I cant measure voltage on 100 picofarad capacitors on multimeter

[ratstar]

What about gigohm resistors - ive seen them on the internet.

 

[Harald Kapp]

There is no use in measuring the voltage on a capacitor - even if you were able to do so with a veeeeeery high impedance instrument. Assuming you had an ideal voltmeter with infinite impedance. All this instrument would tell you the voltage across the capacitor. But regardless of the capacitance, be it 10 pF or 100 µF, the voltage you measure will always be the one you used to charge the capacitor.
to measure capacitance there are several proven methods, like measuring charge/discharge time, measuring impedance at a known frequency etc. Why do you think you need to find a new method?

Excuse my being direct: usually the fine people here really try to help, but your posts seem to be designed to keep us busy without you showing respect for the answers and tips given and frankly without showing some true learning effect.

 

[ratstar]

I'm sorry if I dont seem respectful or grateful, I actually am, maybe I should thank you all now for helping me out. This forum has been invaluable to me already, even tho I've only been here a shortwhile.

Thanks alot!

 

[Alec_t]

What about gigohm resistors

What about them? The resistance of any atmospheric moisture surrounding them immediately makes their high value rather pointless .

 

[HellasTechn]

I doubt it. Do you know of a multimeter with a response time less than 2mS?

I forgot that we where talking about pico farads.

 

[hevans1944]

Electrometers have a high enough input impedance to show the voltage discharge from a 100 picofarad capacitor, but why bother since (1) you probably won't be able to get your hands on one and (2) it sounds like you don't know enough electronics theory to appreciate what you would be trying to measure. The only reasons I can think of for building, instead of buying, capacitors is to learn about how capacitors work, or because commercially available capacitors are too expensive or unobtainable in the values and voltage ratings you desire. Tesla coil builders sometimes make their own capacitors, but there are other forums that discuss that better than this one.

 

[WHONOES]

I Have an "Atlas LCR40" LCR tester. It fits in the palm of my hand. The test leads can be nulled out so that the reading you get is the reading you get. I have had it for many years and it has had a lot of use.

Look it up.

 

[hevans1944]

What about them? The resistance of any atmospheric moisture surrounding them immediately makes their high value rather pointless .

RE: Gigaohm resistors. The few that I have encountered, and successfully used as feedback resistors in electrometer circuits using op-amps with field-effect transistor (FET) input stages, were encased in glass envelopes. The glass envelopes are presumably either back-filled with dry nitrogen gas or evacuated to a hard vacuum to eliminate leakage across the resistor body.
Resistors with values in the teraohm range are available from vendors such as DigiKey. They all appear to have horrible negative temperature coefficients of resistance, and some manufacturers warn NOT to clean the glass envelope because it is coated with silicone (whatever the hell that is!), but you pays your money and you makes your choices.

The glass on the gigaohm range resistors that I used looked like the picture above, but they had to be carefully cleaned with isopropyl alcohol to prevent a leakage path from forming on the outside of the glass in air, as @Alec_t pointed out in his post #24 above. My resistors were supported on Teflon-insulated wiring posts, and connected to op-amps mounted on Teflon sockets. A real PITA to get this set up properly.

After everything was soldered in place, I then had to clean off the solder flux residue, again using isopropyl alcohol and a stiff bristle brush. IIRC, I actually soaked the entire prototype, with the op-amps removed from their sockets, in a tray of alcohol just to be sure everything got cleaned. I may even have used an ultrasonic bath to ensure nothing was left to chance. I was younger then, in the 1970s, still a non-degree electronics technician, eager to please my boss and not make mistakes. I did manage to please my boss, I think because he was "old school" much like me, but I made plenty of mistakes during the twelve years I worked there full-time while pursuing a BEE degree part-time. Fortunately, I learned from the old mistakes and always went on to discover how to make new mistakes. This process continues to this day. And I did finally graduate, ten years and four children later, in the summer of 1978.

 

[Martaine2005]

Fortunately, I learned from the old mistakes and always went on to discover how to make new mistakes. This process continues to this day.

That is the best sentence I’ve read in years and how true it is.
Do you mind if I use it to make myself feel wise?

Martin

 

[hevans1944]

That is the best sentence I’ve read in years and how true it is.
Do you mind if I use it to make myself feel wise?

Martin

@Martaine2005: Sure, go ahead and use it. Put it into practice and you will either become wise or seriously injured or dead. So far, I've been lucky and have acquired more wise than I deserve and only minor (non-serious) educational injuries. Live and learn. Or not.