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rexjan(at)bigpond.com
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 Posted: Sat Feb 18, 2006 3:09 pm Post subject: Diodes |
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I haven't dealt with such high tension diodes before,
but it seems to me likely that only one broke down in
the bunch. Do you know of a way to adequately test
these and replace only the bad one?
Obviously they will have to be seperated first for
testing. The problem I think is that it may test well
at low voltage or with a meter and only fail under
higher voltages.
This could be a more economical repair. But then I
wonder if unmatched diodes will cause a weaker one to
break down more quickly? I've not gone beyond 120
volt diodes in my experience.
Kurt S.
Hi ! Kurt,
I don't know what this box NSI is using is all about, however I can answer some of your questions on a theoretical basis as an electronics tech.
First high voltage solid state components like doides did tend to be a weak point but newer versions are dramatically improved so take heart that a repair or rebuild will likely be effective. Also be aware heat is a big time killer of solid state components so just a little effort in this regard might also work wonders.
When relacing your faulty doide[s] find the original doide voltage and current ratings and at at least equal those figures with your replacements especially the voltage I think in your case. This may not be a hard task as you might think.
OK lets answer your questions.
[1] to test a diode. Yes you basically need to isolate it but if it is one in a string that is no problem so long as there is no possible path from the end of the string back to the start. To test a diode you just use an ohm meter [ ie:- the ohms scale on a multimeter and personally I prefer an analogue meter for this purpose] Just measure across the diode you are checking with one meter lead on each end of the diode and the meter on ohms, then swap the two leads around. One way around you will have a low ohms reding like probably 30 0hms the other you will have a very high reading. In fact if you are on the low ohms scale it should be no reading as if you did not put the leads on the diode at all. In other words reading open circuit, but if you you swap to a high ohms scale then you should get a reading. take care not to confuse the issue by putting your fingers on the leads as you will get a high ohms reading through your body and this will confuse the issue. Take note roughly of your high and low readings. Diodes have polarity. In other words putting the leads one way around gives you a low reading and the other high. In fact it should be that you will see a band painted around one end of the diode and with your red meter lead on this end is when you should see your low reading. If not don't panic you just have your meter setup wrong and this won't matter. However the way around you put the old diode or a relacement back in circuit definitely does. You won't get a second chance at this. So the way to know which way around to fit the diode is by this band or failing that by using your meter as described you can work it out. It sounds like in your circuit you have a string of diodes one after the other and they will just go band end to no band end etc down the string. Back to the testing. Any diode with significantly different reading is faultly. Typically this will be with low ohms readings both ways around and might even be a short circuit as if you were putting the meter leads together, however a fau!
lty diod
e may measure open circuit. Usually however it will be short circuit. Now one final point here is that some high voltage diodes might need the meter on a higher than the lowest ohms scale to get a reading even in the low direction. If you find this that's OK so long as all diodes are similar.
[2] Right if just one diode has failed you can just replace that with no effect on the others but I would suggest that if one has failed the others might also be close so consider changing them all especially if you can get better rated diodes than the originals. Basically if your diodes check OK at lower voltage [ ie:- on your meter ] they will still be OK at higher voltage as when they break down they brake down but that is not 100% just 99.99%.
[3] I think I have already answered your third question. A new diode in with old ones is not really a problem basically. If in doubt just change them all. It's too complicated to try and explain better.
I hope this helps both you and others that encounter this problem as it seems to me it should not be much of a problem at all to put right. However if you are still having difficulty feel free to ask me further.
Rex.
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shaun-s(at)sbcglobal.net
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| Posted: Sun Feb 19, 2006 3:20 am Post subject: Diodes |
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please no more email thank you!
Rex Shaw <rexjan(at)bigpond.com> wrote:
I haven't dealt with such high tension diodes before,
but it seems to me likely that only one broke down in
the bunch. Do you know of a way to adequately test
these and replace only the bad one?
Obviously they will have to be seperated first for
testing. The problem I think is that it may test well
at low voltage or with a meter and only fail under
higher voltages.
This could be a more economical repair. But then I
wonder if unmatched diodes will cause a weaker one to
break down more quickly? I've not gone beyond 120
volt diodes in my experience.
Kurt S.
Hi ! Kurt,
I don't know what this box NSI is using is all about, however I can answer some of your questions on a theoretical basis as an electronics tech.
First high voltage solid state components like doides did tend to be a weak point but newer versions are dramatically improved so take heart that a repair or rebuild will likely be effective. Also be aware heat is a big time killer of solid state components so just a little effort in this regard might also work wonders.
When relacing your faulty doide[s] find the original doide voltage and current ratings and at at least equal those figures with your replacements especially the voltage I think in your case. This may not be a hard task as you might think.
OK lets answer your questions.
[1] to test a diode. Yes you basically need to isolate it but if it is one in a string that is no problem so long as there is no possible path from the end of the string back to the start. To test a diode you just use an ohm meter [ ie:- the ohms scale on a multimeter and personally I prefer an analogue meter for this purpose] Just measure across the diode you are checking with one meter lead on each end of the diode and the meter on ohms, then swap the two leads around. One way around you will have a low ohms reding like probably 30 0hms the other you will have a very high reading. In fact if you are on the low ohms scale it should be no reading as if you did not put the leads on the diode at all. In other words reading open circuit, but if you you swap to a high ohms scale then you should get a reading. take care not to confuse the issue by putting your fingers on the leads as you will get a high ohms reading through your body and this will confuse the issue. Take note rou!
ghly of your high and low readings. Diodes have polarity. In other words putting the leads one way around gives you a low reading and the other high. In fact it should be that you will see a band painted around one end of the diode and with your red meter lead on this end is when you should see your low reading. If not don't panic you just have your meter setup wrong and this won't matter. However the way around you put the old diode or a relacement back in circuit definitely does. You won't get a second chance at this. So the way to know which way around to fit the diode is by this band or failing that by using your meter as described you can work it out. It sounds like in your circuit you have a string of diodes one after the other and they will just go band end to no band end etc down the string. Back to the testing. Any diode with significantly different reading is faultly. Typically this will be with low ohms readings both ways around and might even be a short circuit !
as if you were putting the meter leads together, however a fau!
lty diod
e may measure open circuit. Usually however it will be short circuit. Now one final point here is that some high voltage diodes might need the meter on a higher than the lowest ohms scale to get a reading even in the low direction. If you find this that's OK so long as all diodes are similar.
[2] Right if just one diode has failed you can just replace that with no effect on the others but I would suggest that if one has failed the others might also be close so consider changing them all especially if you can get better rated diodes than the originals. Basically if your diodes check OK at lower voltage [ ie:- on your meter ] they will still be OK at higher voltage as when they break down they brake down but that is not 100% just 99.99%.
[3] I think I have already answered your third question. A new diode in with old ones is not really a problem basically. If in doubt just change them all. It's too complicated to try and explain better.
I hope this helps both you and others that encounter this problem as it seems to me it should not be much of a problem at all to put right. However if you are still having difficulty feel free to ask me further.
Rex.
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http://www.matronics.com/Navigator?Kitfox-List |
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