OT: Using a o-scope with RC circuits

[Steve_M]

I'll start here since there's a few pretty knowledgeable folks that might be able to help me out.

I've got a few SMD capacitors that are now on the loose, meaning that they're sitting on my bench, and I have no idea of what value they are. Some research has led me to info on RC circuits and that I should be able to determine the pf value of the caps with the help of a signal generator and an o-scope. Good news, I happen to be in possession of a Syscomp Circuit Gear.

The problem I'm running into is that I'm not getting any useful readings when the RC circuit is created.

I've got the Circuit Gear signal generator sending a 1K square wave and can see it on the o-scope

My circuit:



Diagram 1: Things look OK

Probe A connected to the signal generator

Probe B connected to the other side of the 10K resistor. No capacitor in the circuit yet.



Diagram 2: With the 200uf capacitor in the circuit, it looks as if the capacitor is basically grounding out the circuit and no matter how much I try to zoom in, I don't see any indication of the capacitor being charged and discharged



Any ideas on what's up?

I was hoping to see something like this:

 

[Bryan Mayland]

The first thing that comes to mind is that a 10k/220uF low pass filter that you have there has a cutoff frequency of 0.07Hz meaning it cuts off every frequency above 0.07Hz. 1kHz is a lot higher so I think you're just attenuating your input to the point you see nothing. For capacitance that large you need something like 100Hz and a 100ohm resistor to see it I think.

EDIT: Also polarized electrolytic caps are straight shorts to ground usually when they have a negative voltage applied to them, which I think also will mess up the results when the function generator goes to negative voltage.

 

[Steve_M]

I ended up trying with a known 0.1uf SMD cap and was able to dial things in. I also had to bring the freq down to 100Hz

Now I end up with:



And I'm now able to properly plot the voltage and time of the capacitor and my calculation looks to be good.

A total charge of 675 mv, with 63% of that being 425.25 mv, which equals 955 us, divided by 9500 (the measured value of the 10k resistor ) and I get 0.1