# Coupled capacitor question

Discussion in 'General Electronics Discussion' started by Googliano, Apr 12, 2011.

1. ### Googliano

3
0
Apr 12, 2011
Hope I can explain this well enough...

A capacitor will block DC in a circuit, but allow AC. Right?

If I have a simple FM transmitter circuit powered by a 9V battery for example, that uses a small mic (like the ones sold at Radio Shack), the 'waveform' that is created from speaking into the mic gets 'imposed' onto the DC(probably not correct wording).

Does the signal that is generated from me speaking into the mic element get blocked by a capacitor in the circuit also? It's still DC right?

I did my best to describe what I'm trying to get at. Just let me know if I need more details.

Thanks

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
That is a working model for a capacitor in series with a signal, yes.

That's a fair enough statement.

Remember that you have an AC signal and a DC level mixed. As you suggested above, the capacitor blocks DC bit allows AC through. Therefore, the AC part goes through, but the DC bias does not. So on the other side of the capacitor you just have the AC signal (this is a little simplistic, but workable).

You could show us the schematic just to be sure we're both talking about the same capacitor

Capacitors have other uses. If placed across a signal (from the signal to ground) they tend to allow DC to pass, but eliminate AC. This is generally the purpose of a larger capacitor in or near your power supply.

3. ### Googliano

3
0
Apr 12, 2011
Thank you very much steve!

The AC signal part is what I was trying to figure out.

Oh and I don't have a schematic. This is just something I have been reading about and wasn't sure about the AC signal part.

Last edited: Apr 12, 2011
4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,411
2,779
Jan 21, 2010
Apologies in advance for the too-large images.

Yes, that's pretty much exactly what happens.

If you draw a graph of voltage over time, with instantaneous voltage on the Y axis and time on the X axis, then a DC voltage looks like this:

An AC voltage looks like this (note that it is symmetrical around zero volts):

If you add them together, you get something that may look like this (what it actually looks like is dependant on the magnitude of both):

Essentially the DC component shifts the AC voltage up or down by some amount.

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5. ### Googliano

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Apr 12, 2011
I now understand. Much appreciated.