for your application, but the circuit is very common.

ak

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Last edited:

crutschow

Joined Mar 14, 2008
23,527
What would happen if I hooked this 40A pwm module up to a 100A car battery tester? Would I be able to dump the full 40A into it, or would that .12 ohm load destroy the PWM module?
The problem is not with the PWM module, it's that the battery tester can likely only carry the load for a short period of time before overheating.
The battery tester is designed for a very short duration test, to determine if the battery has the capacity to start an engine.
 

Thread Starter

zirconx

Joined Mar 10, 2010
48
Your description is very confusing. A load does not source current, it sinks it from whatever the power source is. In you case I guess it is a 12 V battery. The PWM module you link to is not a 40 A load - it is a variable power supply that can supply up to 40 A to its load.
...
Sorry I am probably using incorrect terms. I know the the 40A PWM module is not the load. I would like to use the 40A PWM module to control the amount of current delivered to load. The "100A battery tester" likely has a resistance of .12 ohms to draw 100A at 12v. But I can't put that much load on my circuit. I am wondering if I can use the 40A pwm controller to only allow it to draw 0-40A, or if that will kill the PWM module.

If you are looking for an electronic load, such as a controllable bank of power transistors that can provide a constant current load to a power source, that is a different type of circuit.
I don't think I am looking for that, but maybe we are talking about the same thing. I am trying to create something that lets me vary the load applied to a power supply. But I plan to use a dummy load. And although it would be nice, I'm not aiming to have the current held precisely constant.

The problem is not with the PWM module, it's that the battery tester can likely only carry the load for a short period of time before overheating.
The battery tester is designed for a very short duration test, to determine if the battery has the capacity to start an engine.
True it can only handle loads of 100A for a short time, but I thought it could handle loads of 10a or 20a or whatever for much longer. Also the way the case is designed would make it easy for me to mount a small fan to it.

Thank you for the responses.
 

AnalogKid

Joined Aug 1, 2013
8,153
If you don't need precise regulation, then the circuit becomes even more simple. Here is another grab from the innergoogle that shows the basic concept. It is a variation of the one linked to by DC above. Again, concept only; your circuit would need to be beefed up, and don't even think about dissipating 400 W in a single device. Note that a 40 A load at 12 V is 480 W of heat that must go somewhere. Transistors, resistors, light bulbs, whatever - you're gonna need heatsinks and fans. In the circuit below, the 0.1 ohm sense resistor dissipates 160 W at 40 A.

ak

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