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Meter Multipliers

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I bought a Simpson 1329 AC voltmeter off of eBay, which had been modified by an outside company. I haven't figured out the details of the modification yet. The meter's face reads from 0 - 1500, FS=500uA and to use an external 3 megohm multiplier.

In this context, a multiplier means a resistor in series with a meter. In current-reading meters, a shunt resistor is used in parallel. Volts = series. Amps = parallel.

The meter didn't come with a multiplier or multiplier resistor. As I don't have a need to measure voltages at 1.5kV, dividing by ten gives me a useful range: 0 - 150VAC.

Now, a FS=1mA meter movement has a resistance of about 50 ohms, so this meter, having a 500uA meter movement has a resistance of 100 ohms. In order to drop most of this 150V, we need to put a 300kohm resistor in series with the meter.

500 uA = 0.000500 A = 0.5 mA

1500V / 500uA = 3,000,000 ohms
150V / 500 uA = 300,000 ohms
15V / 500 uA = 30,000 ohm
1.5V / 500 uA = 3,000 ohms
0.15V / 500 uA = 300 ohms
0.015V / 500 uA = 30 ohms
0.0015V / 500 uA = 3 ohms

Now you know some basics on making voltmeter dividers.

Another caveat: if the meter is a rectified meter like Simpson's Rectifier series, voltage near the bottom of the scale may be inaccurate unless at least 0.4 - 0.7 V appears across the terminals of the meter. This is because the diodes have to be "switched" on, or forward biased.

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