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Overloading amperage? Answered

If I have a device that is rated at max 17 amps and 31.5 volts (but capable of operating at any lower amperage or voltage) and I want to run it off a 12v 7amp circuit, will that overload my circuit? Do I need to put a resistor in to limit the load?

What if I need to bring the amperage down to 3.47 amps off the 7 amp circuit... do I need to put in a resistor then, or will my power supply just automatically supply only what it can?



Best Answer 4 years ago

If your device is resistive it will try to draw about 6.5 amps at 12v.

Most power supplies either have a fuse or better ones automatically limit current by reducing the available voltage.

If your PS is a 12v battery you will need a 32 Watt 2.7 ohm resistor for 3.47 amps.


Here is the math ........................

Device resistance = 13.5v / 17A = 0.794 ohms

Total needed resistance = 12v / 3.47A = 3.46 ohms

Add resistance = Total less Device = 3.46 - 0.794 = 2.67 ohm

Power = I^2 x R = 3.47 x 3.47 x 2.67 = 32.1 watts

So I'm thinking that was a typo for the device resistance. Is this correct?

Device resistance = 31.5v / 17A = 1.85 ohms

Total needed resistance = 12v / 3.47A = 3.46 ohms

Add resistance = Total less Device = 3.46 - 1.85 = 1.61 ohms

Power = I^2 x R = 3.47 x 3.47 x 1.61 = 19.39 watts

So then, I would need a 19-20 watt, 1.6 ohm resistor?

Yes you are correct (it was late when I posted)

A 20 W 1.6 ohm Resistor Is the Answer.

You could put two 3 ohm 15 watt resistors in parallel

So then, I have one more question. Sorry! :)

Is the power supplied through the circuit consumed by the resistor and lost, or does the resistor just prevent it from being consumed?

In other words, I want to run 6 of these devices on each 12v 40A rail of the PSU I linked below... Using the resistor(s), that should bring the power consumption of the devices down to 41.64 watts each. That would leave me with 20.82A per rail. However, if the power being supplied through the circuits is being consumed by the resistors, then I'd actually be using 84 watts per device, or 42A per rail, which is over what they are specified for...

Does that make sense? sorry. I'm no electrician lol. Just trying to figure this out.

And actually... now I just had an epiphany, would it also work to just hook up two devices to each 12v 7A circuit? Wouldn't that just give me 3.5A per device (probably close enough to 3.47?), without requiring any resistors?

Sure you could wire them in series.

The resistor wastes the power as Heat to get you the desired output.

There are small modules ( mini inverters ) converters that do not wast the power but step down ( transform ) the power to whatever you need on eBay.

BTW your link does not work.

awesome. Thanks so much

This is the device:


I want to power it with a standard ATX power supply such as this one:


I would be using one of the four circuits on a molex 39-01-2080 connector as described here:


The reason for all this is that I want to try to run 24 of these devices off of one PSU, "undervolting" the devices to achieve a COP close to 3 (i.e. making them move three times the heat as the power being supplied to increase efficiency)

What device ? It matters.