Question on zero-center ammeter

[RLSprouse]

I just did the measurements and calculations using your method. I used a 120kOhm resistor, which just happened to measure exactly 120kOhms, and I measured my test bench supply at +4.987V. Hooking these up in series with the meter movement, I got a reading of 42.5uA on the meter. I measured the voltage across the meter at 0.065V. Using Ohm's law, R = V/I = 0.065/0.0000425 = 1,529.41 or about 1.529kOhms. So my estimate wasn't too far off, but I assume this is a more accurate figure.

[RLSprouse]

Coincidentally, just five minutes ago UPS delivered the 1-0-1 mA meter I ordered from Surplus Sales of Nebraska. It is a beautiful thing, a Simpson panel meter measuring about 4.5" square and *way* more robust than the cheap plastic meters from China. I will go through the resistance measurement process with it now.

[rstofer]

Coincidentally, just five minutes ago UPS delivered the 1-0-1 mA meter I ordered from Surplus Sales of Nebraska. It is a beautiful thing, a Simpson panel meter measuring about 4.5" square and *way* more robust than the cheap plastic meters from China. I will go through the resistance measurement process with it now.

I bought the same meter.  That thing would retail for more than $200 based on meters currently in production but, alas, it isn't listed in the catalog.  It is a superb meter.

The current offering of a 1-0-1 meter shows an internal resistance of 50 Ohms.  For a 5V power supply, we need 5k Ohms to get 1 mA.  We can ignore the 50 Ohms because that is about 1%.  We can use (2) 10K Ohm resistors in parallel.  Or, we could use 4.7K + 270 in series and get a little closer.

I would start with a single 10k just to be certain that I was in the ballpark.  This would give about 0.5 mA.

[Ian.M]

You really need a three digit reading for the voltage across the meter coil to get good results. Also you should ideally compare the current through the meter with another microammeter (or by measuring the series resistor and the voltage drop across it).  However with a series preset to pad the 50-0-50uA meter it up to 1.80K or 2.20K it will be fine as any errors will be compensated for during calibration.

[RLSprouse]

I put the new 1-0-1 mA meter in series with my 5V supply (which measures 4.987V) and a 1kOhm resistor (measuring 0.996kOhms) and the meter indicate about 0.49mA current. The voltage across the meter movement measured 0.050V. So, if I have things right, R(meter) = V/I = 0.05/0.00049 = ~102 Ohms.  I am assuming there is a shunt resistor internal to this meter that results in this low resistance.

[RLSprouse]

Given my previous results, I went ahead and measured the resistance of the meter directly with my Fluke 87V and it indicates an internal resistance of 101.3 Ohms for this meter. The meter is indicating just a hair beyond 1mA current, so I should be able to determine what voltage my Fluke DMM is pumping out to measure the resistance.

[RLSprouse]

I built the circuit from the book (which I posted earlier in this thread) but substituting my new 1-0-1 mA meter movement, and it worked perfectly.

Thanks to you guys for helping me understand all this stuff.