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Old 12th January 2006, 01:50 PM   #1
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Default why two diodes per branch on a full wave rectifier?

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
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Old 12th January 2006, 01:54 PM   #2
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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.
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Old 12th January 2006, 02:00 PM   #3
sreten is offline sreten  United Kingdom
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I assume its due to voltage ratings, not efficiency as PM suggests.

/sreten.
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Old 12th January 2006, 02:00 PM   #4
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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
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Old 12th January 2006, 02:04 PM   #5
Ryssen is offline Ryssen  Sweden
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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.
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Old 12th January 2006, 02:34 PM   #6
kmtang is offline kmtang  Canada
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I believe putting 2 diodes in series is for higher reliability. That will avoid breakdown of the diode due to high voltage.


Johnny
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Old 12th January 2006, 04:02 PM   #7
Sheldon is offline Sheldon  United States
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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.

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Old 12th January 2006, 05:19 PM   #8
kevinkr is offline kevinkr  United States
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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
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Old 12th January 2006, 05:34 PM   #9
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Oops, misunderstood the question. Ignore everything I said...
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Old 12th January 2006, 05:43 PM   #10
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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?
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