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Microwave oven protection diode

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kerogh

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Whirlpool microwave failed with shorted transformer.

I found another (older but same power) Whirlpool in working order but with a component in it that I've never seen in any other oven:
In addition to the usual capacitor/diode voltage doubler arrangement, this unit has a second (double-)diode across the capacitor terminals. It is the same shape/size/color as the other diode but has a symbol printed on it that looks like two diodes connected cathode to cathode (and the anodes going to the cap. terminals) with one of the two diode symbols being smaller than the other (1.5kV PRV) and one being 6kV PRV. The part number is 2X062H/SK5822 and it's listed as a protection diode.

Why would my broken oven (and others that I have worked on) not have this component while the oven that I scrounged the transformer from have it?
 
Microwave ovens use a fairly strange halfwave rectifier arrangement, and a consequence of this is that if the rectifier goes S/C it may not blow the fuse - and this will burn the transformer out. The purpose of the protection diode is to go S/C if the rectifier does - and this then guarantees to blow the fuse and save the transformer.

During normal working it does nothing, and older ovens didn't have them fitted.

To be honest they are a pain, they often go S/C for no particular reason.
 
Thanks Nigel.

Can you explain why the back to back arrangement and why one is rated 6kV and the other 1.5?
Just so I can sleep tonight.
 
The 2X062H contains the two diodes. Only two wires come out of it and it is connected to the two terminals of the capacitor.
 
Here's a circuit, from a Sharp R953, which shows the rectifier (down to chassis) and the protection diode (across the capacitor).
 

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That's exactly my Whirlpool's circuit.
What I can't figure out is what kind of protection that diode arrangement is supposed to provide.
 
That's exactly my Whirlpool's circuit.

I thought it would be.

What I can't figure out is what kind of protection that diode arrangement is supposed to provide.

I've already told you - if the rectifier goes short, the protection diode does as well, and blows the fuse. Without the protection diode if the rectifier fails it may kill the transformer.
 
Well, yes, Nigel. Sort of.
Why the back=to=back arrangement?
Why the different PIV ratings?

Sorry if I'm a PITA but, I'd like to understand.
 
If the diodes short then the transformer sees almost a short in one direction.
It loads the transformer to heavily and blows the fuse.

That's my guess.
 
Last edited:
I'm sorry. I misunderstood your second to last post.
When catching up on a thread I don't start reading from the beginning again. My bad.
 
Hi, been doing repairs on Microwaves for a while now and although I can test nd diagnose various faulty components I don't understand what they are there for, like the diode on the microwave.

The diode that goes from the capacitor to ground, whats it's purpose?

many thanks
 
Hi, been doing repairs on Microwaves for a while now and although I can test nd diagnose various faulty components I don't understand what they are there for, like the diode on the microwave.

The diode that goes from the capacitor to ground, whats it's purpose?

That's the rectifier - the protection diode (where fitted) is across the capacitor.

However, if you don't know what the rectifier is, you shouldn't be attempting repairs to microwave ovens, the most dangerous domestic electrical item there is.
 
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