MP3Car.com - View Single Post

alimh · 10-04-2005, 01:30 AM

Quote: Originally Posted by kbyrd

I don't understand, what will automatically turn off when I turn my computer off? What does the capacitor do?

Sorry for the unclear post...

This is what happened to me:
I would turn on the computer without the 5v connected. Everything would startup fine but no sound (obviously).
Then I connected the 5v (with the computer still on) and "pop" the computer would shut off instantly.

What was happening was the 5v was going back into the (-) channel of the soundcard which was grounded to car ground. So putting 5v right to ground would mean the circuit is shorted.

A capacitor only lets AC pass through. The higher rating it is, the more and more low frequencies it blocks. A DC signal (in our case, the 5v) is a signal with 0 frequency (the lowest of the them all). So any size capacitor would stop that 0 frequency DC from passing back into the soundcard.

Now, in your case, your soundcard may not be grounded right to car ground, but may be grounded to a reference (called signal ground). In any case, there is no harm in putting a really small capacitor (1 nF) between the output of the sound card and the point where the 5v touches the input signal (see my diagram) to be on the safe side.

10-04-2005, 01:30 AM	#8
alimh Newbie Join Date: Jun 2005 Location: Albuquerque Vehicle: VW GTi '03 Posts: 37 My Photos: ()	Quote: Originally Posted by kbyrd I don't understand, what will automatically turn off when I turn my computer off? What does the capacitor do? Sorry for the unclear post... This is what happened to me: I would turn on the computer without the 5v connected. Everything would startup fine but no sound (obviously). Then I connected the 5v (with the computer still on) and "pop" the computer would shut off instantly. What was happening was the 5v was going back into the (-) channel of the soundcard which was grounded to car ground. So putting 5v right to ground would mean the circuit is shorted. A capacitor only lets AC pass through. The higher rating it is, the more and more low frequencies it blocks. A DC signal (in our case, the 5v) is a signal with 0 frequency (the lowest of the them all). So any size capacitor would stop that 0 frequency DC from passing back into the soundcard. Now, in your case, your soundcard may not be grounded right to car ground, but may be grounded to a reference (called signal ground). In any case, there is no harm in putting a really small capacitor (1 nF) between the output of the sound card and the point where the 5v touches the input signal (see my diagram) to be on the safe side.