Measuring current with a panel meter - do I have this right?

Revolvr · Dec 20, 2006

Bit of a noob here, let me know if I have this right.

I am building a variable power supply which can handle 2A max. I have an LED panel meter set to measure 200mV max. I want to use it to measure current drawn by the load.

So I configure it to measure the voltage drop across a shunt resistor. I want the meter to show 2mV when 2A current is flowing.

Then R = V/I = 200mV/2A = 0.1 Ohm for the shunt resistor.
Power = V*I = 200mV*2A = 0.4 Watts

OK so far?

I can find 0.1 Ohm 1/2 watt resistors, but a shunt resistor won't exactly be 0.1 Ohm so how do I calibrate it? A trimmer pot in parallel? But I can't get a trimmer pot that low. Is my best bet to create my own shunt resistor using a long piece of wire that I can calibrate with another ammeter? A 22ga wire has 16.2 Ohms per 1000 ft, so I would need about 6.2 feet - coiled up in the power supply box?

TIA

-- Dan

kchriste · Dec 20, 2006

I can find 0.1 Ohm 1/2 watt resistors, but a shunt resistor won't exactly be 0.1 Ohm so how do I calibrate it?

I would do something like this:

ljcox · Dec 21, 2006

Revolvr said:
Bit of a noob here, let me know if I have this right.

I am building a variable power supply which can handle 2A max. I have an LED panel meter set to measure 200mV max. I want to use it to measure current drawn by the load.

So I configure it to measure the voltage drop across a shunt resistor. I want the meter to show 2mV when 2A current is flowing.

Then R = V/I = 200mV/2A = 0.1 Ohm for the shunt resistor.
Power = V*I = 200mV*2A = 0.4 Watts

OK so far?

I can find 0.1 Ohm 1/2 watt resistors, but a shunt resistor won't exactly be 0.1 Ohm so how do I calibrate it? A trimmer pot in parallel? But I can't get a trimmer pot that low. Is my best bet to create my own shunt resistor using a long piece of wire that I can calibrate with another ammeter? A 22ga wire has 16.2 Ohms per 1000 ft, so I would need about 6.2 feet - coiled up in the power supply box?

TIA

-- Dan

I don't understand why you want the voltage across the shunt to the 2 mV for 2A given that the max current is 2A. This is inconsistent with your calculations.

Your calculation is correct for 200 mV, ie. 0.1 Ohm.

If you can't buy 0.1 Ohm (I bought some 5 Watt 0.1 Ohm resistors from DSE) then you could make it up by connecting ten 1 Ohm resistors in parallel.

eblc1388 · Dec 21, 2006

You do it the other way round by making the measurement fitting the shunt value.

Most panel meters has a small trimmer pot on its pcb to allow the exact calibration of an internal 100mV reference voltage. This usually allow the user to change the reference voltage by 10% either way.

Use ten 1 ohm 1/2W resistors in parallel (you don't want them to heat up and change value too much) and connect a load less than 2A in series with the current measurement range of a DVM.

Adjust the trimmer pot on the panel meter to get an exact value similar to the DVM reading.

Revolvr · Dec 21, 2006

ljcox said:
I don't understand why you want the voltage across the shunt to the 2 mV for 2A given that the max current is 2A. This is inconsistent with your calculations.

Your calculation is correct for 200 mV, ie. 0.1 Ohm.

If you can't buy 0.1 Ohm (I bought some 5 Watt 0.1 Ohm resistors from DSE) then you could make it up by connecting ten 1 Ohm resistors in parallel.

Typo - should say 200mV. I want to keep the 200mV since the panel meter internal voltage divider is already set for that. I do need to move the decimal point though so it reads 1.XXX instead of 1XX.X.

I like the idea of 10 1 Ohm resistors. As another reply said, it would also reduce any temp change. It may also even out the variations so I get something close to 0.1 Ohm.

kchriste · Dec 21, 2006

I like the idea of 10 1 Ohm resistors. As another reply said, it would also reduce any temp change. It may also even out the variations so I get something close to 0.1 Ohm.

You could make the 10th one 1.5hm: or whatever if the resistance is too low. If it is too high you can experimentally add a 11th resistor and swap values until you have it right.
One word of warning though. If you plan on powering the panel meter from the same rectified supply as your variable power supply, note that most of them don't have floating inputs. ie: one input is usually common with the negative supply pin.

Revolvr · Dec 21, 2006

For the variable power supply, I plan on adding a 5V fixed output as well, which will also power the meters. I'm tempted to add +/- 12V outputs too in case I get around to fiddling with op-amps.

The meters are the LED 3.5 digit panel meters from All Electronics. They have +/- inputs separate from the power supply, so they should work.

Cheers,

-- Dan

kchriste · Dec 21, 2006

They have +/- inputs separate from the power supply, so they should work.

Just make sure you do not exceed the common mode range of the meter input then. Could you post a link to the datasheet on these meters or the full make/model number?

Revolvr · Dec 21, 2006

I don't have a datasheet. The model is PM-129B. A search on that number will come up with several links.

-- Dan

Sceadwian · Dec 21, 2006

If you want precision, trim the shunt... Shunts are usually just hunks of copper (or steel) made to a specific dimension, use sand paper, or a dremel tool and grind it down a tad.. The panel meter itself likley has some kind of trim built into it as well.

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