Oscilloscope slow above 100ms per division

[george.okpe.james]

Hi,

I am a beginner and recently purchased an SDS2202X-E oscilloscope. I am testing a simple RC circuit with a 10kOhms resistor and a  470uF capacitor, the time constant for this setup should be 4.7 seconds, so I want to set my oscilloscope to about 500ms per division so I can see the proper charge/discharge cycle, but my oscilloscope becomes very slow, shows a loading bar and the text acquiring, but it still doesn't show the expected waveform, it just a straight line on the level of the charged capacitor.

I believe I am doing something wrong but I don't know what it is, I am just starting out new. I need help.

[StillTrying]

Switch the trigger to AUTO and then press STOP by hand, maybe.

[rstofer]

Your time constant is indeed 4.7 seconds but it takes 6 such periods to get to 99.75% charge.  So, about 30 seconds.  Divide that by 12 divisions and you need to have 2.5 seconds per division.  Better pick 5 seconds/div.  One minute across the screen!

Those are ridiculous numbers.  Try a 0.1 ufd capacitor with that 10k resistor so your time constant is 1 ms.

Look at Reply 54 here:  https://www.eevblog.com/forum/testgear/starter-scope/50/

With the smaller capacitor, set the scope for Single, enable it and then apply voltage to the circuit.  If your trigger value is low enough, you should get a decent graph using the settings given.  A 50 Hz square wave source would be good (83 Hz is better) if you wanted repetitive waveforms but you should be able to get a single shot at 2 ms/div.  You need just 6 ms to charge and you could shift the trigger to the left and use 12 divisions so 500 us per division.

If you use a repetitive square wave, you need 6 ms to charge and 6 ms to discharge so 12 ms total.  If you select 1 ms/div, you should get 1 charge/discharge cycle per screen.  In my example, I picked 2 ms/div to get two complete cycles.

[rstofer]

Given 0.1 ufd and 10k, you want about 83(.3333) Hz.  You might try a 555 timer as the square wave generator.  You can set the frequency with the counter built into the scope.

[tautech]

Hi,

I am a beginner and recently purchased an SDS2202X-E oscilloscope. I am testing a simple RC circuit with a 10kOhms resistor and a  470uF capacitor, the time constant for this setup should be 4.7 seconds, so I want to set my oscilloscope to about 500ms per division so I can see the proper charge/discharge cycle, but my oscilloscope becomes very slow, shows a loading bar and the text acquiring, but it still doesn't show the expected waveform, it just a straight line on the level of the charged capacitor.

I believe I am doing something wrong but I don't know what it is, I am just starting out new. I need help.

Use the dedicated Roll mode button instead of auto Roll mode at slow timebase settings.
Auto Roll mode kicks in above 50ms/div IIRC.

[Doctorandus_P]

I'd think it would be easier to use a much smaller time constant and then use the Oscilloscope in single trigger mode.

For example, use a 1uF capacitor with an 1k resistor for an easy 1ms time constant.
You could also add a 100k discharge resistor parallel to the capacitor so you do not manually have to discharge it.
Why wait 20s for a capacitor to fully charge, if you can do the same in 1ms?

Next step, build an oscillator with a NE555 or a ring oscillator with TTL inverters.

Faster signals are easier to capture, especially if they are repetitive.