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I am trying to measure a voltage of about 2500 at 7 Nanoamps.
I understand that a “normal†voltmeter is useless at 7 Nanoamps. I also believe that a resistive voltage divider would be useless at this low amperage.
I have obtained a Pasco Electrometer. It works great, but maxes out at 100 Volts.
I wanted to use a Capacitive Voltage Divider to bring the voltage down to less than 100 volts, so that I can use my Electrometer to measure the voltage.
The voltage divider setup I am using is on the attached .jpg
For the value of C!, I am using a 10PF (6KV max voltage) ceramic capacitor. For C2 I am using a 1000pf (6KV max voltage) ceramic capacitor.
If I apply 1000 volts to Vin, I should get about 10 volts at Vout.
I have used the divider with 330PF and .033uf polypropylene capacitors, it worked quite well. My problem is that the poly capacitors max out at 2K. I need to go a little higher, probably around 2500 volts.
I ordered some 10f and 1000pf Ceramic capacitors that max out at 6K. I wired them up as shown in the .jpg. For some reason it does not work.
Does anybody know if Ceramic capacitors can be used for this purpose?
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Is your source a DC source or an AC source?
If it is DC then the capacitance of your divider is not doing anything, and it is just a resistive voltage divider made up of the leakage of the caps/wiring/electrometer. If it is AC then an electrometer won't be able to measure the voltage because the net charge delivered to the meter will be 0 without an external rectifier.
It looks like the reason was the leakage. the insulation resistance of the ceramic caps was less than 1/2 of the poly caps.
I am sorry that I don't have enough electrical knowledge to explain why the voltage divider works, I can only tell you that it does. If you try it with a battery and use a regular voltmeter you will see that it works well...I just can't explain why.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
A high-voltage capacitor with low capacitance and low leakage is easy to make. Make a Leyden jar, or just two pieces of metal foil pasted at opposite sides of a plastic sheet. A capacitive divider works, but just for signals that are varying. If you discharge both capacitors and suddenly apply the voltage to be measured (possibly with a large capacitor already charged by it in parallel), a measurement (not very precise) is possible. You can use an electroscope too.
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Capacitive Voltage Divider
