Measuring moving coil meter resistance for zero-center meter

Thread Starter

joelippencott

Joined Jan 8, 2018
3
I want to build a sensitive DC millivolt/microammeter using a 1mA zero-center panel meter and need to find the meter resistance for the calculations required for driving this meter.
I am familiar with the common method of measurement of a standard (zero-left) meter's resistance by driving it to full scale with a battery and a pot and then connecting another pot across the meter terminals, adjusting it for half-scale deflection, and then measuring that pot's resistance.
I searched the Web and all the handbooks and documents I could locate, but still can't find anything on zero-center meters.
What would be the procedure for a zero-center meter?
 

DickCappels

Joined Aug 21, 2008
7,846
The principle applies. Adjust for full deflection in one direction, then add the shunt and adjust for half that amount of deflection.

Just curious: why not use a DVM on the ohms scale and read the resistance directly?
 

Thread Starter

joelippencott

Joined Jan 8, 2018
3
The principle applies. Adjust for full deflection in one direction, then add the shunt and adjust for half that amount of deflection.

Just curious: why not use a DVM on the ohms scale and read the resistance directly?
1) That appears to be the answer I'm looking for. Thank you.

2) I wasn't sure that a direct resistance measurement would give the correct value. I read somewhere that the meter's deflection caused by the applied ohms voltage can affect the resistance depending on the coil position within the magnet, and be inaccurate, hence the method found in most texts. I did find that common method to be identical in three independent publications, two of which were over 50 years old.
I will give this method a try too, to see if it agrees with the first.

Thanks again
Joe
 

OBW0549

Joined Mar 2, 2015
3,566
2) I wasn't sure that a direct resistance measurement would give the correct value. I read somewhere that the meter's deflection caused by the applied ohms voltage can affect the resistance depending on the coil position within the magnet...
The coil resistance of a moving-coil meter does not change with deflection; it is constant.

I've no idea where that "somewhere" was that claimed otherwise, but I suggest you take anything you read there with a HUGE grain of salt; common sense alone should tell you that a coil of wire does not change its resistance in the presence of a magnetic field.
 
The reason ohmmeters used to be avoided when determining the resistance of a sensitive meter is that, in those days, ohmmeters usually ran more current through the measured circuit than a sensitive meter could take. With today's DVMs that is not the case,
 

Thread Starter

joelippencott

Joined Jan 8, 2018
3
The reason ohmmeters used to be avoided when determining the resistance of a sensitive meter is that, in those days, ohmmeters usually ran more current through the measured circuit than a sensitive meter could take. With today's DVMs that is not the case,
It's all making so much more sense now.
Thanks for all the info. I'll be back if I run into any more trouble.
 
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