Oscilloscope slow above 100ms per division - Page 1
Forum section: electronics / beginners

author: george.okpe.james
author_rank: Newbie
timestamp: June 10, 2020, 11:29:27 pm
content: 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.

author: StillTrying
author_rank: Super Contributor
timestamp: June 11, 2020, 12:47:53 am
content: Switch the trigger to AUTO and then press STOP by hand, maybe.

author: rstofer
author_rank: Super Contributor
timestamp: June 11, 2020, 01:18:59 am
content: 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.

author: rstofer
author_rank: Super Contributor
timestamp: June 11, 2020, 01:44:46 am
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: June 12, 2020, 08:10:50 am
content: Quote from: george.okpe.james on June 10, 2020, 11:29:27 pmUse the dedicated Roll mode button instead of auto Roll mode at slow timebase settings.Auto Roll mode kicks in above 50ms/div IIRC.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 12, 2020, 10:40:48 am
content: 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.