Discussion in 'General Electronics Chat' started by grasshopper, Apr 2, 2009.

1. ### grasshopper Thread Starter Member

Apr 2, 2009
11
0
Hi All,

an interesting situation came up when I was measuring the voltages across the individual capacitors in this experiment. I got always got a reading of 0 volts AC ! But when I measured across both of them (in series) i got a reading of 10 volts AC.

The way that the instructions explained it was that the voltage drop across the two capacitors would be equal to the voltage drop across each capacitor individually.

f that is the case, shouldn't I get a reading of 5V across each capacitor? What would be the reason why I am getting a reading of 0V for each individual capacitor?

FYI: I did not have this problem with the resistors. Just the capacitors.

If someone could shed some light on the science behind this, it would very much be appreciated.

Thanks!

2. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
A Capacitor passes AC Current, and blocks DC current. The AC voltage across a capacitor should be low, depending on the frequency: $Z=\frac{1}{2 \cdot \pi \cdot f \cdot C$

What was the AC voltage and frequency, and the value of the capacitors? Were there any other components involved besides the AC Source and 2 capacitors, all in series?

3. ### grasshopper Thread Starter Member

Apr 2, 2009
11
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thatoneguy, thanks for helping me out

the ac voltage source is 15V, frequency is 60khz

the 2 resistors are each 27k

the 2 capacitors are each .1uf

and all are in placed series, as per the instructions of the experiment.

4. ### italo New Member

Nov 20, 2005
205
1
Not two capacitor are created equaly as a consequence in series do not expect the voltage to be the same to be equal you must add a big value of resistance across to equalize them. If you add a 5mfd 5vdc across 10 vdc in series chances are that they will blow up since they have not equal leakage.

5. ### grasshopper Thread Starter Member

Apr 2, 2009
11
0
Hi italo, thank you for the advice,

I added a 15 Mohm resistor across (parallel to) each of the capacitors.

when I now measure the voltage on each capacitor individually, I get a reading of 2.9 V on one and 2.0 V on the other.

But when I measure across both capacitors, I get a value of 8.8 V!

shouldn't I be getting a value relatively close to 5 V?

What concept/theory am I missing/not understanding here?

6. ### Teri Member

Apr 3, 2009
12
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I think you may have a falling tree in the forest situation here Grasshopper. You are changing the voltages in the circuit when you attempt to measure them. Some AC voltmeters have a rather low internal resistance, so when you measure the AC voltage across one of the capacitors, you are actually measuring the voltage across the capacitor in parallel with the internal resistance of your meter. For example, if you are using a 1000 ohms-per-volt meter on the 3 volt range, its internal resistance is 3000 ohms -- very low compared to the rest of the circuit.

Some AC meters meant for measuring audio frequencies also have an internal capacitance in series with the internal resistance to make things even more complicated!

To accurately measure the circuit's voltages, you must use a sensitive (10000Ω/V or greater) AC meter, or an oscilloscope.

7. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Most DVMs are simplier in this reguard, a straight 10MΩ input impedance.

So, what kind of meter did you use?

8. ### grasshopper Thread Starter Member

Apr 2, 2009
11
0
I used a cheap 30 dollar DMM....

but that makes a whole lot of sense... I will try to obtain a much better DVM or try this out with an oscope.... I will let you guys know if this works. Thank you for the help you guys have given so far.

9. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
For a Good DMM, look for a used Fluke 87 series I or III, extra money, get the Fluke 87-V (Latest rev).

There are several used test equipment dealers around with them at double digit prices from time to time. eBay would be the another choice, as long as the seller is in your country, and shipping/handling is reasonable, it's worth it. (Paypal disputes do not refund shipping/handling, or out of country very well)

You can get a used one in excellent shape for under $100, and one with scratches/cosmetic problems that do not effect function for$50-\$75. True RMS from 1Hz-20Khz, Frequency/Duty Cycle, Relative measurements in all modes. Short of an oscilloscope, it is enough accuracy, functions, and reliability for a hobbyist, EE student, or repair professional.