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Old 03-12-2013, 05:31 PM
elengr elengr is offline
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Default half wave rectifier with inductive load

In power electronics we are taught that in negative cycle diode does not turns off in half wave rectifier with inductive load. I know that current through inductor cannot change instantaneously hence current cannot go to zero abruptly but I cannot understand why diode is not turning off.
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Old 03-12-2013, 06:27 PM
JDT JDT is offline
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Basically, it's due to the back emf of the inductor.

To put it another way: an inductor does not like its current to change. So at the end of the half cycle, the voltage across the inductor reverses and the inductor pushes its current through the diode and the transformer secondary.

This is not necessarily a problem. Often, an extra diode is placed between the output of the half-wave rectifier and the 0V rail. This is reverse biased during the conducting half-cycle but carries the inductor current during the non-conducting half-cycle. This prevents the inductor back-emf current flowing through the rectifier and transformer winding.

BTW: this is a simple way to run a DC relay coil from an AC supply. The relay does not buzz or chatter due to this back-emf current flowing through the extra diode during the non-conducting half cycle.
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Old 03-12-2013, 11:57 PM
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ErnieM ErnieM is online now
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Yep, back EMF. Basically the inductor needs to bleed off the energy stored inside it, so it's voltage goes negative to make the diode continue to conduct.

I learned this one the hard way: My first job was in switching power supplies, and a typical transformer secondary to CT diode rectifier (2 diodes) and I saw the voltage on the diode go negative. I saw the phenomena though I did not know why it did that, and never gave it much thought until it came up in an interview for another job.

I did not get that job. <grin>
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Old 03-13-2013, 02:57 AM
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It's called continuous mode operation, or something like that. It is not a fault condition.
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