Sharp Combi Microwave and Strange Diode

[Crabber]

Hi,

I have a combination type Sharp microwave however the microwave function has failed.

I'm new to electronics but have read elsewhere on this forum about a popular fault with Sharp microwaves being a strange protective 'diode' that is positioned across the terminals of the large capacitor together with a second tradionally labelled diode (HVR-1X7) which is connected positive side to one side of the capacitor with the negative side to earth. The strange 'diode' is marked as if it is actually comprised of 2 diodes connected together negative to negative and is labelled 2X062H.

The tradional diode has been tested and appears to work correctly i.e. 7.5v across it when connected to a 12v supply with 220Ω resistor in series.

I have purchased and fitted a replacement 2X062H diode but the microwave still doesn't work. This diode however gives different test results to the original part which behaves under test as a tradional diode i.e. 5.5v across it when connected as above. The replacement 'diode' shows 12.4v in both directions when tested as above as do the 3 other spares I have bought.

How do these diodes work and any suggestions as to the problem with my microwave?

Kind regards,

John

 

[Nigel Goodwin]

I'm a bit bemused why you're checking it with 12V - as it reads 12.4V both ways, presumably that's not enough to make it conduct? - but a protection diode shouldn't conduct, it simply goes S/C under the specific fault conditions that cause it to blow.

However, it's NOT a functioning part of a microwave, and the oven will work fine without it. It's 'function' is to go S/C if the HV rectifier fails, and thus cause the fuse to blow - often the HV rectifier going S/C doesn't blow the fuse, and this can kill the transformer.

I would suggest though that you don't meddle with Microwave ovens - they are VERY, VERY dangerous devices, and can easily kill you.

 

[Crabber]

Hi Nigel and thank you for your reply.
Perhaps I didn't make myself clear enough. The HV rectifier does not appear to have failed as it has 7.5v across it in one direction with my 12v (or more accurately 12.4v) supply and 12.4v in the other direction. Certainly not S/C. The new protective 'diode' appears to work fine as it will not conduct in either direction as the symbol printed upon it would suggest yet the original part does conduct in one direction with 12v (12.4v).

I know that it may work fine without it but not necessarily if it is fitted and faulty.

I note your 14th November 2008 response to a similar query "It's a special diode, created this this specific purpose - no idea how it works, and care even less "

I'll assume that you still don't know and perhaps we should both steer clear of microwaves!!

Has anyone else any ideas?

 

[Crabber]

I think I have found the answer to my question and one that has been raised on this forum previously.

From a posting by Carl Willis who has posted the below on fusor.net forum.

This kind of component is effectively two diodes in series, one with a high reverse breakdown voltage rating and the other with a much lower rating. It is placed in parallel with the HV capacitor in the usual level-shifted magnetron power supply (which uses a separate rectifier in series with the capacitor).

Its purpose in that configuration is to allow the capacitor to charge to a high voltage of the correct polarity only (negative at the magnetron cathode), which is blocked by the high-breakdown diode in the back-to-back arrangement. If the capacitor experiences high voltage of the opposite polarity, the weaker diode in the pair fails. I don't know precisely what condition is being protected against, but my guess is that a high reactive current circulating through the transformer-capacitor pursuant to a failed main rectifier or gassed-out maggie could cause one or both of these components to not "go gentle into that good night." The cap could overheat and burst in a shower of oil, presumably identified as a worse outcome than the need to swap a couple of semiconductors. Or the transformer secondary could melt down before the core thermal protector opens, ruining the component with the highest commodity value. Engineering of a cheap Chinese appliance is all about pushing the envelope and trimming the margins of safe failure.

In any case, I suspect the "protection diode" is only designed to carry current in the permanently-failed state, making it not worth much to you.

In my case this bidirectional diode failed in one direction (as it is supposed to) together with a secondary fuse.

I knew the answer would be out there somewhere.

Many thanks.