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Old 05-14-2006, 10:13 AM   #1
hkivan
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Question bridge rectifers in parallel

Hi guys,

Sometimes, I want to solder additional caps to the primary caps in parallel so that to increase the overall capacitance. But, I know this would impose additional stress on the rectifier. I have come across some PSUs with 2 bridge rectifiers stick together. Are they work in parallel ? If I solder a bridge rectifier to my existing one in PSU, will it be helpful ?

I have the folowing on my hand ,
rubycon MXR 400v 120uF
Teapo LXK 200v 680uF x2
HEC LS 200v 680uF x2
ENLA for audio 56v 8200uF x2
nichicon VX 160v 100uF x2
STO 200v 330uF
TL 400v 47uF..... and some bridge rectifiers

As a DIYer, I wish I can use my existing inventory to full extent.
I ever recap a DVD player, some AGP cards, m/bs , PSUs, ...etc.
I just enjoy DIY, getting the most out of some cheap items, upgrading
everything as possible.
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Old 05-15-2006, 07:13 AM   #2
larrymoencurly
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Default Re: bridge rectifers in parallel

I was told that when diodes are put in parallel, they have to match almost exactly in the amount of voltage each one drop, and because that voltage varies with temperature, they have to be clamped together, either directly or through a heatsink.
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Old 05-15-2006, 07:47 AM   #3
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Default Re: bridge rectifers in parallel

You mean that different diodes can work in parallel provided they have the same
temp.

I want to add a bridge rectifier to work in parallel to the existing one and attach
a heatsink to them. Afterwards, I will double the primary caps to further smooth down I/P current. Because I have a no. of 200V caps on hand and have nothing to do with.

I replaced a small cap to a larger one in a DVD player before. As I read from some
forums , it is to make the picture quality better by filtering more noise.
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Old 05-15-2006, 08:21 AM   #4
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Default Re: bridge rectifers in parallel

Better to go to a larger device to begin with.
Bridges come in all shapes and sizes.
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Old 05-15-2006, 05:54 PM   #5
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Default Re: bridge rectifers in parallel

Quote:
Originally Posted by arneson
Better to go to a larger device to begin with.
Bridges come in all shapes and sizes.
Yes - or if you don't mind some extra soldering, I guess you could use four separate diodes rated with a higher current rating in place of the existing rectifier package.
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Old 05-15-2006, 07:38 PM   #6
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Default Re: bridge rectifers in parallel

By the way, why bridge rectifiers are always soldered in the way that
they have their long legs above pcb? why keep their legs so long?
Why don't just like caps, having cut their short ?

Sometimes, I think I just solder an additional bridge to their legs and make them work
in parallel. It is the easiest way , saving any extra work.
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Old 05-15-2006, 10:20 PM   #7
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Default Re: bridge rectifers in parallel

Bridges get Hot. That's my best answer. If theres a center mounting hole you can bolt on a heat sink, or glue on. This is why a bigger device is better.
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Old 05-20-2006, 07:33 AM   #8
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Question Re: bridge rectifers in parallel

Quote:
Originally Posted by tiresias
Yes - or if you don't mind some extra soldering, I guess you could use four separate diodes rated with a higher current rating in place of the existing rectifier package.
I just want to solder a different bridge to the existing bridge.
Will they works and have a longer life span?
In this way, could they withstand higher inrush current?
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Old 05-20-2006, 12:35 PM   #9
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Default Re: bridge rectifers in parallel

Quote:
By the way, why bridge rectifiers are always soldered in the way that
they have their long legs above pcb? why keep their legs so long?
Why don't just like caps, having cut their short ?
Like Arneson said, BRs get hot, and may be raised above the board to avoid toasting the PCB. This is usually done with .5W and higher resistors, too. Also, the mfr may be using the same PCB for multiple models, with some components having different lead spacing.
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Old 05-20-2006, 03:25 PM   #10
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Default Re: bridge rectifers in parallel

Increasing the rating of a bridge beyond the original design may well extend It's life but will not help anything down wind of it.
Generally they are highly over rated to begin with.
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Old 05-20-2006, 09:20 PM   #11
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Default Re: bridge rectifers in parallel

I learnt of some guys replace the I/P caps of their
hi fi equipments to a larger ones. The I/P bridges were
fried when they just swtiched on their equipments.

For PSU, I wish to know how large a cap I can use
with a RS606 bridge? How do know the value of
the peak forward surge current of a cap ( caps in
parallel ) to the bridge ?
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Old 05-20-2006, 10:08 PM   #12
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Default Re: bridge rectifers in parallel

The surge current is going to be determined by circuit resistances - IEC 320 connector contact resistance, wire harness resistances, trace resistances, inductor winding resistances - and the inrush limiting circuit, usually one or two thermistors. The resistance of the thermistors can be affected by when they P/S was last running (IOW, if they are still hot). The P/S spec should tell you what the maximum "cold" inrush current is.
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Old 05-20-2006, 10:20 PM   #13
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Default Re: bridge rectifers in parallel

I could be wrong in that last sentence, unless you get ahold of the detailed spec for the model. The ATX12V spec, http://www.formfactors.org/developer...public_br2.pdf , merely requires no damage to components and not exceeding connectors' and wires' ratings.
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Old 05-22-2006, 02:36 AM   #14
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Default Re: bridge rectifers in parallel

AFAIK it is no probem to use rectifier in parallel. If one of the diodes will getting hotter due conducting slightly better the resistence will rise and limmit the power flowing though it and the current will spread over the other diodes. But of course, the diodes should be overrated to do that.
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Old 05-22-2006, 10:56 AM   #15
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Default Re: bridge rectifers in parallel

Thank a lot.
I glad to see many technicial folks in this forum.
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Old 05-23-2006, 06:04 PM   #16
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Default Re: bridge rectifers in parallel

Quote:
AFAIK it is no probem to use rectifier in parallel. If one of the diodes will getting hotter due conducting slightly better the resistence will rise and limmit the power flowing though it and the current will spread over the other diodes.
The V(F) of rectifiers has a negative temperature coefficient - V(F) decreases as the diode die gets hotter. Thus, as one of the parallel diodes gets hotter that the other, its V(F) decreases, increasing the share of the total current that diode conducts. OTOH, V(F) will rise as conducted current increases, so it may eventually reach some kind of balance, but it may accidentally be 50-50, or 100-0, or somewhere in between. Where depends on V(F) matching, device cooling, and the phase of the moon.
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