Why two diodes per branch on a full wave rectifier?

photo_marc · 12th January 2006, 01:50 PM

I have seen several designs that use a center tap transformer and two identical diodes in series on each hot leg wired as a full wave rectifier.
What is the advantage of this over just one diode per hot leg?

I have attached a diagram of a ciruit that uses this for reference
Marc

pinkmouse · 12th January 2006, 01:54 PM

One diode per rail does indeed work, but only at 50% of the efficiency, so half your transformer power is wasted. Think about it for a while, drawing out the waveforms may help to understand.

sreten · 12th January 2006, 02:00 PM

I assume its due to voltage ratings, not efficiency as PM suggests.

/sreten.

tubetvr · 12th January 2006, 02:00 PM

In that attached circuit diagram the reason is probably to meet the requirements on reverse voltage, 2 diodes in series can withstand twice as high voltage as one diode. Note! it is adviceable to connect high value resistors in parallell with each diode in order to equalise the voltage obver each diode.

Regards Hans

Ryssen · 12th January 2006, 02:04 PM

Quote:

One diode per rail does indeed work, but only at 50% of the efficiency

Is it the same for a switched powersupply,like in Behringer DCX2496.
they only use 1 diod.

kmtang · 12th January 2006, 02:34 PM

I believe putting 2 diodes in series is for higher reliability. That will avoid breakdown of the diode due to high voltage.

Johnny

Sheldon · 12th January 2006, 04:02 PM

With the 275-0-275 transformer, the diode will see a peak inverse voltage or PIV of 275*1.4*2 = 770V. The rating for the diode is 1000V, I think. So the extra diode is supposedly unnessary, but perhaps the designer just wanted a bigger safety margin, as other have suggested. Note that with a bridge and a 275V secondary, the PIV is 275*1.4 = 385.

Sheldon

kevinkr · 12th January 2006, 05:19 PM

In an instance like this one might put two diodes in series with a snubber resistor across each, the purpose would be generally to provide some additional margin for line transients. (load dumps, distant lightning strikes, inductive surges) I highly recommend this, diodes are cheap..

Kevin

pinkmouse · 12th January 2006, 05:34 PM

Oops, misunderstood the question. Ignore everything I said...

leadbelly · 12th January 2006, 05:43 PM

Don't forget this technique Eli mentions (although not the case for the AX84 amp in question):

http://www.diyaudio.com/forums/showt...599#post610599

I want to to try it, but a 10 pack of SiC's from Digikey is too pricey for me. Anybody want to split?

12th January 2006, 01:54 PM	#2
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	One diode per rail does indeed work, but only at 50% of the efficiency, so half your transformer power is wasted. Think about it for a while, drawing out the waveforms may help to understand. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

12th January 2006, 05:19 PM	#8
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	In an instance like this one might put two diodes in series with a snubber resistor across each, the purpose would be generally to provide some additional margin for line transients. (load dumps, distant lightning strikes, inductive surges) I highly recommend this, diodes are cheap.. Kevin __________________ www.kta-hifi.net

12th January 2006, 05:34 PM	#9
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	Oops, misunderstood the question. Ignore everything I said... __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

12th January 2006, 05:43 PM	#10
leadbelly diyAudio Member Join Date: Dec 2002 Location: Calgary, Alberta	Don't forget this technique Eli mentions (although not the case for the AX84 amp in question): http://www.diyaudio.com/forums/showt...599#post610599 I want to to try it, but a 10 pack of SiC's from Digikey is too pricey for me. Anybody want to split? __________________ Aerodynamics are for people who can't build engines. Enzo Ferrari

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12th January 2006, 02:00 PM	#3
sreten diyAudio Member Join Date: Nov 2003 Location: Brighton UK	I assume its due to voltage ratings, not efficiency as PM suggests. /sreten.

12th January 2006, 02:00 PM	#4
tubetvr diyAudio Member Join Date: Jan 2003 Location: Now back in Sweden	In that attached circuit diagram the reason is probably to meet the requirements on reverse voltage, 2 diodes in series can withstand twice as high voltage as one diode. Note! it is adviceable to connect high value resistors in parallell with each diode in order to equalise the voltage obver each diode. Regards Hans

12th January 2006, 02:34 PM	#6
kmtang diyAudio Member Join Date: Nov 2004 Location: Vancouver	I believe putting 2 diodes in series is for higher reliability. That will avoid breakdown of the diode due to high voltage. Johnny

12th January 2006, 04:02 PM	#7
Sheldon diyAudio Member Join Date: Dec 2003 Location: San Diego	With the 275-0-275 transformer, the diode will see a peak inverse voltage or PIV of 2751.42 = 770V. The rating for the diode is 1000V, I think. So the extra diode is supposedly unnessary, but perhaps the designer just wanted a bigger safety margin, as other have suggested. Note that with a bridge and a 275V secondary, the PIV is 275*1.4 = 385. Sheldon

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