Bridge Rectifier Help

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Skyone
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Bridge Rectifier Help

Post by Skyone » Mon Jul 24, 2006 8:35 pm

I'm new to working with AC circuits and I'm looking for some help with rectifiers.

I have this GBU8M rectifier. I need it to convert AC to DC (like any bridge rectifier does). I need it to take in 120V from the wall and convert it into DC.

My question is, if I'm using the GBU8M, and feed 120V AC into it, will I get 120V DC out of it? Or will there be a large tolerance range?

Also, once I have my ~120V DC, I need to be able to work with the voltage. Can anyone suggest a good way to get ~120V DC down to 10V-20V DC?

Thanks
Sky

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Post by marshallh » Mon Jul 24, 2006 9:53 pm

Transformers only work with AC. So, you need a step-down transformer to bring down the 120v to a low AC voltage. Then, you run it through your bridge rectifier, and then your 7805/whatever. Don't forget filter caps!
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Post by superdeformed » Mon Jul 24, 2006 10:16 pm

Someone correct me if I'm wrong, but wouldn't rectifying 120VAC (off the mains) get you around 170VDC minus the voltage drop from the diodes? (I've read that the rectified voltage is based off the peak value, not RMS, don't really know a whole lot about AC though)

Anyways, as marshallh said you'll want to use a transformer to drop the voltage first. It's easier and safer to deal with (less components at high voltage) than trying to regulate down to 10-20V from >120VDC.

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Post by gannon » Mon Jul 24, 2006 10:29 pm

IIRC you need large decoupling capacitors to get the max boost from ac->dc conversion

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Post by Skyone » Mon Jul 24, 2006 10:34 pm

I think I'll end up taking 120v AC down to 17v AC. Then I'll feed that 17v into a rectifier along with a 1000uF Electrolytic capacitor. I should get ~20v DC after the capacitor, and I'll feed that through a switching regulator to get down to 12v DC. Thanks for the help guys.

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Post by timmeh87 » Tue Jul 25, 2006 7:11 am

yes, the AC voltage is stated in RMS (root-mean-square). so the peak voltage is ~1.4 times the RMS votlage.

however, when you smooth it out with the filter caps, the peaks fill in the valleys so to speak, and you end up with roughly the RMS voltage anyways.

sky, the switching regulator step seems pointless, why dont you just step it down to 12 in the first place
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Post by Skyone » Tue Jul 25, 2006 12:49 pm

timmeh87 wrote:yes, the AC voltage is stated in RMS (root-mean-square). so the peak voltage is ~1.4 times the RMS votlage.

however, when you smooth it out with the filter caps, the peaks fill in the valleys so to speak, and you end up with roughly the RMS voltage anyways.

sky, the switching regulator step seems pointless, why dont you just step it down to 12 in the first place
So should I do from 120AC to ~8.6AC then feed it through a rectifier? I was just using the switching regulator to get exactly 12v, because after going through a transformer, rectifier stage and capacitor, you can never be too exact.

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Post by timmeh87 » Tue Jul 25, 2006 4:45 pm

nono. the way its set up is that you dont need to ever do any weird calculations. the ac voltage value (RMS) is its "equivalent dc voltage" so to speak. just treat it like normal.

and true. but then a 7812 makes more sense (To me). less expensive, smaller, less complicated. no noise problems.
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Post by Skyone » Tue Jul 25, 2006 6:58 pm

A 7812's max current draw is what, 1A-1.5A? I need a good 2.5A regulator. I can settle for linear however.

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Post by gannon » Tue Jul 25, 2006 7:03 pm

ATC1084 is a 5A adj. lin. regulator IIRC

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Post by Skyone » Tue Jul 25, 2006 7:17 pm

gannon wrote:ATC1084 is a 5A adj. lin. regulator IIRC
Digikey?

EDIT: Not digikey...

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Post by gannon » Tue Jul 25, 2006 7:31 pm

jameco #299743

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