Maker Pro
Maker Pro

I cant measure voltage on 100 picofarad capacitors on multimeter

ratstar

Aug 20, 2018
485
Joined
Aug 20, 2018
Messages
485
Is this because the power is draining out of the capacitor before the multimeter can read it?
 

ratstar

Aug 20, 2018
485
Joined
Aug 20, 2018
Messages
485
Its not in a circuit yet, I'm just charging the cap up with the power supply directly, the volt reading on the electrolytic is working, but the little ceramic cap is just displaying millivolts as if its not even charged up. The farad reading is about 100 picofarad, so what am I doing wrong?
 

Audioguru

Sep 24, 2016
3,656
Joined
Sep 24, 2016
Messages
3,656
Calculate the capacitor discharge time. Your voltmeter might be 20M ohms x 100pF = 2000us which is 2ms. Your voltmeter cannot show anything for only 2ms but if it did you will not see it.
 

ratstar

Aug 20, 2018
485
Joined
Aug 20, 2018
Messages
485
Yes, thats what I was thinking. So to measure a picofarad cap you need a special kind of multimeter?
 

Audioguru

Sep 24, 2016
3,656
Joined
Sep 24, 2016
Messages
3,656
I cannot think of any reason to measure the voltage on such a low value capacitor.
Instead you might be able to measure the voltage of the thing that charges it.
 

ratstar

Aug 20, 2018
485
Joined
Aug 20, 2018
Messages
485
An oscilloscope could read the volts tho right, just multimeters cant.
 

HellasTechn

Apr 14, 2013
1,579
Joined
Apr 14, 2013
Messages
1,579
Why do you need to measure the voltage of a charged capacitor? In practice the voltage will equal the supply voltage. At least for a certain time period.
There are meters that can calculate capacitance, ESR and the voltage drop (in certain freq).

Measure it with the source attached?
There is no point in doing that. He will ony measure the supply voltage.
 

Nanren888

Nov 8, 2015
622
Joined
Nov 8, 2015
Messages
622
There is no point in doing that. He will ony measure the supply voltage.
Yes. Exactly. :) Exactly the point. Is that not what he seems to want to measure?
The point was to either point out that this capacitor charges to the supply voltage, or to tease from him what he actually wanted.
Since he claims no ability to measure the fall with time and has suggested no interest in time at all, the voltage charged to is the only thing left.
 

ratstar

Aug 20, 2018
485
Joined
Aug 20, 2018
Messages
485
I'm actually making my own capacitors, so I dont know if they are functioning or not, I get farad readings out of them, but they are too small to test for real. If I hooked em up to an oscillator I can tell, because the led will turn on.

So I've got an oscillator circuit on the way. :)
 
Last edited:

HellasTechn

Apr 14, 2013
1,579
Joined
Apr 14, 2013
Messages
1,579
Yes. Exactly. :) Exactly the point. Is that not what he seems to want to measure?
That is not what i ment. If you hook a cap in parallel with a power supply and measure the voltage across it (with the supply still on it) then you are mearly measuring the voltage that the supply is giving you not the voltage that the cap can hold.
 

HellasTechn

Apr 14, 2013
1,579
Joined
Apr 14, 2013
Messages
1,579
I'm actually making my own capacitors, so I dont know if they are functioning or not, I get farad readings out of them, but they are too small to test for real. If I hooked em up to an oscillator I can tell, because the led will turn on.

So I've got an oscillator circuit on the way. :)

Thats probably the best approach. Still you could do an ohms test with an analog multi meter. you should see the resistance rising as the capacitor is charging up. YOu will not be able to figure out it's capacitance value but it will give you an idea that is is actually storing something.
 

Alec_t

Jul 7, 2015
3,587
Joined
Jul 7, 2015
Messages
3,587
Still you could do an ohms test with an analog multi meter. you should see the resistance rising as the capacitor is charging up.
I doubt it. Do you know of a multimeter with a response time less than 2mS?
 

ratstar

Aug 20, 2018
485
Joined
Aug 20, 2018
Messages
485
Maybe you could put a 1k resistor to the probe, then times the readings by 1000.
 

Audioguru

Sep 24, 2016
3,656
Joined
Sep 24, 2016
Messages
3,656
Maybe you could put a 1k resistor to the probe, then times the readings by 1000.
The input resistance of a multimeter is 20 million ohms. Adding 1k will make a VERY small difference of 0.005%.
The discharge time of 2 thousands of a second is too small for a multimeter to measure the voltage. Also your vision will not be able to see the numbers.
 

kpatz

Feb 24, 2014
334
Joined
Feb 24, 2014
Messages
334
Some multimeters have a capacitance measurement setting, and maybe also ESR and leakage.

If you're going to be making capacitors, I'd get a meter that can measure them properly. Measuring charge voltage on a small capacitor isn't going to tell you much, other than that it can hold a charge at all.

Also, with small capacitors, you'll have the inherent capacitance of the meter components, leads and probes to contend with.
 

ratstar

Aug 20, 2018
485
Joined
Aug 20, 2018
Messages
485
haha woops
The input resistance of a multimeter is 20 million ohms. Adding 1k will make a VERY small difference of 0.005%.
The discharge time of 2 thousands of a second is too small for a multimeter to measure the voltage. Also your vision will not be able to see the numbers.
 

bertus

Moderator
Nov 8, 2019
3,304
Joined
Nov 8, 2019
Messages
3,304
Hello,

An 1X probe of an oscilloscope has an input impedance of 1 Meg Ohm, 20 times lower as the 20 Meg Ohm of the multimeter.
An 10X probe of an oscilloscope has an input impedance of 10 Meg Ohm, still 2 times lower as the 20 Meg Ohm of the multimeter.

Bertus
 
Top