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Cliff Jones
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Join date : 2010-11-22

PostSubject: Analog Meters    2010-12-06, 11:21 pm

How to's on using discarded and new meter movements.
Suppose you get meter without any information on the full scale voltage, and you want to know what it is, what do you do?

Heres a couple of hints to see if its working first.
Use a 9volt battery in series with your finger tips. Your body has enough resistance to limit the current through the meter. If that shows any meter pointer movement, then you have a working meter.


First object is find out is the meter AC or DC? If the pointer moves upscale with the battery in both positions regarding polarity. Then it is AC.

If it moves the pointer up and reversing the battery and it moves down or vs/vs then you have a DC meter.

Next step is to consider what is the safe current to be so as not to burn up the meter making it useless.
(One thing I learned on the job, was to use two potentiometers in this method.) Wink

So as a starting point, we turn to our faithful system of logic and ohms law. Most meters manufactured have a sensitivity of 50 microvolts or more. Now if we use a 9volt battery we have to limit the current to no more than 50 microvolts, to be safe. (sometimes even less)
So using ohms law
voltage = 9volts (I personally would use a penlight cell at 1-1/2 volts)
current = 50 microvolts
So we need to know the resistance required.
R = E/I = 9v/50 micro-amps= 9/5e-06 = 180,000 OHMs, = R1
so use a 500Kohm Potentiometer and adjust it for highest resistance.
Then hook the Pot (potentiometer), voltmeter and and battery  in series.
Then slowly move the pot (R1)to a lower resistance till the meter pointer is at the highest number on the pointer.
Use a multimeter to test the meter voltage at the terminals, this will be the maximum voltage swing allowed for the meter being tested.
Next use another pot of approximately the same value (R2), but in parallel to the meter movement connections.
Also start out with the pot adjusted to the highest resistance, and slowly decrease the resistance, till the pointer is now at half scale point.
Now measure the parallel pot after removing the pot from the circuit.
So now resistor(R2) resistance is now the same resistance value as the meter.(Rm)
To find the ohms per volt rating of the meter, use this formula
ohms/v = ohms per volt = 1/Ifs
Ifs = full scale current in amps

I'm a Science Thinker, Radio Tinkerer, and all around good guy. Just ask Me!
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