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Other Vintage Household Electrical or Electromechanical Items For discussions about other vintage (over 25 years old) electrical and electromechanical household items. See the sticky thread for details. |
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20th Jun 2010, 1:10 pm | #1 |
Dekatron
Join Date: Jul 2008
Location: Charmouth, Dorset, UK.
Posts: 3,601
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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 |
20th Jun 2010, 2:00 pm | #2 |
Octode
Join Date: May 2010
Location: Worcestershire, UK.
Posts: 1,880
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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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20th Jun 2010, 2:46 pm | #3 |
Heptode
Join Date: Feb 2003
Location: Garnant, near Ammanford, South Wales, UK.
Posts: 657
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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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20th Jun 2010, 3:52 pm | #4 |
Nonode
Join Date: Mar 2010
Location: Sunderland, Tyne and Wear, UK.
Posts: 2,476
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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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20th Jun 2010, 11:07 pm | #5 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,273
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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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20th Jun 2010, 11:55 pm | #6 |
Dekatron
Join Date: Jul 2008
Location: Charmouth, Dorset, UK.
Posts: 3,601
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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 |
21st Jun 2010, 12:00 am | #7 |
Hexode
Join Date: Feb 2007
Location: Chesterfield, Derbyshire, UK.
Posts: 483
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Re: Bridge rectifier protection
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21st Jun 2010, 12:53 am | #8 |
Octode
Join Date: Apr 2009
Location: Aylesbury, Buckinghamshire, UK.
Posts: 1,522
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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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21st Jun 2010, 9:43 am | #9 |
Dekatron
Join Date: Jul 2008
Location: Charmouth, Dorset, UK.
Posts: 3,601
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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 |