Need help with filtering out AC voltage after rectifier.

Thread Starter

AndrejaKo

Joined Sep 6, 2010
16
I have a 220 V 50 VA to 2* 12 V 2.1 A transformer followed by KBL10 (1 kV, 4 A) Graetz bridge rectifier. After rectifier, I get around 10 V of AC, which I'd like to eliminate. After doing some research in the Internet, I'm thinking about using a choke to block the AC part of the voltage.

I'm thinking right now about using Radiohm 42H22 30 00 or 42V25 30 00 chokes (3 A, 1.2 mH, 70 mΩ per winding). Would they be good enough for my use? If not, what inductance should I use, or even better, how would I calculate needed inductance? I'm pretty new to that field.
 

Kermit2

Joined Feb 5, 2010
4,162
What size of filter capacitor do you have on the DC line?

You need at least 470u or more to bypass that AC signal to ground and leave you much steadier DC
 

wayneh

Joined Sep 9, 2010
17,496
I'm a little confused as to why there would be much AC ripple at all without a load. Shouldn't the cap be acting as a peak detector, and basically hold steady?
Where is the 10v ripple coming from?

The ripple will increase under load, depending on the RC time constant of the filter cap plus load. Under enough load, the cap will have little effect on the size of the ripple, which will just be the incoming pulses.
 

Thread Starter

AndrejaKo

Joined Sep 6, 2010
16
The first location at which the ripple shows is at the rectified output. I was thinking about blocking it between capacitor and rectifier
 

wayneh

Joined Sep 9, 2010
17,496
The first location at which the ripple shows is at the rectified output. I was thinking about blocking it between capacitor and rectifier
So the "ripple" was actually just the RMS (meter read) DC voltage coming off the naked (unfiltered, no cap) rectifier? That's pretty much what you'd expect and isn't anything to worry about.
 
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