# 3 phase midpoint connected rectifier

Discussion in 'General Electronics Chat' started by shespuzzling, Oct 11, 2012.

1. ### shespuzzling Thread Starter Active Member

Aug 13, 2009
88
1
Hello,

Why can't 2 diodes conduct at once in a 3 phase 3 pulse rectifier? When 2 of the phase voltages are positive, I don't see why only one diode conducts.

Thanks!

2. ### mlog Member

Feb 11, 2012
276
36
It would help if you would post a schematic.

3. ### JDT Well-Known Member

Feb 12, 2009
658
87
The diode on the most positive phase will conduct only. For a very brief instant, when two phase have the same voltage, two diodes will be conducting.

I assume you are talking about a 3-phase (half wave) rectifier.

4. ### shespuzzling Thread Starter Active Member

Aug 13, 2009
88
1
Yes a half wave rectifier, here's a schematic http://www.eece.ksu.edu/~starret/589/man/Efig3.gif

Does only one diode conduct because the other 2 are actually reverse biased? How can you figure that out? Is there a way to prove it by circuit analysis?

Thanks!

5. ### gerty AAC Fanatic!

Aug 30, 2007
1,231
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Don't forget the phases are 120° out of phase from each other.

6. ### Dodgydave AAC Fanatic!

Jun 22, 2012
7,511
1,239
Only one diode will conduct per phase cycle, the diode with the most voltage takes first phase, then as its phase decays, and the other phase increases, the second diode takes over, then onto the third phase , then back to the first, the same on the negative cycles.

in a full bridge, only two are ever in full conduction( pos and neg cyles) otherwise you would be shorting out the phases.

7. ### crutschow Expert

Mar 14, 2008
19,746
5,523
If you view the voltage waveform for the three phases on one graph (such as here) you will see at which point each phase has a higher voltage and polarity as compared to the other phase the diode is connected to. That will tell you which diode is conducting.