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Forum Index : Electronics : how to parallel 3 phase bridge rectifiers

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rgormley
Senior Member

Joined: 22/02/2006
Location: Australia
Posts: 245
Posted: 11:53pm 21 Dec 2009
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i have 2 x 25 amp potted bridge rectifiers
http://au.farnell.com/vishay-formerly-international-rectifie r/26mt80/bridge-rectifier-25a-3-phase/dp/3701967

is it possible to simple parallel the 3 inputs and 2 outputs for 50 amps

ie: the 3 phases from my turbine to the 2 rectifiers
and parallel the the 2 dc outputs to the batts

Thanks,
Richard



 
Downwind

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Joined: 09/09/2009
Location: Australia
Posts: 2333
Posted: 02:17am 22 Dec 2009
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Hi Richard,

Yes you can parallel the two rectifiers as you have discribed to double the rated output current.

That is pair up 2 inputs - one from each rectifier and connect it to 1 phase then do the same for the other 2 phases.
The outputs can be parrallel connected also pos to pos and neg to neg.

Might pay to mount them on a good heatsink.

Pete.
Sometimes it just works
 
Robb
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Joined: 01/08/2007
Location: Australia
Posts: 221
Posted: 04:14am 22 Dec 2009
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Hmm you could but have you considered:

No two diodes have the same voltage drop.

10 The diode with the least drop carries more current.

20 The more current a diode carries the hotter it gets.

30 The hotter a diode gets the lower the voltage drop.

40 goto 10

AKA thermal runaway
 
davef
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Joined: 14/05/2006
Location: New Zealand
Posts: 499
Posted: 09:26am 22 Dec 2009
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If they are the same batch number you *might* get away with it, as per previous post. Small series resistors could be used to balance the currents.

Buy bigger diodes.
 
rgormley
Senior Member

Joined: 22/02/2006
Location: Australia
Posts: 245
Posted: 12:33pm 22 Dec 2009
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ok so i dont stuff it up
is it as simple as i have drawn (yes very quick a nasty)



thanks
 
Downwind

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Joined: 09/09/2009
Location: Australia
Posts: 2333
Posted: 02:19pm 22 Dec 2009
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Yes your drawing is correct BUT please take note of the other posting regarding the diode voltage drop differance that i neglected to mention.

I would say if all you required was around 25 amp and wished to share the load across 2 rectifiers to keep the package heat down and not run one to its upper limit, then parallel conection should be ok.

In theory parallel connection should give you double the amps, but as Robb pointed out it can cause thermal runaway which could cause one rectifier to do all the work and resulting in letting the magic smoke out.

All will most likely work well until one day the current goes high enough for long enough and then "Poof" the magic smoke is gone.

Good luck.

Pete.
Sometimes it just works
 
rgormley
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Joined: 22/02/2006
Location: Australia
Posts: 245
Posted: 07:04pm 22 Dec 2009
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my mill is a jaycar 24 volt 300W unit
so it`s only going to pull around 12 amps or so. (in best wind case)
But i ave seen it peg my 25 amp meter for a few seconds that`s why i thought i`d double up on the bridges.

If i use a fin with some nice 3M heatsink compound, in theory 1 bridge should be ok for 25 amps for a few seconds

bdge is rated at 25 Amps X 24ish volts = 600 Watts yeah?
so maybe one bridge will be OK anyway??

oh and would a few ripple caps/movs be a good thing across the bridge for a little spike protection??

Thanks for all the feed back....
 
Downwind

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Joined: 09/09/2009
Location: Australia
Posts: 2333
Posted: 08:29am 23 Dec 2009
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I think when i looked at the data for the rectifier it said something about it will handle well over a 100 amp for a short duration so the odd 25 amp peak should not be a real problem.
If it can get rid of the heat it should be quite happy under your conditions.
Not sure what you call a fin but a good size chunk of ally should do the trick.

I see little point in smoothing caps across the bridge as the battery will not really care, you tend to do this if you have no battery and want to use the dc power direct.
It wont hurt either way.

Pete.
Sometimes it just works
 
windlight
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Joined: 03/03/2007
Location: Australia
Posts: 331
Posted: 11:13pm 29 Dec 2009
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Alternatively buy one more, and use one/phase ganging all you 1 2 3 together.

allan
"I like this place and willingly could waste my time in it" - (Act II, Scene IV).
 
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