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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I am using a vintage voltmeter that is 1000 ohm/volt (1ma for full deflection). I am using this DC meter to measure AC voltage. An AC and DC voltage is measured across a resistor. The AC voltage is isolated with a 470uf capacitor that then goes through a schotky full-wave rectifier (0.2v drop). The rectified voltage is maintained with a capacitor. The measuring DC voltmeter clearly draws some current with throughs off the accuracy of my measurements as the voltage being measured is no more than 5v and the current through the measuring resistor is about 10-15ma.
Is there any way around this and still maintain my vintage voltmeter?
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Steve,
I need a specfic value for the circuit. In this case, the resistor being used for the voltage being measured is 268 ohms. Of course, the volt meter @ 1000 ohm/volt has a significantly low resistance. Can I modify my measuring resistor so its value in parallel with my voltmeter is 268 ohms?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
Suppose you want the meter to read 5 Vrms full-scale = 1 mA dc average. For a full-wave rectified sinewave, rms current = (peak current)/SQRT(2) avg current = (peak current) x 2/pi so (rms current) = (avg current) x pi/(sqrt(2) x 2) = (avg current) x 1.11 so for 1 mA avg, Iac = 1.11 mArms and 5 Vrms/1.11 mArms = 4.5 kOhm. (in series with the meter)
For 268 ohms total, R = (268 x 4500)/(4500 - 268) = 285 Ohms
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vintage voltmeters
