How to Measure DC Current Larger than Meter Rating

I have a current meter that has a 10A MAX rating. Is there a circuit that I can build which can scale this so I can measure up to 40A?

You can measure the voltage across an external shunt resistor.
For example, if the minimum sensitivity scale of your meter is 200mV then a 0.2V / 40A = 5mΩ resistor will give a 200mV reading on your meter for 40A.

Depending upon the accuracy you need, the 5mΩ resistor can be just a short piece of wire of the proper length. Calculate the length from a wire gauge resistance table.

If you know or may measure internal resistance of meter, Ra, add a resistor in parallel with meter connector with value R= Ra/3. That will extend range to 40A; This resistor must support current and heat developed in it.

iop95 said:

If you know or may measure internal resistance of meter, Ra, add a resistor in parallel with meter connector with value R= Ra/3. That will extend range to 40A; This resistor must support current and heat developed in it.

Click to expand...

You can also add a resistor in series with the movement so that the meter effective resistance becomes 4*R.

This is a more accurate method and avoid hunting for a accurate shunt... Most common meters (I am talking about good old analog meters that have a coil moving in between the poles of a magnet) take less than a mA and you really need not worry about the heat in the resistor. Most old analog meters are 10k/V.

This (series addition of ) will not work for the fancy digital meters. I do not know why. The shunt method probably will.

I needed to read very high Amperes. I purchased 'inductive' meters from an automotive supply house. (JC Whitney, a few dollars each, I don't know about availability at this time). It's really a magnetic needle, like a compass. You hold it against the wire. No direct connection. The needle moves right to indicate positive flow, left for negative flow. Just the thing if you need to read alternator current, or starter current draw in the hundreds of A.

Accuracy is questionable. So you would need to adjust distance of the magnetic needle, until it reads 10A when your DMM reads 10A. Then assume the magnetic needle gives correct readings for greater than 10A.

I found I could make it more sensitive, by holding a very strong neodymium magnet near the needle. Any semblance of calibration was lost, of course.

I appreciate all the great advice. I found this kit on ebay (see attached). It’s under $10USD so I’ll give a try. I also found these hall effect sensors on Digikey for about $1/pc. I’ll try those as well. I have attached a circuit which shows what crutschow described.

I’ll test both setups and provide some feedback. Thanks for all the responses.:grin:

c_mitra said:

You can also add a resistor in series with the movement so that the meter effective resistance becomes 4*R.

Click to expand...

That would be fine for small changes, like converting from 10A to 12A.

For x4 to 40A, the shunt will need to dissipate x16 the power and that might be a problem if the existing shunt is conservatively rated.

My own very cheap 10A reading digital multimeter places a 10 second maximum with 15 minute rest between readings, and I have already cooked that shunt.

Safest way is a proper 50A shunt that develops 50mV. That is a common shunt size, and with a millivoltmeter the scale will read directly in amps.

**broken link removed**