# diodes in parallel

Discussion in 'Homework Help' started by electromaniac89, Apr 20, 2011.

1. ### electromaniac89 Thread Starter New Member

Apr 20, 2011
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2 DIODES ARE CONNECTED IN PARALLEL AND THE INPUT TO THIS CIRCUIT IS A CONSTANT CURRENT SOURCE. What is the reason for overheating of the diode?

2. ### beenthere Retired Moderator

Apr 20, 2004
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Dec 26, 2010
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One could try to guess the sense of the original question, which may well have something to do with poor current sharing between diodes in parallel (current "hogging"). On the other hand, wouldn't it be better if the question were explained more fully, so that we do not have to engage in such detective work?

I'm tired of seeing so many cryptic fragments of questions posted like this. Why oh why can't people take the trouble to set out their problems in a few clear sentences? Surely it cannot just be a question of English as a second language. I am left to wonder whether this reticence is born of laziness, or a desire to avoid putting the question more fully so that teachers etc. will not be able to detect them on the web.

4. ### electromaniac89 Thread Starter New Member

Apr 20, 2011
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Ok Mr Principal (Adjuster) if you cant answer the question that is fine

Keep going again Can see that you had a great day and wants to vent your spleen on the forum. Shame on you

5. ### electromaniac89 Thread Starter New Member

Apr 20, 2011
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Hi beenthere,

Making it more precise, the diodes are connected in parallel . To this combination the input is a constant current source. I understand due to diode mismatch, one diode gets turned on faster than the other, hence overheats...thinking logically...But cant think of the reason in terms of physics why the diode heats up

Dec 26, 2010
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Passing a forward current I through a diode results in a voltage drop Vf. There is therefore a power loss I*Vf, which turns up as heat in the diode. Unfortunately, the diode Vf for a given current is not identical even for devices of the same type, and to make matters worse it can be expected to reduce with increasing temperature.

Diodes need to be very well matched to operate in parallel, as otherwise the current will not share well. The relatively sharp current/voltage characteristic typical of a diode means that differences in Vf at a given current of the order of millivolts can turn up as significant current imbalances when the diodes are in parallel.

The diode(s) getting the greater share of the current tend to heat more than their neighbours, leading to a further reduction in their forward voltage, so that they take even more of the total current. In unfavourable circumstances this may result in overheating.

7. ### Georacer Moderator

Nov 25, 2009
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Adjuster, If I were you I wouldn't bother answering to mr. Too-Good-To-Listen-To-Advice here. We don't owe answers to anybody and if they don't like our rules they can go play elsewhere. For a moment I thought of answering electromaniac89's question, but then, after re-reading his answer to you I felt I shouldn't even write two words.

@ electromaniac89
When a Super Moderator and a Senior Member ask you for more information and point you to the right direction of asking help, you 'd better listen closer.