Analog meter resistor sizing

Yesterday surfaced a question regarding sunt and resistor divider sizing for analog panel meters.

Actually I was a bit rude, while answering as I feel, that if somebody unable to calculate a sunt or a resistor divider for a panel meter, not the best idea to start to repair audio amplifiers.

Anyway, I think, I need to apologize for this. If you jump to electronics, you need to start somewhere.

The question: calculate a divider for 15V and a sunt for 30A. As we figured out, it needed for two unknown meters.

To replicate the problem, I picked an analog meter from my own parts collection, to show, how to do this.

First you need to know the parameters of the meter. Specially you need two things:

The DC resistance of the meters coil

The current needed for the full scale

First, I measured the DC resistance. Yes, I know, it is not a gentile thing to directly connect a panel meter to an ohm meter, but it probably will not kill it, and we will have the figure needed:

For measuring the current needed for the full scale you need a power supply (or some kind of battery), an amper meter able to measure in sub mA range, the meter to be measured and a potentiometer. For sizing the potentiometer you need to take into consideration that the analog meter (Deprez) itself usually in 20-100uA range.

I choose a 10k potentiometer. The meter in question is a 100uA one (you need to guess for the first measurement, what you have), the 10k + 1.15k according to Ohm's law will give us 1.15V (U=I x R, 0,0001 A x 11500 ohm).

Connect the panel meter, the potentiometer, your own amper meter in series to the power supply. Set the power supply to a lower voltage what you calculated above, and turn the potentiometer into the highest settings before you switch it on. If your power source has current limit, set it to let say 1mA, or the lowest possible setting to protect you meters, just in case of any error.

The result: R = 1150 ohm, I = 90mA (we don't need more precise than this as the meter itself have far bigger error than our rounding)

When it is done, switch it on and turn the potentiometer until you read full scale on your analog meter:

Now we have the values needed for the divider and sunt calculation.

So the divider. I choose 20V scale. For this you need a series resistor to the meter. Back to Ohm's law:

R = U/I, 20V / 0,00009A = 222.222 ohm

Subtract the coil resistance give 221.072 ohm. I choose a 220k resistor, just to show the result:

(the resistor need to be connected to the meter in series obviously)

Next - current measurement (it is easier to calculate this way):

I choose 1A full scale (I neither have a 30A capable supply nor appropriate sunt, so 1A is good enough for the calculation) We need the voltage drop across the meter at full scale: U=I x R, 0,00009A x 1150 = 0.1035 V.
From this the sunt would be R=U/I, 0.1035 V / 1 A = 0.1035 ohm
(Yes, I left out the resistance of the coil from the calculation, but the effect of it negligible)

I choose 0.1 ohm resistor, what I have:

Actually, we are done.

The thing above is not calibrated. To create a good meter, you need to do a minimal adjustment to get as precise result as possible.

For voltage measurement choose a slightly bigger resistor than calculated, and place a 10 times bigger 10 turn trimmer in parallel with the drop resistor before you connect in series with the meter. With this you can adjust the meter (yes I know, it is not the best possible solution, but good enough for this)

For the current measurement, choose a slightly bigger sunt resistor and connect a trimmer in series with your meter before connecting it in parallel with the sunt for adjustment.