# Basic electronics question.... AC to DC



## greco (Jul 15, 2007)

This concept came up in another thread re: cooling an amp with a fan.

My question is purely academic...I will not be trying this using an amp as a source of power.












My question is: 
*How can one diode and one cap "convert" the AC current to DC?
*
I thought that a 4 diode bridge and possibly more than one cap would be needed.

Is it because of the low AC voltage involved (i.e., 6.3 volts) ?
and/or because the DC fan will operate happily on less than optimal "ripple free" DC?

Thanks.

Cheers

Dave

PS I just went back to look at the site that I copied the diagram from and found out that it is *bcmatt's *(blog?) site.
Great site *bcmatt*...Congrats !!
http://yeomansinstruments.blogspot.com/


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## dtsaudio (Apr 15, 2009)

It's called a half wave rectifier. A pretty decent explanation (with video and diagrams) can be found here.
http://www.wisc-online.com/objects/ViewObject.aspx?ID=SSE402

And yes, the fan doesn't need really smooth DC


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## greco (Jul 15, 2007)

dtsaudio said:


> It's called a half wave rectifier. A pretty decent explanation (with video and diagrams) can be found here.
> Half-Wave Rectifier
> And yes, the fan doesn't need really smooth DC


Thanks for the response and for the link. 

While we are on this subject....out of academic curiosity.....
Would the fan run any better (i.e., more RPM or whatever) if the AC was fully rectified?

Last question...I promise.

Thanks

Dave


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## dtsaudio (Apr 15, 2009)

Possibly. The DC may be slightly higher, and being cleaner it should be quieter. One other thing, is full wave (bridge rectified) DC is easier on the filter cap than half wave (less ripple current)


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## greco (Jul 15, 2007)

dtsaudio said:


> Possibly. The DC may be slightly higher, and being cleaner it should be quieter. One other thing, is full wave (bridge rectified) DC is easier on the filter cap than half wave (less ripple current)


Many thanks for answering my question. 
This is very interesting to learn...even though I know it is basic theory (somehow I missed it in my reading).

I promised only one more question...I shouldn't have.

Is there any easy way of determining when half wave rectification can be used for any given DC circuit...or what factors determine the absolute need for full wave rectification?

Cheers

Dave


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## dtsaudio (Apr 15, 2009)

There's no real hard and fast rule, however full wave rectification is smoother, and requires less filtering. So anything dealing with audio signals, or where noise is a consideration should be full wave. Half wave is used in less critical applications. Also current is a consideration. Ripple rises as DC current is drawn, so the more current the more ripple. This greatly affects the capacitor size. More ripple means more capacitance and higher capacitor voltage rating. It may actually be cheaper to use a bridge with smaller capacitance than a half wave with bigger capacitor.
You often find half wave in the bias circuit for tube amps as there is very little current drawn, and therefore only a small cap is needed. It simplifies the circuit without affecting performance.


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## greco (Jul 15, 2007)

Again, many thanks for your answer to my question. You explained everything with clarity and ease of understanding.
I'm thinking of breadboarding a simple half wave rectified circuit and using my scope to see what happens to the trace when I make minor changes to the components. I have a little transformer which steps down to about 4 VAC on the secondary.

Cheers

Dave


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