Voltage Measurement on a Capcitor

A DC voltmeter reads the average voltage. The average voltage of an 8 volts p-p sawtooth, riding on a base of -12, should be

-12 + (0.5 * 8) = -8,

close to what you're seeing.

John

Your volt meter is probably reading the RMS voltage, which is the peak voltage (of the sawtooth), divided by the square root of 3 (with a little bit if DC voltage added in too). Brian

I'm analyzing a circuit which contains a capacitor. The capacitor is fed a constant current and is discharged periodically. The capacitor is a non-polarized .01uf capacitor connected on one side to a -12v supply and the other side to the constant current source. If I measure voltage on one pin, I get approximately -7.5v, the other pin measures around -11.94. When I hook an oscilloscope up to the pin with the -11.94, I see a flat line which I presume would be the -11.94 v source voltage. When I hook up to the -7.5v side, I see a sawtooth ramp which is 8 volts high. I expected to see both of these outcomes. What I'm trying to figure out, being new to both electronics and using oscilloscopes, is what the -7.5v volt meter value represents.

I'm sure this is a really dumb question and when I see the answer I'll probably go 'Doh!'

Thanks.

But DC voltmeters (usually) read average, not RMS.

The RMS value of this waveform (an 8v p-p sawtooth whose max negative excursion is -12) must be greater than 8, because the average is 8. The RMS is actually 8.33.

John

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com... > On Sun, 12 Dec 2004 18:11:41 -0600, "Brian" > wrote: >

If you subtract the effective RMS voltage of a sawtooth wave that has a peak voltage of about 8 volts, from the - 12 volts DC, you come out with an effective RMS voltage of 7.38 volts. While this is not an AC voltage, it would take an AC voltage of this value to get the same heating effect. Since his meter is set to read DC voltage, he will get a negative reading. 12V - 4.62V = 7.38V Brian

If you're using a voltmeter when you get the -7.5V reading then you're probably seeing either the average or rms value (depending on the type of voltmeter you have). The meter's response is too slow for you to see the variations in the voltage -- whereas the scope is fast enough.

Also, be aware that the meter and/or scope can affect the reading if the current source is small (in value). With low-amperage current sources the current drawn by the measuring equipment can have an effect on the actual voltage in the circuit. For example, I recently was measuring the voltage on a capacitor that was tied to a non-varying current source. When I hooked up a voltmeter, I saw the capacitor's voltage changing -- even though I knew that its voltage had stopped changing (before I connect the meter). The voltmeter was drawing current from the capacitor and thus caused its voltage to change.

Bob

------------------------------ Do you have the AC switch on on your scope inputs??

-Steve

Ok, that's what I suspected. There was some doubt about whether the cap was really discharging completely back to -12 before ramping again, but given the wave is 8v p-p and the -7.x v is close to the average if the swing was from -12 to -4 I feel pretty comfortable that the cap is draining to -12 before recharging.

Thanks.

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com... > On Sun, 12 Dec 2004 22:21:57 -0600, "Brian" > wrote: >

Your Right.