# How to measure the transient response of a capacitor

Discussion in 'General Electronics Chat' started by SamEricson, May 2, 2015.

1. ### SamEricson Thread Starter Member

Apr 25, 2015
196
0
What kind of test circuit would I need in order to test the transient response of a capacitor?

When I put 3 capacitors in parallel, it adds the capacitance and storage. Does this make the transient response of a capacitor faster or slower?

2. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
Well, what would make sense to you? If I add three 55 gallon barrels in parallel under a rain spout, is it going to take more time or less time to fill them?

Instead of asking about circuits to test something, you might consider looking up just what a capacitor is and how they work and what they do.

3. ### #12 Expert

Nov 30, 2010
16,665
7,310
I was stuck at home with a broken car yesterday, but I can't entertain you today. There is no point in answering your questions because you don't have any background in the subject about which you inquire, you don't read what you are told to read, and you don't learn from the answers.

4. ### SamEricson Thread Starter Member

Apr 25, 2015
196
0
My electronic books and google only talk about the RC time constant. I don't need a test circuit to measure the RC Time constant. I need a test circuit that measures the transient response of a capacitor

5. ### WBahn Moderator

Mar 31, 2012
18,085
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What is it that you mean by "measure" the transient response? Do you understand what the term "measure" even means?

6. ### SamEricson Thread Starter Member

Apr 25, 2015
196
0
yes measure the time of the transient response of a capacitor using an oscilloscope.

apply a repetitive “step” voltage to the input of the RC circuit to display the transient response of the circuit. A good approximation of the transient response may be obtained using a square-wave excitation since it is periodic and may be regarded as a series of positive and negative step voltages.

7. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
What do you mean by "measure the time of the transient response"?

How can you hope to measure something when you can't even articulate what it is you want to "measure"?

8. ### SamEricson Thread Starter Member

Apr 25, 2015
196
0
The ESR in capacitors is a resistance, the aging of a capacitors ESR goes up. The transient response is the switching on and off time of the capacitor + ESR. You use a function generator STEP waveform or use a momentary switch to switch the voltage on and off. Connect it to the oscilloscopes trigger input as well as the capacitor. Monitors the capacitors output on Oscope ch1. and "capture the event" of the transient response using a storage oscilloscope. Set up the Oscilloscope to single shot triggering and trigger polarity set to positive.

Did i articulate

9. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
No, because you still haven't indicated what the "measure" is. Let's say that you have successfully captured the transient response on the oscilloscope, what is it that constitutes your "measure" of that response?

10. ### SamEricson Thread Starter Member

Apr 25, 2015
196
0
I captured the transient response of the capacitor

Do you want me to say that I didn't capture the transient response and I don't know what the transient response is because i don't know what I'm doing?

The charging time is a transient time at a peak voltage

Every capacitor material using the same value has a different charging time, why is that? you can have the same value and use different types of electrolytic capacitors and measure the charging time. You have the same value but the charging times and decay times are different why is that?

I'm not talking about the RC time contant, I'm leaving the external resistor out of the subject.

Capacitor material types
electrolytic 30D
Electrolytic 109D
Ceramic Capacitor
Silver Mica
Monolithic ceramic
Mylar
Paper In oil
Paper wax
Tantalum
Polycarbonate
Polyester
Polyester film and foil
Polyester metallized film
Polymer film
Polypropylene
Polystyrene
etc.

You can have the same capacitance value but change the capacitors material type and you will have a different charging time, why is that?

11. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
Perhaps it would help if you sketched a typical transient response that you would capture and show what your "measure" of it is.

12. ### SamEricson Thread Starter Member

Apr 25, 2015
196
0
It depends on the type of capacitor material, the capacitors + ESR time constant, the Step waveform pulse width time

The Step waveform pulse width has to match or be greater then the capacitor+ESR time constant.

The Type of capacitor material changes the charging time and discharging time, any reason why?

13. ### JWHassler Member

Sep 25, 2013
204
34
OK, I'll bite.

The material changes the behavior because it changes the capacitance .
It's still an RC time-constant.

14. ### SamEricson Thread Starter Member

Apr 25, 2015
196
0
but using the same capacitance value , the "only difference" is changing the capacitor material

The charging time and discharging time are different even tho the capacitance value is the same, the only difference is the capacitor material

15. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
How do you know the capacitance value is the same?

And, again, please provide a sketch of your measure of charging time and discharging time.

Apr 25, 2015
196
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17. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
Why should I care about seeing a picture of a particular LC meter?

I would much rather see a sketch of your "measure" of the transient response, but I'm about to give up on that.

18. ### ramancini8 Member

Jul 18, 2012
447
119
Capacitor suppliers issue data sheets that show the frequency response of the capacitor, and frequency response has a direct correlation to transient response. The transient response of an aluminum electrolytic cap (frequency response = 100KHz) is much worse than a mica cap (frequency response = 100MHz). I can't tell you what the response time is because I never had a use for that measurement.

All capacitor dielectrics change size when stressed, especially by voltage, but the return to normal size. This determines what sample/hold performance will be.

19. ### SamEricson Thread Starter Member

Apr 25, 2015
196
0
True good point

The Frequency of the step waveform changes the transient response

When capacitors are Molded, Dipped, WET, Metallized. What is the difference between them?

20. ### ronv AAC Fanatic!

Nov 12, 2008
3,393
1,449
Some have higher ESR and/or inductance depending on how they are made.