[Boatanchors] Clarification ...

[Bry Carling]

Series and parallel resistor networks. That’s all we are looking at with meters for voltage or current. These are very easy to understand and there are good articles that explain them in ways the average ham can follow. Just look in older ARRL Handbooks, or search for web articles of the same.

Best regards - Bry Carling, AF4K





On Oct 5, 2018, at 5:48 PM, Michael D. Harmon <mharmon at att.net<mailto:mharmon at att.net>> wrote:

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> <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
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