[Boatanchors] Clarification ...

[Michael D. Harmon]

The problem with external shunts and multipliers is this:

OK, I have a 1 mA meter ("I") with a 68 ohm internal resistance ("R").  
If I hook it directly to my "meter test" box (a 9V battery in series 
with a 200K 10-turn pot) and increase the pot to the point that the 
meter is at full scale (1 mA through the meter), and measure the 
voltage  ("E") across the meter, I measure 68 millivolts.  That's just 
Ohms Law!  If I have an I of .001 (1 milliamp) across an internal 
resistance of 100 ohms, I will have a voltage of 100 millivolts.  If I 
add a shunt across the meter, the voltage will divide between the meter 
coil and the shunt, causing a DECREASE in the voltage across the meter.  
If I increase the value of the shunt, the voltage across the meter will 
rise to 68 millivolts and never go higher.

Now, if I put a resistor (multiplier) of 32 ohms in series with the 
meter coil, making a TOTAL resistance of 100 ohms, all that's going to 
happen is that the meter will still drop 68 millivolts and the external 
resistor will drop the rest.

The point of all this discourse is that a 1 mA meter movement with an 
internal resistance of anything other than 100 ohms internal resistance, 
will never show a voltage across the resistor of 100 mV.

Mike, WB0LDJ



On Oct 4, 2018, at 21:21, Michael D. Harmon <mharmon at att.net 
<mailto:mharmon at att.net>> wrote:

I have the parts of an old Simpson Model 375 ammeter.  Some time in its 
checkered past, it was dropped, thrown, or run over, smashing the meter 
movement beyond repair.  I decided to save the shunts and reconstruct 
the meter in a new box.

When I downloaded the manual, I discovered that the original meter was 
described two different ways.  In the electrical specs at the front of 
the manual, the meter movement is described as a "1 mA annular 
instrument with integral shunt".  The (external) current shunts for each 
range are designed to provide a 100 mV drop at full scale on the meter 
movement.  In the schematic on the back page however, the meter is shown 
as a 100 mV meter.

If the meter is designed to be 1 mA full scale, and provides a 100 mV 
drop, then according to Ohms Law, the internal resistance must be 100 
ohms. I have boxes of meters, but I have NOT found a 1 mA meter that has 
an internal resistance of 100 ohms!  I have measured them all!

  If you try to use a microammeter as a millivoltmeter, using the the 
multiplier formula (R= Efs/Ifs -Rm), most of the time you end up with a 
negative number (depending on the internal resistance of the meter under 
test)!  You just can't build a millivolt meter out of a microammeter 
without some trick of mathematics!

What am I missing here???  Was this some kind of special meter?  I have 
Jim Tonne's meter face design  program, but it doesn't do much for me 
until I find a suitable meter movement!

Anyone know where I can find a 1 mA meter movement with a 100 ohm 
internal resistance??

Thanks for the help!
Mike Harmon, WB0LDJ
mharmon at att dot net