[testing trival]  find a faulty diode in series diode bridge rectifer - Page 1
Forum section: electronics / projects

author: sagitis
author_rank: Newbie
timestamp: January 26, 2011, 11:49:00 am
content: yeah I came acorss a trival question here so might as well share it,** I declare this is not some stupid assignment or what. I already came up a solution but not sure is this the best way to test. I am happy that you point out my mistakes or give me hints for a better solution.The question is find a faulty diode in a full wave bridge rectifier made from a series of diodes (assume 25 on each branch in a high voltage system)while applying a voltage a diode has around 0.7voltage drop, ( total voltage drop 0.7*25 =15.5V)Thus I should apply, a resistor on the positive output and +16VDC battery across AC input and at the end of the resistor.If across one branch of the diodes does not have a voltage drop of 15.5V. There is a faulty diode in that branch.Similar technique should be applied on the negative side but positive 16VDC should be apply on the negative output.To find out the faulty diode, the best way is to test the middle voltage at the branch, if it is okay then test the upper middle voltage of the branch etc. (yeah using the number sorting methode in programming,but I don't know what is it called)The only concern I have is the break down voltage of the diode. I know that it varies for different diode but assuming on a high voltage system, those diode should be able withstand 100V (140 diodes) reverse am I correct. I don't have a IC dictionary here is, so excuse my ignorance.This is the only method I could think of , all inputs are appreciated.

author: Zad
author_rank: Super Contributor
timestamp: January 26, 2011, 04:09:25 pm
content: Yes, sounds perfectly valid and logical to me. Diodes do tend to fail open circuit, which are a whole lot easier to findP.S. Don't forget to discharge the smoothing caps.

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2011, 06:31:26 pm
content: Quote from: Zad on January 26, 2011, 04:09:25 pmAre you sure? Both open and closed circuit failure modes are common in my experience.

author: Polossatik
author_rank: Frequent Contributor
timestamp: January 26, 2011, 08:23:06 pm
content: Most bad (mainly small signal) diodes I came across where indeed open. The more they where "fried" the opener they whereNot saying this is significant enough sampleor that the same goes for more heavy duty diodes...

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2011, 08:54:07 pm
content: Quote from: polossatik on January 26, 2011, 08:23:06 pmYes heavier duty diodes are less likely to become fired open, what often happens is the fuse kicks in after they're fried enough to conduct in both directions but before they completely meltdown. The worst culprits are TRIACs and SCRs which often do this.

author: sagitis
author_rank: Newbie
timestamp: January 28, 2011, 01:09:51 am
content: thanks for the reply, this question was given by a senior technicians.He said my answer was correct but did not consider what if it is for a 500KV system whcih was the main point of the question.I searched some high voltage diode characteristic but still have no idea why my testing method is wrong. Using small voltage does not kill you anyway.