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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I set up what I thought was an AC filter capacitor, but it doesn't seem to be doing what I thought it should. The capacitor is 470uf. I have about 100ma of DC current going through the resistor, and I can add a 3vac 60hz waveform. I have two probes, P1 and P2 attached as shown, connected to my oscilloscope and voltmeter. I want the capacitor to block the DC component. However, I can still see the DC on the scope and on my voltmeter if I have it set to DC. What am I doing wrong?
EDIT: I seemed to fix the problem by going from 470uf to a 0.1uf ceramic. I guess the 470uf never came to equilibrium with the amount of current. Now that I have this fixed, how would I apply the forumula Zc = 1/2pi*f*C to get the right capacitance value? Or, does it not matter what Zc is in this case as I am measuring the voltage across both the resistor and capacitor?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
The capacitor is in series with your 'scope probe input. A typical 'scope probe input looks like 10 Mohm in parallel with 10 pF so if the capacitor is 0.1 uF frequencies below 0.16 Hz will be attenuated ( 1/2.pi.R.C) 10 nF would attenuate frequencies below 1.6 Hz 1 nF ........ 16 Hz etc. etc.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I see that, but how does this apply to what is the right capacitance value to block DC and allow the 60hz signal to pass? When I used 470uf the DC was still getting through; when I used 0.1uf the DC was blocked. If I increased the DC component I saw it rise for a moment on the scope, but it quickly came back to zero.
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AC filter capacitor
