hello,
i would like to ask on which formula should i used in order to measure the capacitance of the capacitor. i was only given the frequency, signal level and bias value.

i have read about the formula and the information on internet already provide the capacitor value.
I have to supply ac source so how can i know which value of voltage should be able to be used?
is there formula for that?

thank you.

 

Is this a question related to your studies?

The formula (and there are several) depends upon the circuit in which the capacitor is measured. Let's see your circuit or at least a description of it.

 

Is this a question related to your studies?

The formula (and there are several) depends upon the circuit in which the capacitor is measured. Let's see your circuit or at least a description of it.

yes. it is for my studies. the circuit have only ac signal connected to a capacitor. the ac signal is 100kHz, signal level of 50mV and 0V bias.
also, how can i know how much voltage should i apply to measure capacitor? if there is formula for that?

 

drive the capacitor with a resistor in serial and measure the voltage across the resistor to get at the impedance of t he capacitor, thus its capacitance.

 

MOD NOTE: Moved to Homework Help from Projects

 

yes. it is for my studies. the circuit have only ac signal connected to a capacitor. the ac signal is 100kHz, signal level of 50mV and 0V bias.
also, how can i know how much voltage should i apply to measure capacitor? if there is formula for that?

WHAT CIRCUIT?

We are NOT mind readers. There are many, many circuits that can be used to measure capacitance as well as many techniques. Throw us a bone and show us the circuit YOU are supposed to be using.

 

I thought it was pretty clear that the circuit is a voltage source and a capacitor. There's only one way to make a circuit out of two components.

The answer is: measure the current drawn from the source, calculate the impedance using the source voltage, then calculate the capacitance using the impedance and the frequency.

 

I thought it was pretty clear that the circuit is a voltage source and a capacitor. There's only one way to make a circuit out of two components.

The answer is: measure the current drawn from the source, calculate the impedance using the source voltage, then calculate the capacitance using the impedance and the frequency.

may i know how to measure the current drawn from the source?

my attempt is :
I=E/Xc,
is this right?

and also, what is means by signal level 50mV? why is it needed? i had searched about it but still confused.

 

If this is for homework, I assumed the question was theoretical. If you need a practical way to measure it, things get more complicated. You would need a method of current sensing.

Yes, I=E/Xc.

As for the signal source: there are two basic ways capacitors are characterized. Their impedance to AC signals and the voltage they develop in response to a DC current. Either way you need a signal source, both for measuring and for the capacitor to be a useful part of a circuit. A signal level of 50mV means a signal source with an amplitude of 50mV.

 

If this is for homework, I assumed the question was theoretical. If you need a practical way to measure it, things get more complicated. You would need a method of current sensing.

Yes, I=E/Xc.

As for the signal source: there are two basic ways capacitors are characterized. Their impedance to AC signals and the voltage they develop in response to a DC current. Either way you need a signal source, both for measuring and for the capacitor to be a useful part of a circuit. A signal level of 50mV means a signal source with an amplitude of 50mV.

how about the value of voltage that i should supply to the ac source?how can i know suitable value to supply? is there any way that we can calculate how much voltage should be supply? i searched about this in internet but the available examples already provide the ac voltage value.

 

In Post #10 fatimahfaisal wrote:
how about the value of voltage that i should supply to the ac source? how can i know suitable value to supply?

Any voltage that is adequate for you to observe the current but not so high that it can damage the instruments and components being used.

In Post #10 fatimahfaisal wrote:
is there any way that we can calculate how much voltage should be supply?

That depends on how you plan to measure the current and the capabilities of the instrument you are going to use.

In Post #10 fatimahfaisal wrote:
i searched about this in internet but the available examples already provide the ac voltage value.

Those values are a good for a start.

Remember to use a sine wave only.

 

In Post #10 fatimahfaisal wrote:
how about the value of voltage that i should supply to the ac source? how can i know suitable value to supply?

Any voltage that is adequate for you to observe the current but not so high that it can damage the instruments and components being used.

In Post #10 fatimahfaisal wrote:
is there any way that we can calculate how much voltage should be supply?

That depends on how you plan to measure the current and the capabilities of the instrument you are going to use.

In Post #10 fatimahfaisal wrote:
i searched about this in internet but the available examples already provide the ac voltage value.

Those values are a good for a start.

Remember to use a sine wave only.

yes. i will use sin wave. i use lcr meter to measure this capacitance value. i have to calculate in theory first so that i can be sure that the output displayed in my program is correct.
that is the reason i am asking because i only provided with freq,bias and signal level. i wonder how much ac voltage should be supplied.

 

The larger the amplitude of the AC part of the signal the better your accuracy is likely to be, just as long as it is not so much that it can hurt something.

What kind of capacitor will you be measuring? (e.g. Ceramic disk, aluminum electrolytic, film, etc.)

 

The larger the amplitude of the AC part of the signal the better your accuracy is likely to be, just as long as it is not so much that it can hurt something.

What kind of capacitor will you be measuring? (e.g. Ceramic disk, aluminum electrolytic, film, etc.)

i will be using ceramic and aluminum electrolyctic. does the value will differ when using different types of capacitor?

 

No, but if you detect and DC current through the electrolytic capacitor you might want to add some bias and see if that changes the result.

 

No, but if you detect and DC current through the electrolytic capacitor you might want to add some bias and see if that changes the result.

if i put Vm (sin wt) , Vm=12V will it be okay? as the freq =100kHz, Signal level=50mV and 0V bias. will it be okay?
i still confused on how to use suitable value

 

You can use a peak voltage of 12 volts as long as the capacitor is rated for that voltage.

You understand the relationships among capacitance, reactance and frequency so you will be able to adjust the test parameters as necessary to obtain a meaningful measurement.

50 millivolts sounds small for common bench equipment. 5 volts might be a better starting point.

 

hello,
i would like to ask on which formula should i used in order to measure the capacitance of the capacitor. i was only given the frequency, signal level and bias value.

i have read about the formula and the information on internet already provide the capacitor value.
I have to supply ac source so how can i know which value of voltage should be able to be used?
is there formula for that?

thank you.

Hi there,

If you have a sine voltage source and place it across a capacitor then you have to measure the current in order to calculate the capacitor value. The formula is fairly simple:
i=Vc*w*C

where w=2*pi*f where f is frequency in Hertz, and Vc is the AC voltage across the cap, and you can solve that for C.

There are a couple of catches here though. One is that the applied signal level has to be ok for the cap, the other is that the frequency has to be right for the value of the cap or else you will not get discernible readings (not large enough to measure properly).
This usually means you should have some way to change the frequency and the applied voltage level. For example if you are going to measure 0.001uf you will need different settings than if you are going to measure 1uf because there is a 1000 to 1 difference between those two so if you have the setting for 0.001uf such that you read 1ma, then for the 1uf cap you would have to read 1 amp, which is a lot for testing. Similarly, if you measured a 10uf cap you'd have to measure 10 amps which is too high for this kind of test, but changing the test parameters will help bring this back into a reasonable range.

Alternately, you can use a resistor in series with the cap. The cap voltage is then
Vc=Vs/sqrt(w^2*C^2*R^2+1)

where Vc is the voltage measured across the cap and R the resistance in series, and Vs is the applied AC voltage, and the cap value is then:
C=sqrt(Vs^2/(Vc^2*w^2*R^2)-1/(w^2*R^2))

If you measure the voltage across the resistor it is:
Vr=Vs*w*R*C/sqrt(w^2*C^2*R^2+1)

and if R is small compared to w*C we then have the approximate:
Vr/R=Vs*w*C

and that can be solved for C also (but only if R is small):
C=Vr/(Vs*R*w)

Note that however you do it, it is best to alter the applied signal in order to get a good reading on the measuring equipment. You can predict this with the above formulas and if you know the approximate value you can set the parameters beforehand, and if you dont know the approximate value then you can apply a variable test signal until you start to get good readings.

It is also sometimes good to look at the phase too in order to see if you have any inductance with that capacitance, but i will assume you are just testing regular caps for now.

Inductors can be done almost the same way, but power inductors often need the correct bias to check properly, and they also have to be tested for saturation.