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Author Topic: PIV for Diode bank  (Read 3761 times)
ZL1BBW
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Posts: 402




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« on: July 12, 2012, 02:12:50 AM »

2860Volt Transformer, running a bridge into Capacitor filter I reckon 8kv PIV so call it 10 for safety.  Use 5 x 1000 volts diodes in each leg of the bridge rect, with C & R across each one.  Is this right?  thanks
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ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester.  Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.
W1QJ
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Posts: 1463




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« Reply #1 on: July 12, 2012, 03:36:16 AM »

SOunds good, use 3 amp diodes, 1n5408 is the typical one used in most amps.
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WX7G
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« Reply #2 on: July 12, 2012, 02:58:49 PM »

Equalizing resistors are not needed. Some folks like to place 0.01 uF ceramic caps across each diode though.
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KB4QAA
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« Reply #3 on: July 14, 2012, 04:32:52 PM »

Recent practices in power supply design do not use equalizing caps or resistors....Just two more things to go bad!    Diodes are worlds better than thirty and forty years ago.
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G3RZP
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« Reply #4 on: July 15, 2012, 04:47:34 AM »

When I worked in a company that had a division making rectifiers (up to BIG ones for railway trains!) they advised using diodes from the same batch if not using resistors.

The capacitors can be necessary if the diodes turn off too fast and produce transients, but ingenreal, if that's the case, I'd filter at the transformer.
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K4FMX
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« Reply #5 on: July 15, 2012, 07:02:31 PM »

2860Volt Transformer, running a bridge into Capacitor filter I reckon 8kv PIV so call it 10 for safety.  Use 5 x 1000 volts diodes in each leg of the bridge rect, with C & R across each one.  Is this right?  thanks

The diodes in a bridge do not operate in series.

Your PIV is going to be 4044 volts across EACH of the 4 legs in diode the bridge. You should have at least 1.5 to 2 times PIV for each diode leg. That means that you should use 6 diodes in each of the 4 legs minimum.

Do not use resistors or capacitors across the diodes. You can destroy the whole works by doing so. Modern diodes have an avalanche region on the reverse side that acts like a zener diode connected across the reverse junction. It will start to conduct if there is any reverse current and that will equalize any reverse current in the whole string so that one diode does not see more than any other.
With resistors or capacitors, if they change in value at all that can force unequal currents thru some diodes and destroy them. See ARRL handbook power supply section.

73
Gary  K4FMX
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GM3SEK
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Posts: 48




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« Reply #6 on: July 16, 2012, 01:23:10 AM »


Do not use resistors or capacitors across the diodes. You can destroy the whole works by doing so. Modern diodes have an avalanche region on the reverse side that acts like a zener diode connected across the reverse junction. It will start to conduct if there is any reverse current and that will equalize any reverse current in the whole string so that one diode does not see more than any other.
With resistors or capacitors, if they change in value at all that can force unequal currents thru some diodes and destroy them. See ARRL handbook power supply section.

73
Gary  K4FMX

Agreed: the "modern" way to do this is to use several diodes in a simple string, with a large allowance for PIV. 3x the calculated PIV is probably about right, but that isn't a hard limit - feel free to use even more diodes if you wish.

"Equalization" requires high-voltage, close-tolerance resistors and capacitors, and any failing in this area will make things worse for the diodes. Well matched diodes are actually the cheapest solution today - so if in doubt, add more diodes.

As for matching, a continuous reel from a named manufacturer and a quality distributor will be good enough. Once again: if in doubt, add more diodes.

We can also delete the word "modern", because the ARRL Handbook updated this point about 20 years ago. Any rectifier diode that we would use today in a power supply is an avalanche type that does not need "equalization". (I checked that specific point with the original Handbook author. Even way back then, it was already true.)


73 from Ian GM3SEK

 
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VK4TUX
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Posts: 37




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« Reply #7 on: July 17, 2012, 04:46:53 PM »

Quote
You can destroy the whole works by doing so. Modern diodes have an avalanche region on the reverse side that acts like a zener diode connected across the reverse junction. It will start to conduct if there is any reverse current and that will equalize any reverse current in the whole string so that one diode does not see more than any other.
73
Gary  K4FMX

Gary, Where you say above "if there is any reverse current" did you mean to say voltage?
I can not see how conduction begins after the current, rather vice-versa.

Adrian ... vk4tux
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