modified 5 years ago

Please help - Capacitor dc-offset

02:34:03

When I start simulation with switch closed, circuit 2&3 removes DC bias successfully (after adding 1M resistor). When I start simulation with switch OPEN and TIME=0, then, CLOSE switch after few seconds, circuit 2 & 3 doesn't remove DC bias, even though I added 1M resistor.  Somebody in different forum said I should add small resistor in series at output of signal generator (circuit 3) so it can allow capacitor to discharge (I didn't understand this anyway, if somebody understands logic for doing this please can you explain), but after adding 10 Ohm resistor in circuit 3, still not removing DC bias. Please help

published 5 years ago

PrathikP

5 years ago

You need to add a resistor at the output. If current doesn't run through the capacitor, it's not going to do any DC blocking

Robert_Kidd

5 years ago

Yep. EC voltmeter is ‘perfect’ instrument having infinite resistance - so as far as circuit operation is concerned it’s not there. Add a load, say 1MOhm.

dB7

5 years ago

Thanks guys, I added 10M resistor, but same thing happens.. Please see updated description

Robert_Kidd

5 years ago

It’s all a matter of timing :-). With the values chosen the time constant is t = c x r = 300 x 0.000001 x 1 x 1000000 = 300 seconds. As the timebase can only be set to a maximum of 10ms you’re going to wait a long time before you see the end result.

Robert_Kidd

5 years ago

Reduce the time constant by reducing capacitor to 30uF and resistor to 1kOhm. Set timebase to 10ms, now switch on.

Robert_Kidd

5 years ago

If you go back to your original circuit, remove all the voltmeters, the put just one voltmeter across the capacitor. Switch on. You’ll now see the voltage across the capacitor very slowly increasing. As this increases, correspondingly the voltage across the resistor is decreasing. Eventually all the dc is across the capacitor and none across the resistor - it just takes forever! Again you could view this better if you reduced the component values. Hope all is clearer now.

Robert_Kidd

5 years ago

10 Ohm - nonsense:-)

Robert_Kidd

5 years ago

Does that help you?

dB7

5 years ago

It helps thanks Robert_kidd sorry for late reply have been v busy with work. I think I have another q related to this when I have chance to do some more simulations.. will you see it?

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