Microwave fuse burnout

[Nigel Goodwin]

Hi Nigel,
One would assume that the reason for their use is cost; but I would guess that they're no less expensive than glass??
......... john

They are generally more expensive - the 'claimed' reason for their use if that they supposedly don't explode when they blow - yet in my considerable experience they are no better than normal glass fuses in that regard, and far more likely to fail for no reason.

 

[debe]

When i used to do warranty work for Sharp apliances. We were told if you sell parts to a customer dont tell them how to fit them. You may become liable if they do it wrong & try & sue.

 

[rjenkinsgb]

They are generally more expensive - the 'claimed' reason for their use if that they supposedly don't explode when they blow - yet in my considerable experience they are no better than normal glass fuses in that regard, and far more likely to fail for no reason.

In small consumer-size fuses there are glass ones that are sand filled; it may be that there are ceramic ones that are not filled, so not actually any better.

I can only guess that there are a lot of cheap low quality or "fake HRC" ones in consumer goods.

In the industrial side, I have seen a lot of cracked or burst plain glass fuses, but never a single cracked ceramic one, no matter how large the overload.

Any genuine "HRC" fuse is filled with fine silica sand or some similar material. That absorbs some of the heat generated as the wire blows and prevents a plasma arc forming between the fuse element ends as the circuit is broken.

It's the formation of an arc that causes the fuse body to crack or burst.
If a glass one blows under an extreme overload and does not shatter, it's not unusual for the glass to have a dark shiny coating afterwards - that's re-condensed metal from the fuse element, that was sustaining a plasma arc for some time.

 

[KeepItSimpleStupid]

Search for the Microwave wattage test with water. It could be the magnetron.

 

[Nigel Goodwin]

Search for the Microwave wattage test with water. It could be the magnetron.

If the mag was duff it would need to be S/C to blow the fuse, so would be faulty all the time.

The power test is simple enough to do though, you just need an accurate thermometer and a couple of calibrated beakers.

However, there's no point just to see if it's working - a far simpler 'yes/no' test is simply to place a cup of cold water in the oven, run it on full power for 1 minute - then take the cup out and place your finger in the water. The water should be warm, but not hot - nice simple test.

 

[Nigel Goodwin]

In small consumer-size fuses there are glass ones that are sand filled; it may be that there are ceramic ones that are not filled, so not actually any better.

I can only guess that there are a lot of cheap low quality or "fake HRC" ones in consumer goods.

In the industrial side, I have seen a lot of cracked or burst plain glass fuses, but never a single cracked ceramic one, no matter how large the overload.

Any genuine "HRC" fuse is filled with fine silica sand or some similar material. That absorbs some of the heat generated as the wire blows and prevents a plasma arc forming between the fuse element ends as the circuit is broken.

It's the formation of an arc that causes the fuse body to crack or burst.
If a glass one blows under an extreme overload and does not shatter, it's not unusual for the glass to have a dark shiny coating afterwards - that's re-condensed metal from the fuse element, that was sustaining a plasma arc for some time.

Microwave oven fuses are full of sand, as were the ceramic fuses fitted by Sharp in their (CRT) TV's for a number of years, both commonly blew to pieces, leaving the remains of the sand all over the board

I presume the 'industrial' fuses you refer to are a LOT better made?, and considering what some of them cost they certainly should be.

 

[Shinnen]

Hi,
The ceramic fuse I took apart contained a pure white powder. I wouldn't think that sand would be so white.
.......... john

 

[Nigel Goodwin]

Hi,
The ceramic fuse I took apart contained a pure white powder. I wouldn't think that sand would be so white.
.......... john

Are you a cocaine smuggler?

Various types of sand though are pure white - but who knows what the Chinese might put in their fuses.

 

[Shinnen]

Hi Nigel,
No, not cocaine.
White sand eh? Ok.
.......... john

 

[Shinnen]

Well gentlemen, it's now exactly one month since I replaced the fuse, and it blew again this morning.
My wife tells me that, when she opened the door, the light came on, then went off. She had not attempted to start the heating process, only opened the door. At the time, the oven had not been used for about 16 hours. So, I'm looking for guidance on how to. proceed, simplest options first. As always, any help is appreciated.
..... john
.

 

[picbits]

Try changing the bulb, it could be on it's way out. Our fridge bulb in our office took out our whole ring main when it blew.

 

[Nigel Goodwin]

Try changing the bulb, it could be on it's way out. Our fridge bulb in our office took out our whole ring main when it blew.

It certainly couldn't be 'on it's way out', bulbs don't do that - but incandescent bulbs often go S/C as they fail, this sometimes even sometimes causes the glass envelope to be destroyed, and often trips the MCB (as in your case).

For what the OP has described, it sounds like an issue with the safety interlock switches on the door - generally as you open the door two switches (in series) open and remove all power to the magnetron, a fraction of a second later a third switch closes, and this shorts out the supply to the magnetron. This is basically a crowbar system, and causes the fuse to blow if the first two switches don't open properly.

As it's a critical safety feature, it's not something an amateur should be messing with.

 

[Shinnen]

Hi Nigel,
So, that's it. Are there any tests I can perform to determine what the cause is, or if it's what you suspect? Are there any simple things I can repair/replace?
Thanks,
........ john

 

[Nigel Goodwin]

Hi Nigel,
So, that's it. Are there any tests I can perform to determine what the cause is, or if it's what you suspect? Are there any simple things I can repair/replace?
Thanks,
........ john

As an ex-microwave oven engineer, I would simply have replaced all three door switches and realigned it correctly - then I would have power and leakage tested it, as we were required to do so by regulations. Probably only one switch would be faulty, but if it's intermittent it's hard to tell which one, and the others are likely to have been 'strained' as well - so it's best to change them all.

 

[Shinnen]

Thanks again Nigel,
So, you're relatively satisfied that the door switches are the problem? And, you don't recommend that an amateur attempt the repairs? (I'm not offended. I only want your honest opinion.)
........ john

 

[Nigel Goodwin]

Thanks again Nigel,
So, you're relatively satisfied that the door switches are the problem? And, you don't recommend that an amateur attempt the repairs? (I'm not offended. I only want your honest opinion.)
........ john

Yes John, it's a crucial safety area and not something to mess with - no legitimate supplier would even sell you the parts.

 

[Shinnen]

OK. Thanks for your advise. I'll buy a new unit.
Is there any particular brand you prefer over the others?
...... john

 

[Nigel Goodwin]

I've got a 40 year old Toshiba one

Sharp are now Vestel (they bought the rights to the name off Sharp), I've no idea who makes Toshiba (their TV's are Vestel), Panasonic used to be decent - again, no idea if they still are.

It's difficult to know who makes what these days, and nothing is very well made - my Toshiba cost a LOT of money - which is probably why it's still going (I've changed the magnetron once in it).

 

[Shinnen]

OK ........ thanks again.
...... john

 

[Shinnen]

Hi Nigel,
I put another fuse in. This time it burned out in two days, same way, when my wife opened the door, the light went out. Is this consistent with your appraisal about it being the door switches?
Thanks,
...... john
P.S. Maybe the problem is my wife opening the door.