DC blocking capacitor

[nickkka]

I was just wondering whether a capacitor's value has any effect in terms of dc blocking?

 

[Optikon]

I was just wondering whether a capacitor's value has any effect in terms of dc blocking?

Technically speaking, NO. An _ideal_ 1 pF capacitor will block DC just as well as a 1F capacitor. They are both OPEN circuit at DC.

However, if the capacitor must PASS AC, then the value matters. Now it is an impedance problem to solve.

 

[bananasiong]

The reactance of the capacitance depends on the frequency. DC has no frequency, so the reactance is infinite ideally.

 

[nickkka]

Well you see I'm using an electret condenser microphones..and so it happens I'm using this capacitor to block DC component from the output signal.

I used 10uF first and shows nothing on the oscilloscope for the output signal, but when I changed it to 10nF the oscilloscope shows output which is picked up by the microphone.

 

[bananasiong]

10 uF is usually electrolytic capacitor, it is polarized, did you connect it well?

 

[nickkka]

Yes it is polarised and yes again I did check the connections twice.

 

[bananasiong]

Not sure
But there should be signal to the output right? Maybe the capacitor is not working?
I have tried an audio amplifier circuit where the 220 uF capacitor connected via the output to the speaker, it worked well.
The dc blocking capacitor in the FM transmitter is 330 nF and also worked fine.

 

[Hero999]

Here's the formula for capacitor impedance:
[latex]X_C = {1 \over 2 \pi FC}[/latex]

 

[stevez]

Just a thought, could the capacitor have failed open?

After several failures of electrolytic capacitors I am now applying a DC voltage across them for a time before I use them in my projects. Not the process called forming. Failures have been such that smoke/heat results and I nearly ruined my hard work. My stock of capacitors is new stuff but some as much as 15 yrs old. I don't test all capacitors as completely as I could though I do check w/meter for shorts.

 

[TheVictim]

Most electret mic circuits have the cap doing double duty by blocking the DC supply voltage from your amp input AND passing the audio. I found .1uF to work just fine.

 

[audioguru]

If the resistance that the mic and capacitor are feeding is 1k ohms, then the 0.1uF capacitor will cut all bass frequencies. 1600Hz will be 3dB down.
If the resistance that the mic and capacitor are feeding is 10k ohms, then the 0.1uF capacitor will cut most bass frequencies. 160Hz will be 3dB down.
If the resistance that the mic and capacitor are feeding is 100k ohms, then the 0.1uF capacitor will have a flat response down to about 80Hz, and 16Hz will be 3dB down.

 

[TheVictim]

Sounds like I should be using bigger caps.

 

[Sceadwian]

Don't forget what capacitively coupling a signal does to it. The lower the capacitance value the more you de-emphasize the bass and mid range and emphasize the highs. This is especially pronounced on small capacitors at audio frequencies.