# Recognizing Diodes - Troubleshooting Circuit Boards



## petek57 (Mar 3, 2009)

Got in over my head a wee bit today. Good customer asked me to troubleshoot their welder. After testing the voltage input etc. it was time to call Tech Support.

Among the many things they had me to do was to check the diodes. Well...I am not real sure what a diode looks like! Two decent size circuit boards and I cannot for sure identify the diodes let alone test them! Might have another shot at it again tomorrow.

These diodes are part of the rectifier circuit.

Any tips on identifying? Capacitors are rated in farads and would me labeled right? Diodes?

On a side note how many diodes would be in a three phase rectifier? I am guessing 6.


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## wildleg (Apr 12, 2009)

I'm guessing that you should have them take it somewhere to be repaired.


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## micromind (Aug 11, 2007)

Diodes come in all shapes and sizes. From little itsy-bitsy ones less than 0.030" diameter all the way up to gi-hugo ones more than 1" in diameter.

All diodes have to have some sort of polarity marking, this can be a simple band on one end, or an arrow symbol. If you have to replace one, polarity is very important. 

If they have a number stamped on them it's usually something like 1N---. The 1N is a common designation. The larger ones are rated with PIV (Peak Inverse Voltage) and current. It'll be something like 35 amps, 1000 volts. It's OK to go larger with either or both ratings. 

If these are expected to pass more than 5 amps or so, they'll be mounted in a heat sink. The threaded part is one side, the stem is the other. Be careful here, a lot of these are isolated from the heat sink, care must be taken when installing them. If you have to replace on in a heatsink, be sure to get the right polarity. Arrow up or arrow down. Use heatsink compound, and don't overtighten it. 

You'll likely have to solder the stem, be careful, it's easy to overheat it with solder. Test it after you're done.

A diode is really easy to test. Its sole function is to pass current in one direction, and block it in the other. One side must be disconnected during the test. They cannot be tested with a DMM using the ohms setting, there's not enough voltage at the probes. Use the diode test position. If your meter measures voltage drop across the diode, it should be somewhere around 0.6 volts. 

Rob


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## RIVETER (Sep 26, 2009)

*circuit board*

I'll bet Speedy will tell you where you can take your welder.


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## petek57 (Mar 3, 2009)

*Thanks!*

Will study this before I head back to this job:thumbsup:



micromind said:


> Diodes come in all shapes and sizes. From little itsy-bitsy ones less than 0.030" diameter all the way up to gi-hugo ones more than 1" in diameter.
> 
> All diodes have to have some sort of polarity marking, this can be a simple band on one end, or an arrow symbol. If you have to replace one, polarity is very important.
> 
> ...


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## Mr.C (Dec 20, 2009)

Also if they are press packs and your testing them not installed , make sure you put them in a vice and compress them when you test them. One more note like the guy above said make sure you look at the picture on them in that welder you typically have normal and revers biased diodes that look exactly the same but are oppisite in the way the let current pass through them. the cathode of one will be threaded and on the other identical lookig one the anode will be threaded. But don't shy away from a job like this its really quite simple once its in front of you and you got your meter out.


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## mikeh32 (Feb 16, 2009)

if you are asking, then you might want to have some one else do it. its a pain in the ass to solder some of those boards. i used to design interface boards for cash registers.


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## Mr.C (Dec 20, 2009)

Try fixing this, part of the enjoyment of being an electrician is discovering new circuits and troubleshooting them, were born this way we got to find out how everything works and then figure out how to fix it, if someone left us in a room with something broke and told us don't touch that, as soon as they left the room we couldn't help but start to messing with it. Hey give it a shot you can do it welders aren't that tricky, and if you come across a diode or cap or flux time capacitor you don't have any clue about Google it, I do this all the time everyday I come across a drive or welder or processor that I have never seen before and You just go as far ad you can t/s and the get the make and model and Google if you need some more info. But yea you can fix this welder I know this because you were smart enough to ask a question, so have fun with it and make a Buck.


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## electromanj (Dec 18, 2007)

Much applause to MR C and micromind for their encoragement! Never shy away from a new learning experience! If you are smart enough to seek advice from those that know than you are smart enough to find the answer. That is how we learn.


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