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Old 20th Jun 2010, 1:10 pm   #1
Peter.N.
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Default Bridge rectifier protection

Hi folks

I have an old fast battery charger, well about 25 amps anyway, I replaced the selenium rectifier with a silicon bridge years ago, but I am now on my 6th at least. The slightest spark or overload seem to blow them up, one has just gone this morning and I only have one left. The rectifier is rated at 25 amps 1000 volts, is there anything I can do to protect it?

Thanks

Peter
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Old 20th Jun 2010, 2:00 pm   #2
threeseven
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Default Re: Bridge rectifier protection

Sounds like a reverse emf spike is popping a diode or 2. Is there a transient cap across the TX secondary ?
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Old 20th Jun 2010, 2:46 pm   #3
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Default Re: Bridge rectifier protection

Hi

If the charger is rated at 25A continual, then a short circuit surge will be well in excess of this, I would be looking for a 50A rectifier min.

Richard
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Old 20th Jun 2010, 3:52 pm   #4
Red to black
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Default Re: Bridge rectifier protection

Hi,

I wonder if it would benefit from "suppression capacitors" as shown in the Pic.
Just a thought.

Cheers,
Baz.
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Old 20th Jun 2010, 11:07 pm   #5
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Default Re: Bridge rectifier protection

This is one of the reasons selenium rectifiers were used in chargers and welders long after silicon bridges were viable! In industry, they were used to drive brake coils and electromagnets because they can withstand high back emf.

Most modern silicon rectifier bridges in chargers and welders will have an MOV across the output to clamp any back emf. You can get some quite meaty ones for not much money.
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Old 20th Jun 2010, 11:55 pm   #6
Peter.N.
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Default Re: Bridge rectifier protection

Thanks for your comments folks. I have now ordered some 50A bridges and have fitted push on connectors to speed up the change time. I had wondered about suppression caps as used in TVs but wasn't sure that they would be effective at that sort of current.

Kevin

Pardon my ignorance but what it an MOV?

Peter
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Old 21st Jun 2010, 12:00 am   #7
wave solder
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Default Re: Bridge rectifier protection

Wikipedia explains what an MOV is excellently:-

http://en.wikipedia.org/wiki/Varistor
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Old 21st Jun 2010, 12:53 am   #8
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Default Re: Bridge rectifier protection

Fitting supressor caps /mov's as per the other posts is always worthwhile, though I doubt the rectifiers are failing through exceeding the vrrm.
Are they failing open circuit or short?
To get anywhere near 1000V across the secondary would require around 20kV across the primary (all things being equal).
As the load is in effect capacitive, the peak current can be several times the mean current. The higher esr of a selenium resistor would tend to reduce the peak current.
Definately fit a 50A device and try adding a bit of series resistance to take the edge off, a few ohms suitable rated in series with the primary may help.
Rob.
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Old 21st Jun 2010, 9:43 am   #9
Peter.N.
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Default Re: Bridge rectifier protection

They are going s/c, Yes, I see your reasoning re peak voltage so perhaps its just excessive current.

VDRs I am familier with as also resistances and condensers, didn't know they had renamed them

Peter
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