Does this even make sense? Measuring voltage of a capacitor

[berkeman]

But does that voltmeter have this thing called "MOSFET"? It doesn't feel natural to include the word "MOSFET" in my answer.

I doubt it. But that only shows the panel meter. What is the make and model number of the voltmeter? What are its specifications? Especially its input resistance in voltage measurement mode...

We had a discussion about input impedance of voltmeters recently somewhere here (in EE?), and this wikipedia page ended up being helpful. See if it helps you (sorry if you've already seen it or been pointed to it):

http://en.wikipedia.org/wiki/Multimeter

.

 

[flyingpig]

Yeah I know what that is, but we didn't use it.

But how should I answer the original question?

Is the voltage reading steady? Why might you expect this voltage to vary?

It isn't steady because the resistance on the bench voltmeter is low

 

[berkeman]

Yeah I know what that is, but we didn't use it.

But how should I answer the original question?



It isn't steady because the resistance on the bench voltmeter is low

Correct. Although I think I would call the resistance "non infinite" or "finite" or something, rather than "low". You could use some of the description from the wikipedia page about input resistance (with attribution to wikipedia.org, of course) in your report.

 

[flyingpig]

But I still don't understand what is this MOSET thing. How does this let the current from the capacitor to be "read"?

 

[berkeman]

But I still don't understand what is this MOSET thing. How does this let the current from the capacitor to be "read"?

Not current -- voltage. The MOSFET can be configured as an amplifier or follower to sense the input voltage and drive the rest of the meter's measuring circuitry.

http://www.google.com/images?hl=en&...mplifier&gbv=2&aq=0&aqi=g6&aql=&oq=mosfet+amp

.

 

[flyingpig]

But I thought voltage doesn't "move"

 

[berkeman]

But I thought voltage doesn't "move"

Voltage does not move per se. I'm not sure I'm understanding your perspective here.

The external large capacitor has a charge on it, related to the capacitance and the voltage. When you connect the voltmeter's initially uncharged leads to the capacitor, a small current flows in the leads to charge up the input capacitance of the MOSFET preamp in the FET VOM. That puts (almost) the same voltage on the VOM input as was on the capacitor to start with. Very little current flows for the rest of the measurement (only MOSFET gate leakage current). When you disconnect the VOM leads from the external capacitor, they are initially charged up still to that same voltage, but the voltage droops to zero fairly quickly (based on the leakage current and input capacitance).

 

[flyingpig]

Can you not use acronyms...? I hardly know what those technical words mean.

 

[berkeman]

Can you not use acronyms...? I hardly know what those technical words mean.

So you didn't actually read the wikipedia page that I linked for you a few pages back then...

 

[flyingpig]

No I know what a multimeter is, we have one of those.

 

[berkeman]

No I know what a multimeter is, we have one of those.

So what acronym is not defined for you?

 

[flyingpig]

Mosfet, vom, fet

 

[berkeman]

Mosfet, vom, fet

Re-read the wikipedia article please. Thank you.