# Real time Capacitor "Time constant" observation using an osciloscope

Discussion in 'General Electronics Chat' started by Mozee, Sep 21, 2017.

1. ### Mozee Thread Starter Member

Jul 23, 2016
85
8
Hello,
I want to observe the charging capacitor voltage curve using the Rigol DS1054Z oscilloscope but I want it to be in real time e.g. to watch the curve being created not just a straight line going up and down and then to be able to use the cursor function to find out the time. I've setup the circuit and I'm probing around the capacitor, I'm new to oscilloscopes so a good explanation or setup by step on how to setup my o'scope would be great.

2. ### MrChips Moderator

Oct 2, 2009
18,155
5,700
Simple. You need a square wave generator feeding a low-pass or high-pass RC filter circuit.
Connect CH1 to the output of the square ware generator.
Connect CH2 to the output of the RC circuit.

3. ### Mozee Thread Starter Member

Jul 23, 2016
85
8
Hey friend,
I do understand the circuit configuration, I just don't know how to set my oscilloscope (RIgol DS1054z) to record and display the waveform.
When I select the "Utility" function then "Record" then when recording the waveform all I see is a straight line on the graticule goes up indicating that the voltage is increasing but I want this representation to look like a curve slowly increasing in amplitude until the cap. is fully charged then it will look like a straight line again at the max voltage.

4. ### MrChips Moderator

Oct 2, 2009
18,155
5,700
Get back to first principles.
What is the value of R and C?
What is the time constant RC?
What is the HORIZONTAL TIME/DIV setting on your scope?

For example, if R = 1kΩ and C = 0.1μF
RC = 100μs

Set your HORIZONTAL TIME/DIV for 500μs/DIV to get an overview.
Set your square wave generator to 500Hz.

Then change HORIZONTAL TIME/DIV for 200μs and 100μs for an expanded view.

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5. ### MrSoftware Senior Member

Oct 29, 2013
1,303
387
There are plenty of people here who can tell you better than I can, but try this as a starting point.

Make a rough estimate for how long it will take to charge your cap to about 90% voltage, and set the time divions (x direction) to about the same value. So if you calculate 5us then set it to 5us/division. Now set the trigger to about 75% of the peak expected voltage, and set it to rising edge. Set the scope to "single". Then turn on the circute and see what it catches. It might be difficult to catch your signal this way though, because any noise will trigger your one and only shot at it, and you might end up looking at the noise and not the signal.

A better way is to feed the cap from a square wave generator. Use the same setting as above, but set the scope to "run" mode. Now it should trigger on every rising edge, and you should see your signal in more-or-less real time. Scope the square wave generator by itself, and then when connected to the cap to see the difference. Add a resistor between the generator and the cap to slow the cap charging down, and put the probe between the resistor and the cap to see the results.

The "auto" setting on the scope can be hit and miss, sometimes it works but I find it's more reliable to use manual settings, at least with my scope.

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6. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,183
418
At very slow sweep speeds, the scope will go into a "graph" or "scroll" mode, which is perfect for what you want.

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7. ### Mozee Thread Starter Member

Jul 23, 2016
85
8
Thanks a lot guys.
@MrSoftware That what I was exactly looking for. I did so and after some failure due to noise I started getting the scope to trigger and I was able to view the charging curve of the capacitor. This is a totally new thing for me because I have very little knowledge regarding O'scopes.
Thanks again

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8. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,183
418
You might want to check out my ebook, "Digital Storage Oscilloscopes for Ham Radio," published by ARRL