Microwave failure modes - and fuse types
Roger Mills
believe to be about 10 years old.
The microwave section has just failed, although the grill function still
works.
On investigation, I found a blown fuse behind the control panel. It's a
glass fuse - 20mm long x 5mm dia - and is rated at 250v 10A.
The actual marking says M10D. The M apparently means medium speed to
blow - somewhere between an F (fast blow) and T (slow blow - time
delay). I've no idea what the D signifies.
Two questions:
1. Since *something* must have caused the fuse to blow, what's the
likelihood that replacing the fuse will fix it? [Fuses *can* die of old
age, but what else may have caused it?]
2. Since I can't find an identical use, what's the best type to use?
Maplins have T10's and also some ceramic jobbies - apparently intended
for microwaves.
--
Cheers,
Roger
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Please reply to Newsgroup. Whilst email address is valid, it is seldom
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NT
> In my holiday flat there's a Neff built-in combi-microwave which I
> believe to be about 10 years old.
>
> The microwave section has just failed, although the grill function still
> works.
>
> On investigation, I found a blown fuse behind the control panel. It's a
> glass fuse - 20mm long x 5mm dia - and is rated at 250v 10A.
>
> The actual marking says M10D. The M apparently means medium speed to
> blow - somewhere between an F (fast blow) and T (slow blow - time
> delay). I've no idea what the D signifies.
>
> Two questions:
>
> 1. Since *something* must have caused the fuse to blow, what's the
> likelihood that replacing the fuse will fix it? [Fuses *can* die of old
> age, but what else may have caused it?]
>
> 2. Since I can't find an identical use, what's the best type to use?
> Maplins have T10's and also some ceramic jobbies - apparently intended
> for microwaves.
Something's not right with this picture. A microwave shuold not use a
glass fuse, they need the rupturing capacity of a sand filled ceramic
fuse.
NT
Roger Mills
It's possible that the fuse which blew was not the original - I haven't
owned the device from new. But the markings on the PCB next to the
fuse-holder say F1[1] and M10.
What are the special characteristics of a sand filled ceramic fuse? I
can certainly get some from Maplin which would fit - but would they fail
to provide adequate protection in some other way?
[1] Presumably Fuse 1 - there's another 1 amp stripy fuse labelled F2,
which *didn't* blow
NT
Sand filled ceramics have much higher rupturing capacity than glass
fuses. And the consequence of failure to blow isnt just electrical,
but potentially severe internal burns of the end user.
NT
Arfa Daily
"Roger Mills" <watt....@gmail.com> wrote in message
news:99af8n...@mid.individual.net...
I've seen glass fuses on the mains input of microwaves, although equally,
ceramics are also quite common. Check to make sure that nothing has happened
to the door interlock switch mechanism. One of the switches places a short
across the output of the other one when the door is opened. It's a sort of
'belt and braces' thing. One switch opens to isolate the supply when the
door opens, and the other closes to place a short across the open one, just
in case it hasn't opened. The fuse-blowing situation can occur if the switch
contacts on either one weld themselves up, or if a switch comes loose on its
bracket, or the mech that works it breaks. You would be ok with a T rated
fuse. If you want to put a ceramic in, you should be able to get a T10A
without too much problem.
Arfa
chudford
> "Roger Mills" <watt.ty...@gmail.com> wrote in message
>
> - Show quoted text -
Not the fuse the OP is describing but there is usually a glass fuse on
the HV side of the microwave.
My microwave has just failed and I found this fuse had blown.
I replaced it with a fuse off Ebay (there are lots to choose from).
Unfortunately the problem is more severe than a blown fuse as the
replacement also blew.
So I guess the is a fault with the magnetron circuit.
When time permits I will be stripping the microwave down to see what
failed out of interest.
A word of warning, don't fiddle around in the microwave circuits
unless you know which bits not to go near.
Geo
wrote:
>
>On investigation, I found a blown fuse behind the control panel. It's a
>glass fuse - 20mm long x 5mm dia - and is rated at 250v 10A.
Since the fuse is/was glass, was the glass blackened or sprayed with
metal internally?
If so then "something" has blown it and the replacement you buy will
blow as well.
If not then it /may/ have been just tired of life.
Roger Mills
>
> Sand filled ceramics have much higher rupturing capacity than glass
> fuses.
Sorry, I'm not sure I understand the term "rupturing capacity". Are you
talking about the physical disintegration of the glass (or whatever)
tube - or simply about the overload current which can be sustained
before it blows?
And the consequence of failure to blow isnt just electrical,
> but potentially severe internal burns of the end user.
>
I'm not sure I understand that either! What circumstances do you have in
mind?
docholliday
> On 27/07/2011 15:36, NT wrote:
>
>
>
> > Sand filled ceramics have much higher rupturing capacity than glass
> > fuses.
>
> Sorry, I'm not sure I understand the term "rupturing capacity". Are you
> talking about the physical disintegration of the glass (or whatever)
> tube - or simply about the overload current which can be sustained
> before it blows?
>
circuit may be many times higher than the rated capacity of the fuse
(which is the maximum current it can pass indefinitely), and the
rupturing capacity is the current the fuse can safely break. When the
fuse blows there will be a momentary arc between the ends of the fuse
element, and for higher rupturing capacity fuses the sand fill helps
to extinguish this safely. As an indication, the standard BS1362 fuses
used in UK 13 Amp plugs have a rupturing capacity of 6000A, while
glass fuses can only break currents of the order of 100A.
There's lots more at
http://wiki.diyfaq.org.uk/index.php?title=Fuse.
Mike
Andy Dingley
> What are the special characteristics of a sand filled ceramic fuse?
High rupturing capacity - the ability to interrupt a large fault
current and isolate it quickly, rather than allowing an arc to be
sustained across the ends of the melted wire. Mostly an issue for high
voltage or inductive loads - for a 20mm fuse, inductive loads, such as
a big transformer, are the likely reason.
NT
> On 27/07/2011 15:36, NT wrote:
>
>
>
> > Sand filled ceramics have much higher rupturing capacity than glass
> > fuses.
>
> Sorry, I'm not sure I understand the term "rupturing capacity". Are you
> talking about the physical disintegration of the glass (or whatever)
> tube - or simply about the overload current which can be sustained
> before it blows?
neither. If fault current is above the rupturing capacity of the fuse,
the fuse cant break the circuit.
> And the consequence of failure to blow isnt just electrical,
>
> > but potentially severe internal burns of the end user.
>
> I'm not sure I understand that either! What circumstances do you have in
> mind?
Interlock failure causes fuse blows. If the fuse doesnt blow, then a
faulty interlock means sometimes the thing will cook with the door
open. Unless you want to be food, thats not a good idea.
If the main fuse blows, always check the shorting resistor. Sometimes
that also fails, and sometimes that means no working protection
circuitry, but it still cooks.
NT
Peter Parry
wrote:
>1. Since *something* must have caused the fuse to blow, what's the
>likelihood that replacing the fuse will fix it?
Quite good, certainly worth a try. One other failure mode which can
occur and will blow the fuse is that the door micro switches fail -
not usually difficult to diagnose or fix.
>2. Since I can't find an identical use, what's the best type to use?
>Maplins have T10's and also some ceramic jobbies - apparently intended
>for microwaves.
Any slow blow or time fuse is fine. Even if the fuse ruptures the
microwave case will contain the glass fragments.
Roger Mills
It's blackened for about 80% of the length of the visible glass envelope
- suggesting a rapid overload.
Other posts are suggesting that this could possibly be caused by a
problem with the interlock switches on the door.
I will investigate next time I'm at the flat. The only problem is that
it's mounted quite up in a built-in unit - above a conventional oven -
and it's bl**dy heavy!
Graham.
"Peter Parry" <pe...@wpp.ltd.uk> wrote in message news:ps3137l5hgsj5fl6p...@4ax.com...
As the glass fuse did not vaporise and was only partially blackened I suspect the door
microswitches did not crowbar the supply (but if it tripped the MCB as well then I wouldn't be so sure)
I think it is more likely to be, in order of likleynes:
HV diode short-circuit
HV capacitor short-circuit
Magnetron cathode (filament) to anode (chassis earth) short circuit.
Generally speaking a microwave can run for a long time with any of the
above faults without blowing the mains fuse, because the transformer is run
at near saturation so there isn't as great an increase in primary current as you
might imagine.
Apart from not heating the food the only symptom is a slightly louder buzz from
the tranny than normal.
Microwave ovens are one of the easiest of domestic appliances to fault-find and repair IIHO,
(Panasonic inverter models exempted!)
*Unplug* from mains and *discharge* the big HV capacitor with a couple of well insulated screwdrivers
before delving inside, I can't emphasise those enough. Use a multimeter on ohms range to test the components
above.
--
Graham.
%Profound_observation%
Arfa Daily
"Graham." <m...@privacy.com> wrote in message
news:j0v6jq$gc4$1...@dont-email.me...
Understanding, of course, that a conventional multimeter is unlikely to
produce enough voltage on any of its ohms ranges, to turn on the HV diode,
so this device is likely to read open circuit, even if it's ok ...
Agreed though, that a short circuit diode will be picked up ok.
Arfa
Windmill
> *Unplug* from mains and *discharge* the big HV capacitor with a couple
> of well insulated screwdrivers before delving inside, I can't
> emphasise those enough. Use a multimeter on ohms range to test the
> components above.
But bear in mind that the diode is likely to be actually about 10
diodes in series, so if it's OK it won't conduct in either direction
unless the multimeter applies a voltage of more than maybe 6 volts.
That fooled me once into thinking that the diode was open-circuit.
(The meter, not my own, had only a 1.5 volt battery.)
--
Windmill, Use t m i l l
Til...@Nonetel.com @ O n e t e l
. c o m
Mr. Benn
"Arfa Daily" <arfa....@ntlworld.com> wrote in message
news:eaIYp.48346$Z04....@newsfe07.ams2...
The HV is the reverse voltage that the diode is rated at, not forward
voltage, so you can still check it with a multimeter.
Graham.
"Mr. Benn" <inv...@invalid.invalid> wrote in message news:j1r3k4$ogd$1...@dont-email.me...
The depletion voltage required to forward bias the diode is going to be many times
higher than the typical 0.7v for a single silicon junction.
--
Graham.
%Profound_observation%
Graham.
Vf is 12 volts on this diode
http://pdf1.alldatasheet.com/datasheet-pdf/view/149585/ETC1/HVR-1X-01A.html
so a popular multimeter with a PP3 battery won't forward bias it.
An old Avo 8 on the other hand might just do it.
I think they go very leaky rather than OC
--
Graham.
%Profound_observation%
Roger Mills
> In my holiday flat there's a Neff built-in combi-microwave which I
> believe to be about 10 years old.
>
> The microwave section has just failed, although the grill function still
> works.
>
> On investigation, I found a blown fuse behind the control panel. It's a
> glass fuse - 20mm long x 5mm dia - and is rated at 250v 10A.
>
> The actual marking says M10D. The M apparently means medium speed to
> blow - somewhere between an F (fast blow) and T (slow blow - time
> delay). I've no idea what the D signifies.
>
> Two questions:
>
> 1. Since *something* must have caused the fuse to blow, what's the
> likelihood that replacing the fuse will fix it? [Fuses *can* die of old
> age, but what else may have caused it?]
>
> 2. Since I can't find an identical use, what's the best type to use?
> Maplins have T10's and also some ceramic jobbies - apparently intended
> for microwaves.
I'm now back at the flat, and have done the following:
1. Replaced the blown fuse with a 10A ceramic jobbie.
The result was that the panel lit up again, and I could open and close
the door without problem.
However when I tried to operate the microwave (on its second from
highest setting) the internal light came on as normal, it made
encouraging noises (albeit a bit louder than usual) for a few seconds
and then the fuse blew again.
2. I then tried a T10 glass fuse.
The panel lit up again and opening and closing the door was again ok.
This time, I tried operating the microwave on its lowest setting - and
it ran for the time I had set (about 15 seconds) without problems. I
then tried it on its second from lowest setting, and the fuse
immediately blew!
Does this additional information help to narrow down the likely
underlying problem?
ARWadsworth
Time to take the wife shopping for a new one:-)?
--
Adam
Roger Mills
Possibly. Problem is that it's a built-in combi microwave (which are
ridiculously expensive compared with free-standing ones[1], it's not a
current model (about 11 years old, at a guess) and there's a matching
single oven - so a new one ain't quite going to match, even if it fits
the space! It would be better to mend it if it doesn't cost the earth.
[1] A Neff built-in combi is of the order of £400, whereas Lidl have got
a free-standing microwave with grill on offer at £40 on Thursday!
BigWallop
> On 27/07/2011 10:35, Roger Mills wrote:
> > In my holiday flat there's a Neff built-in combi-microwave which I
>
> The panel lit up again and opening and closing the door was again ok.
> This time, I tried operating the microwave on its lowest setting - and
> it ran for the time I had set (about 15 seconds) without problems. I
> then tried it on its second from lowest setting, and the fuse
> immediately blew!
>
> Does this additional information help to narrow down the likely
> underlying problem?
>
> Roger
>
Start with a word of warning. Never operate a microwave oven while it is
empty. Always have, at least, a half cup of water inside. Also, unless
stated on the front of the oven, never put metal inside the oven. Not even
crockery (plates, cups, saucers etc.) with metallic design features.
Another very common microwave oven killer, is the frozen pack of Sunday
Bacon, which often contains metal foil in the packaging and is thrown in the
micro' to defrost.
Microwave Ovens don't really have a low, medium or high setting, they only
have "ON" or "OFF" periods for set times, depending on which option you
choose. Setting the oven on low, and only running it for a short time set,
may not have been long enough to show a fault.
Example:
High setting starts the oven immediately, and remains on until the set time
expires.
Medium starts the oven immediately, but only runs for, let's say, 10
seconds, then it switches off for a few seconds, then back on again, until
the set time runs down.
Low setting may not start the heating cycle for a few seconds, then it will
cycle the on / off periods even more frequently, thus giving the food only
short blasts of energy at a time, rather than constantly like the high
setting would.
My advice would be. Unplug the oven from the mains. Look inside at the
walls, roof and around the base for any small scorch marks left by arcing
metal which may have inadvertently been placed in the oven. The marks could
be as small as a group of black dots, like ink dawbs from a ball-point pen.
If you do see marks like that, then get a new oven. The wave guide has
probably been blackened and will, more than likely, not be repairable.
Replacing the whole 'innards will be more expensive than a new oven.
If you don't see marks on the inside of the oven, and make sure you have
checked it thoroughly, then a repair, to something simple like the timer or
control circuit, may not be an expensive option. Choose a reputable
engineer for these types of repair, as a bodge can seriously damage health
and damage property.
Who knows, it could be something silly like beef fat dripped across the
turntable motor. But I am not going advise that you take this type of
appliance apart by yourself. They contain large amounts of stored energy,
even when unplugged and lying dormant for years, and can kill very easily.
So get the Yellow Pages out.
Good luck with it.
--- Posted via news://freenews.netfront.net/ - Complaints to ne...@netfront.net ---
NT
> "Roger Mills" <watt.ty...@gmail.com> wrote in message
Sorry but most of this is not good advice. The HV capacitor certainly
can kill though.
NT
whisky-dave
Seemed quite reasonable to me, although saying HV capacitors can kill
is rather misleading.
Cars can kill too, but it's the charge on the capacitor that can kill
not the capacitor itself
well unless swallowed or inserted somewhere it really shouldn't be ;-)
Andy Burns
> Seemed quite reasonable to me, although saying HV capacitors can kill
> is rather misleading.
I think the point is they can kill someone who is unaware of the charge
they (can) hold even when switched off
> Cars can kill too, but it's the charge on the capacitor that can kill
> not the capacitor itself
It's usually the momentum of a car that kills, not the car itself.
NT
Probaly the biggest shortcoming of this group is the tendency of
people with no electronic expertise to give advice on the susject.
Telling people never to operate an oven empty is a classic, it only
ever applied to the very first microwave oven model many decades ago.
NT
m...@privacy.net
whisky-dave <whisk...@gmail.com> wrote:
> Seemed quite reasonable to me, although saying HV capacitors can kill
> is rather misleading.
Can easily be lethal to the unaware!
Metal foil is often used in microwaves to shield areas where the food is
thinner to avoid overcooking those parts. Generally metal isn't a problem as
long as not resonant and close to other metal parts, resulting in sparking. I
regularly leave the spoon in when making gravy without a problem.
The majority of microwaves do pulse for lower than full power, however, my
last two have used inverters and have reduced the power to the magnetron in
the same way yhat switched mode power supplies regulate.
The failure of the first of my inverter microwaves was due to my forgetting
to add the milk/water to my porridge. It never recovered from the resultant
fire.
It is important that some load that will absorb the microwaves is present in
the oven cavity. This is almost without exception water in a liquid form
(rather than as ice which doiesn't absorb microwaes), hence adding a teaspoon
of water to frozen veg before cooking.
--
B Thumbs
Change lycos to yahoo to reply
Andrew Gabriel
<m...@privacy.net> writes:
>
> The failure of the first of my inverter microwaves was due to my forgetting
> to add the milk/water to my porridge. It never recovered from the resultant
> fire.
For a few weeks, I had no kitchen due to ripping out and fitting a
new one. I moved one of the floor-standing cupboards into the dining
room, and stood the microwave, kettle, and toaster on it, and that
was my kitchen, plus the fridge-freezer.
I bought a frozen multi-pack of microwave chips, which I had a
few of. However, a year or two later, I found the last one still
in the freezer. Of course, it was well past its best-before date,
but what can go wrong with chips? Popped them into the microwave,
and switched on. Within a few seconds, they explode into flames.
Manage to get them out of the microwave quickly, and only damage
was an internal coating of soot, and a burning smell which took
a couple of weeks to leave the house.
On closer inspection, they had completely freeze-dried, leaving
basically a mixture of starch powder and oil. Hence the fire.
I did wonder if "best-before" was perhaps understating things.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]