sitting up Rigol1054z to observe the Real time Capacitor "Time constant"

[Mozee]

Hey folks,
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 as voltage slowely increases and decreases, and then to be able to use the cursor function to find out the charging time . I've setup the circuit and I'm probing around the capacitor, I'm new to oscilloscopes so a good explanation or step by step on how to setup my o'scope to real-time record the waveform?

[ataradov]

What exactly have you setup? And what exactly is not working as expected? What value of the capacitor did you pick? How do you charge it?

[Mozee]

My setup is a 9V battery connected in series with a 22K resistor and a 100uF capacitor and a switch. When the switch is pressed the cap. starts charging slowly and I want to observe that charging curve. I haven't used oscilloscopes before thus I don't know how to setup my O'scope to display the charging curve. I want it to be displayed exactly as this

[ataradov]

Just set auto triggering and a slow time-base. You will see a current  scan position go by. Press your button when it is at the beginning of the screen.

The time constant of your setup is 2.2 seconds, so it is not going to be that slow, but you should still see it.

[Damianos]

Try with the following settings:
- Vertical sensitivity: 2V/div.
- Vertical position: 3 divisions lower from the center.
- Signal and trigger coupling: DC.
- Trigger channel: the same that is used for the measurement.
- Trigger mode: Single.
- Trigger type: edge.
- Trigger direction: up.
- Trigger level: "a few millivolts above zero".
- Horizontal scale: 1sec/div.
- Horizontal position: near the left side of the screen.
- While the scope says that is waiting, close the switch

[Mozee]

Thank you guys
@Damianos , It worked thanks a lot. It would be great if you could explain to me why did I choose those triggering settings.

[danadak]

- Trigger channel: the same that is used for the measurement.
- Trigger mode: Single.
- Trigger type: edge.
- Trigger direction: up.
- Trigger level: "a few millivolts above zero".

The mode setting does a one time sweep/sample collection and stops. So
what you see onscreen is the results of the switch closure to full charge.
Depending of course if your sweep rate is ~ 5 time constants or more.


https://en.wikipedia.org/wiki/RC_circuit


Regards, Dana.

[Damianos]

Thank you guys
@Damianos , It worked thanks a lot. It would be great if you could explain to me why did I choose those triggering settings.

Some explanation:

- Vertical sensitivity: 2V/div.
- Vertical position: 3 divisions lower from the center.

These two are to place the signal on the screen in a way to have a big vertical image. Play also with the "fine adjustment" of the vertical sensitivity.

- Signal and trigger coupling: DC.

... because the signal is slow.

- Trigger channel: the same that is used for the measurement.

Using this it is not needed a separate connection for the trigger...

- Trigger mode: Single.

By selecting this, there is only one scan, so at the end of it you have a stable image of the signal to make your measurements. Note that the signal is not repetitive.

- Trigger type: edge.
- Trigger direction: up.
- Trigger level: "a few millivolts above zero".

We want the signal capture to start when it begins to increase from zero.

- Horizontal scale: 1sec/div.
- Horizontal position: near the left side of the screen.

As already mentioned, the T=RC is about 2.2 seconds and the phenomenon ends after a few times this amount. So by this adjustment we try to have the entire progress on the screen. To have more details of parts of the signal, try to use the "horizontal zoom" functionality of the oscilloscope.

- While the scope says that is waiting, close the switch

I don't think that this needs an explanation!

Regards,
Damianos