# Bridge rectifier

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#### Gednal

##### New Member
Hi there,

Wondered if any one can help out with this please.

I have a bridge rectifier used on a 24 DC power supply unit.
The Input is 3-phase AC.

How do I calculate what level of 3-phase AC Volts is required at the rectifier input, to provide 24 VDC output ?

Appreciate the help, Cheer's
Paul

#### forumlicker007

##### New Member
Why can't you use just single phase line? What's the AC voltage? What's the rating of transformer?

#### Gednal

##### New Member
Hi,

This is on an existing application that is designed for 480 VAC 3-PH input into a rack mounted Power supply. I have no info on the transformer installed, no Part number on it. All I can tell you is that there is a Semikron SKD 31/08 rectifier mounted on a large heatsink and there is 24VDC, 20A output.

My question is more out of curiosity, of what level of 3-ph AC would be required at the input to the rectifer.

Thanks
Paul

#### forumlicker007

##### New Member
I couldn't understand what you mean by "level". The rectifier circuit probably given the 3Phase supply directly to a set of 1N4007 diodes and there'll be a X'former after that, no doubt it'll be something complicated circuitry. Might contain MOSFET's with heatsinks.

#### be80be

##### Well-Known Member
Are you sure the transformer is three phase most of the time if you need 24 volt from 480 three phase you would use just two of the phases and step that down for your low voltage. I looked up this and you sure are using a three phase bridge here a datasheet http://www.electro-tech-online.com/custompdfs/2009/04/d31.pdf Your wanting what 480 three phase transformer low voltage out put to get 24 volts from your bridge it should be about 26 to 28 volts per phase

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#### Gednal

##### New Member
Hi Again,

Appologie's for the poor terminoloy by using "level", I meant 3-phase Vrms really.

Yes, I also thought it unusually to use 3-phase, but someone did design it that way.

Anyway, I don't want to make a big issue out of it sorry, I was just curious of what 3-Phase Vrms (line-line) input that the bridge would need to provide the 24VDC output.

Thanks
Paul

#### be80be

##### Well-Known Member
about 26 to 28 volts per phase to bridge to get 24 volts dc out But I don't add it if you want there the math

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#### rmn_tech

##### Member
Hi,

This is on an existing application that is designed for 480 VAC 3-PH

Gednal is obviously in the U.K. as this is the standard 3 Phase here we do not need 2 or three phases in the home here as in the U.S.A. so this is an industrial application and Gednal is just curious.

Also is the transformer 3 Phase in to 1 Phase out, or 3 Phase in to 3 Phase out. This makes a big difference to your rectifier circuit.

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#### Gednal

##### New Member
Hi,

Well this is an Industrial application in the USA that uses 480 VAC 3-Phase delta. There is three phases input to the transformer, and three phases directly out of that to the "3-Phase" rectifier, then 24VAC, 20A output with a smoothing cap etc.

Additionally, from another source I'm told that the Vrms at the input to this particular rectifier is "roughly" ........Vrms= Vdc / 1.35. So for 24VDC output, input Vrms required would be 24/1.35=17.77 Vrms (line-line).

Yes, I'm purely curious as I didn't know what I should expect on the Input side.

I'm also curious if they used 3-phase to cut down on the Line Amps / Conductor size and....to get a better "quality" output ampltidue after rectification and before smoothing out. Just a thought.

Appologie's for maybe talking about 3-phase on an electronics forum....but there are diodes involved : )

Lastly, I'm a UK guy, working on German equipment, in the USA !!.

Cheer's

#### be80be

##### Well-Known Member
I'm also curious if they used 3-phase to cut down on the Line Amps / Conductor size and....to get a better "quality" output ampltidue after rectification and before smoothing out. Just a thought.
That's it smoother power and smaller wiring.

#### be80be

##### Well-Known Member
Your transformer has to put out 26volts per phase to get 24volts from your bridge you posted

#### Mr RB

##### Well-Known Member
Assuming you got some fat caps on the bridge output the voltage is the same as any bridge under light loading;

(1/2 the peak to peak voltage in) - (2 * rect drop)

#### Leftyretro

##### New Member
I don't think the voltage out of the transformer/bridge combination would be any different then with single phase full wave bridge rectification for the same turns ratio, just develops a higher and easier to filter ripple frequency and less current flowing in any given transformer winding Vs single phase. At least that is my gut feeling.

Lefty

#### Diver300

##### Well-Known Member
The peak voltage on 3-phase is exactly the same as single phase. You need to be wary of the fact that 3-phase voltages are often measured phase-phase.

For example, UK mains is 230 V AC single phase, or 400 V AC 3-phase. The phase - ground voltages are 230 V AC either way, so any of the 3-phases can be used as a single phase supply.

As for rectification, the more phases, the less the ripple. Any odd number of phases can be doubled with a full bridge or centre-tapped windings to give less ripple. This is common with single phase transformers that are usually used with a full bridge or centre-tapped so that the ripple frequency is 100 Hz with 50 Hz mains.

With 3-phase, transformers can be wound with each phase centre-tapped, and then 6 diodes will give 300 Hz ripple. Or a 3 phase winding that is isolated can be connected to a 6 diode full bridge, to give 300 Hz ripple. That is what is done in car alternators (obviously they are variable frequency).

When 3 or more phases are rectified, the voltage never dips to zero, so in some cases smoothing isn't needed at all. 6 phase rectification only has 14% ripple with no smoothing.

#### Gednal

##### New Member
Thanks for all that.

"Your transformer has to put out 26volts per phase to get 24volts from your bridge you posted".........Thanks be80be.

I'll also read up on the ripple thanks Diver300.

Cheer's

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