RC circuit phase difference and Oscilloscope

[aurosunil]

I want to see the phase difference between current and voltage in a simple RC circuit on a oscilloscope. How to do it? Is it possible?
It can be shown in XY mode but can it be shown in normal mode?
any idea will be appreciated
thanks
aurosunil

 

[Roff]

This will work if your input rise and fall times are long enough for the op amp to handle them. For a 741 op amp, this is around 10usec. You can use a wider bandwidth op amp if you want to use faster risetimes.
Sine waves will work pretty well up to around 10kHz for a 741.
This will only work for peak currents up to the output current capability of the op amp.

A much simpler approximation is to forget the op amp and add a series resistor in the ground lead which is much smaller (<1%) than the R in the R-C circuit. The voltage across it will be proportional to the current.

 

[aurosunil]

This will work if your input rise and fall times are long enough for the op amp to handle them. For a 741 op amp, this is around 10usec. You can use a wider bandwidth op amp if you want to use faster risetimes.
Sine waves will work pretty well up to around 10kHz for a 741.
This will only work for peak currents up to the output current capability of the op amp.

A much simpler approximation is to forget the op amp and add a series resistor in the ground lead which is much smaller (<1%) than the R in the R-C circuit. The voltage across it will be proportional to the current.

Hi Ron,
what i am interested in is that there is a simple circuit having two components, a resistor and a capacitor. now as the capacitor current will be leading the capacitor voltage, i want to see two waveform i.e. current and voltage waveform on the oscilloscope showing the phase difference.
i hope i made my self clear.

thanks
aurosunil

 

[Nigel Goodwin]

what i am interested in is that there is a simple circuit having two components, a resistor and a capacitor. now as the capacitor current will be leading the capacitor voltage, i want to see two waveform i.e. current and voltage waveform on the oscilloscope showing the phase difference.
i hope i made my self clear.

That's what he drew for you, the resistor and capacitor are labelled R and C, the other few bits are to generate the current reading for the scope.

I thought it was a rather nice solution!, my first thought was a small resistor in the bottom of the capacitor (as he also mentioned), but his opamp solution was far better.

 

[aurosunil]

I am sorry, i did't understand that.i am totally confused now. what i wanted to know is how to see current and voltage waveforms on oscilloscope, in a circuit which consists of just two elements a registor R & a capacitor C, and supply votage. When i saw an op-amp in the circuit first question that came to my mind was what an op-amp is doing there ? so the clarification of the problem on my part.
Regarding op_amp circuit:
1. i have to make two circuits and take oscilloscoe inputs from Vcap and Vres?
2. Can't we see current waveform on oscilloscope?

 

[Russlk]

I am sorry, i did't understand that.i am totally confused now. what i wanted to know is how to see current and voltage waveforms on oscilloscope, in a circuit which consists of just two elements a registor R & a capacitor C, and supply votage. When i saw an op-amp in the circuit first question that came to my mind was what an op-amp is doing there ? so the clarification of the problem on my part.
Regarding op_amp circuit:
1. i have to make two circuits and take oscilloscoe inputs from Vcap and Vres?
2. Can't we see current waveform on oscilloscope?

1. Yes
2. The oscilloscope only shows voltage waveforms, so you have to somehow convert current to voltage. That is what Ron's circuit does.

 

[aurosunil]

Thanks a lot for all the clarification. But
1. if Oscilloscope only shows voltage waveform then why use op-amp at all. why not take scope inputs from resistor and capacitor directly? this way, one need to construct only one circuit?

2. I feel there is some flaw in my understanding of this but what, i am not aware of. Can any one throw more light on the topic?

3. further how a young learner who has not studied the op-amp can be shown practically the phase difference in capacitor or inductor at all.

 

[Nigel Goodwin]

Thanks a lot for all the clarification. But
1. if Oscilloscope only shows voltage waveform then why use op-amp at all. why not take scope inputs from resistor and capacitor directly? this way, one need to construct only one circuit?

As Russelk just explained, a scope can only show voltages, the opamp is used to convert the current to a voltage - so you can't read the current from the circuit with out adding something to produce a voltage from it.

Your requirement is to compare the voltage and current - so you have to produce a voltage from the current to show it on a scope.

 

[aurosunil]

Yes that is correct. but voltage across a resistor is proportional to the current through the resistor. So if one channel of scope is fed from resistor another from the capacitor it must show the Vr and Vc waveforms ?
it is really puzzling to me.

 

[Nigel Goodwin]

Yes that is correct. but voltage across a resistor is proportional to the current through the resistor. So if one channel of scope is fed from resistor another from the capacitor it must show the Vr and Vc waveforms ?
it is really puzzling to me.

Where are you planning connecting the ground of the scope?, then think about how to measure voltage and current.

 

[aurosunil]

Yes that is correct. but voltage across a resistor is proportional to the current through the resistor. So if one channel of scope is fed from resistor another from the capacitor it must show the Vr and Vc waveforms ?
it is really puzzling to me.

Where are you planning connecting the ground of the scope?, then think about how to measure voltage and current.

i left the ground floating. and it worked. however earlier it was not showing and hence the question in the first place. I don't have any explaination.
even tried with inductor- resistor circuit there also it showed the phase difference.

Any way thanks a lot to all of you.

 

[Nigel Goodwin]

i left the ground floating. and it worked. however earlier it was not showing and hence the question in the first place. I don't have any explaination.
even tried with inductor- resistor circuit there also it showed the phase difference.

If you leave the ground floating, all you will get is masses of mains hum pickup, you MUST have the scope ground connected as it's the reference that the inputs work from.

 

[aurosunil]

If you leave the ground floating, all you will get is masses of mains hum pickup, you MUST have the scope ground connected as it's the reference that the inputs work from.

yes, you are very true, scope did picked up some noise or hum but it showed clearly the sine waves and phase difference.