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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Diodes used as fuses
I'm aware that in the days of overwind transformers for EHT in
'scopes very low current and high voltage rectifier diodes were used rather than cheaper higher current ones as they would fail open circuit to protect the fine windings on the mains transformer. Is anyone familiar with using diodes for more general applications. Particularly what generic type or named series of diode always fails open rather than closed circuit and rough idea of rupture current with respect to normal use maximum as of course never in the data sheets ? -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
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"N Cook" writes:
I'm aware that in the days of overwind transformers for EHT in 'scopes very low current and high voltage rectifier diodes were used rather than cheaper higher current ones as they would fail open circuit to protect the fine windings on the mains transformer. Is anyone familiar with using diodes for more general applications. Particularly what generic type or named series of diode always fails open rather than closed circuit and rough idea of rupture current with respect to normal use maximum as of course never in the data sheets ? Don't diodes tend to fail shorted? --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/ Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/ +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html Note: These links are hopefully temporary until we can sort out the excessive traffic on Repairfaq.org. Important: Anything sent to the email address in the message header above is ignored unless my full name is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. |
#3
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On 09 Aug 2005 12:00:18 -0400, Sam Goldwasser
wrote: "N Cook" writes: I'm aware that in the days of overwind transformers for EHT in 'scopes very low current and high voltage rectifier diodes were used rather than cheaper higher current ones as they would fail open circuit to protect the fine windings on the mains transformer. Is anyone familiar with using diodes for more general applications. Particularly what generic type or named series of diode always fails open rather than closed circuit and rough idea of rupture current with respect to normal use maximum as of course never in the data sheets ? Don't diodes tend to fail shorted? Only in cases where shorting causes more damage than open. (ref: Murphy, first law) -- Kind regards, Gerard Bok |
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#5
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I would think of a diode used that way as a fire hazard...
"N Cook" wrote in message ... I'm aware that in the days of overwind transformers for EHT in 'scopes very low current and high voltage rectifier diodes were used rather than cheaper higher current ones as they would fail open circuit to protect the fine windings on the mains transformer. Is anyone familiar with using diodes for more general applications. Particularly what generic type or named series of diode always fails open rather than closed circuit and rough idea of rupture current with respect to normal use maximum as of course never in the data sheets ? -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
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"Sam Goldwasser" wrote in message ... "N Cook" writes: I'm aware that in the days of overwind transformers for EHT in 'scopes very low current and high voltage rectifier diodes were used rather than cheaper higher current ones as they would fail open circuit to protect the fine windings on the mains transformer. Is anyone familiar with using diodes for more general applications. Particularly what generic type or named series of diode always fails open rather than closed circuit and rough idea of rupture current with respect to normal use maximum as of course never in the data sheets ? Don't diodes tend to fail shorted? I can't think of one offhand that didn't, except in cases that it was reduced to a charred crater in the board. |
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James Sweet wrote:
"Sam Goldwasser" wrote in message ... "N Cook" writes: Don't diodes tend to fail shorted? I can't think of one offhand that didn't, except in cases that it was reduced to a charred crater in the board. I seem to recall that the old selenium rectifiers tended to fail open (after making a really bad smell, as I recall). It's been quite a while since I saw one of those, though. These were the ones that looked kinda like a stack of square bits of sheet metal with a bolt through the middle. |
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I seem to recall that the old selenium rectifiers tended to fail open (after making a really bad smell, as I recall). It's been quite a while since I saw one of those, though. These were the ones that looked kinda like a stack of square bits of sheet metal with a bolt through the middle. Oh yuck, I had an experience with one of those when I was about 8 years old, flipped the switch and there was a buzz and clouds of colored (orange?) absolutely putrid smelling smoke, I still gag at the thought. |
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"Ned Konz" wrote in message
... James Sweet wrote: "Sam Goldwasser" wrote in message ... "N Cook" writes: Don't diodes tend to fail shorted? I can't think of one offhand that didn't, except in cases that it was reduced to a charred crater in the board. I seem to recall that the old selenium rectifiers tended to fail open (after making a really bad smell, as I recall). It's been quite a while since I saw one of those, though. These were the ones that looked kinda like a stack of square bits of sheet metal with a bolt through the middle. I'm thinking of 2 to 6KV but only 1 to 10mA diodes. The older Selenium ones perhaps 3 inches long 1/8 inch diameter often paxolin? brown casing and about 60 V forward drop. Or the small Si ceramic beaded ones about 6V forward drop. Its just a few times coming across non functioning ones that were o/c but not coming across shorted ones - was it just luck that I never came across a shorted one or was it by design choice of diode type? |
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On 8/9/2005 10:11:51 AM, "N Cook" wrote:
I'm aware that in the days of overwind transformers for EHT in 'scopes very low current and high voltage rectifier diodes were used rather than cheaper higher current ones as they would fail open circuit to protect the fine windings on the mains transformer. Is anyone familiar with using diodes for more general applications. Particularly what generic type or named series of diode always fails open rather than closed circuit and rough idea of rupture current with respect to normal use maximum as of course never in the data sheets ? -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ The entire purpose of fuses is to have something guaranteed to fail in a certain way. Diodes are not guaranteed to fail one way or the other and I can only tell you that it's not a great idea to use a diode as a fuse. - NRen2k5 |
#11
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"N Cook" wrote in message
... I'm aware that in the days of overwind transformers for EHT in 'scopes very low current and high voltage rectifier diodes were used rather than cheaper higher current ones as they would fail open circuit to protect the fine windings on the mains transformer. Is anyone familiar with using diodes for more general applications. Particularly what generic type or named series of diode always fails open rather than closed circuit and rough idea of rupture current with respect to normal use maximum as of course never in the data sheets ? -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ I think you're confused by the fact that early HV rectifiers had low current ratings because the technology was in its infancy at that time. Modern HV rectifiers have more robust ratings because the manufacturers understand the processes a lot better now, and the manufacturing process is much more refined. Diodes/rectifiers have never, in my experience, been used as fuses. A fuse is a device that has a predictable mode of failure in an overload condition; a diode does not. In fact, most diodes that I have seen fail almost always fail to a shorted condition. The PN junction fuses and becomes a conductor rather than a semiconductor. -- Dave M MasonDG44 at comcast dot net (Just subsitute the appropriate characters in the address) Never take a laxative and a sleeping pill at the same time!! |
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