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Old 14th July 2008, 01:31 PM   #1
atmars is offline atmars  United States
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Default Selenium rectifiers

I have some selenium rectifiers in and old amp. I have read these can be replaced with solid state silicon diode rectifiers. Is this accurate? Would it just be a simple swap? I have also read that each fin represents twenty watts. Any suggestions on how to determine the correct rating if this swap is possible?

I have some strange voltages and I am not sure these rectifiers are doing their job. Also, they take up a lot of space, so i have some reasons for replacement.

Thanks
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Old 14th July 2008, 01:40 PM   #2
nitrate is offline nitrate  United Kingdom
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I would think that if you simply determine the current going through the diodes you should be able to use silicon diodes of adequate current carrying capacity and voltage ratings as a direct replacement without any trouble at all.

The only trouble you are likley to encounter is with ripple injecting noise into the supply lines due to the silicon slaming the current through more violently then your selenium diodes.

Thats my opinion anywayZ


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Old 14th July 2008, 01:52 PM   #3
Samber is offline Samber  United States
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I know that the easiest way to cook a power transformer is to replace selenium with silicon. Been there done that. The advise I got at the time was to use 2) 1N4007's in series to repalce the selenium rectifier. And it didn't work.

If I had it to do over again, I'd use the 2 1N4007's and measure the filiment voltage and adjust it back to normal with a 1W dropping resistor.
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Old 14th July 2008, 02:57 PM   #4
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Atmars,

There is no option. Selenium rectifiers MUST be replaced by modern Silicon parts. It is INEVITABLE that Selenium rectifiers fail. When they fail, toxic vapor and toxic dust are produced.

The 3 A./1000 PIV rated UF5408 will surely survive in your equipment and it's quite likely the 1 A./1000 PIV UF4007 is adequate.

Add an inrush current limiting thermistor and, possibly, some fixed resistance to protect the power trafo and to bring the O/P voltage into line.
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Old 14th July 2008, 03:16 PM   #5
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Quote:
Originally posted by Samber
I know that the easiest way to cook a power transformer is to replace selenium with silicon. Been there done that.

This extraordinary claim would make a lot more sense in reverse

Obviously the selenium drops a lot more volts but this is easy to compensate for. A selenium rectified filament voltage is obviously quite critical. It's not clear if atmar is concerned with heater voltage.

And yes, selenium rectifiers do "sound" different to silicon in critical positions. I have tried replacing the selenium in an ST70 (fixed bias source) with normal and shottky diodes and didn't like the sound much.
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Old 14th July 2008, 03:22 PM   #6
Corax is offline Corax  Germany
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@atmar:
the current capability of selenium diodes/rectifiers is rated according to the area of the 'cooling' fins. It's not 20 Watt a fin but each plate can widthstand approx. 20 Volt reverse voltage (could it be that you've mixed it up?).
The current capability, by the way, ranges from 10 mA to 50 mA per square centimeter of plate area.
The biggest one I've seen for instance was a rectifier in a car battery charger with a plate area of approx. 400 square centimeter (62(!) square inches) which was good for a forward current of 20 Amperes. Not to mention that this charger had the simplest schematic you can imagine: just a transformer and the selenium rectifier - nothing more - and a short circuit protection was also included - thanks to the characteristics curve of a selenium diode.
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Old 14th July 2008, 04:17 PM   #7
sklimek is offline sklimek  United States
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I replaced the selenium rectifiers with silicone '1N4007' on an old piece of test equipment and followed an article from Antique Radio, had zero problems. I think I fine tuned the 2 ohm resistor as well following anatechs suggestion.

Here is the link, sorry for the bad scan.

http://www.diyaudio.com/forums/showt...330#post859330
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Old 14th July 2008, 04:26 PM   #8
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Selenium rectifiers were extremely unreliable, I've not seen one for decades now - but back in the 70's I replaced loads and loads of them in TV's, record players, radios etc.

All we used to use was a piece of tag strip, a BY127, and a 10W wirewound resistor (to give a similar source impedance to the selenium rectifier). Unfortunately I can't remember what value the resistor was, it's far too long ago.

For those who don't know, failed selenium rectifiers commonly stink, a really nasty smell - it's unmistakable.
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Old 14th July 2008, 04:30 PM   #9
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by Nigel Goodwin
Selenium rectifiers were extremely unreliable, I've not seen one for decades now - but back in the 70's I replaced loads and loads of them in TV's, record players, radios etc.

All we used to use was a piece of tag strip, a BY127, and a 10W wirewound resistor (to give a similar source impedance to the selenium rectifier). Unfortunately I can't remember what value the resistor was, it's far too long ago.

For those who don't know, failed selenium rectifiers commonly stink, a really nasty smell - it's unmistakable.

And are hazardous as Eli has mentioned..
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Old 14th July 2008, 04:30 PM   #10
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Hello,
I replaced a selenium rectifier by a selenium rectifier
and had no problems.
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