Time Delay Fuse
Zen83237
well check the obvious. The fuse in the plud was ok but a fuse on a circuit
board inside had blown.
The assistant in Maplins said it was a time delay fuse and I had to buy a
pack of 10. The microwave is now working. �150 saved and I have 9 spare
fuses.
So to a non electronics person can somebody explain the need for a time
delay fuse?
Kevin
Andy Wade
> So to a non electronics person can somebody explain the need for a time
> delay fuse?
See http://en.wikipedia.org/wiki/Inrush_current (transformer) and
http://en.wikipedia.org/wiki/Fuse_(electrical)#Speed
--
Andy
Andrew Mawson
"Zen83237" <zen8...@zen.co.uk> wrote in message
news:4b9a799b$0$2489$db0f...@news.zen.co.uk...
It takes a while to blow so start up surges don't take it out. Usually
the fuse wire is 'slugged' with a blob of solder (placed in the
middle) that gives the wire more thermal inertia to prevent rapid
heating. Sometimes they incorporate a spring element.
AWEM
Harry Bloomfield
They are simply designed to allow an initial current surge without
blowing. Your fuse obviously had worn out by the switch on surges.
--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk
NT
Beware of microwaves like this. The fuse in question is heavy duty
enough that it doesnt normally fail without good reason. When it does
fail, a common scenario goes like this:
1. Interlock switches fail
2. this results in a short across the supply, via the fuse and a short
current limiting resistor.
3. Both fuse and resistor blow
4. User replaces fuse
5. You now have a microwave with a dangerous interlock, plus no
working interlock failure protection system.
6. Pray you dont get cooked.
NT
Zen83237
"NT" <meow...@care2.com> wrote in message
news:6519d893-46cc-447b...@e7g2000yqf.googlegroups.com...
NT
Not sure that I understand. You mean the interlock between the power and
door opening whilst the power is on. One thing that notice was that I had
inadvertantly blocked off the louvres allowing cooling air in (or hot air
out ) so may be wondered if something got a bit warm.
Kevin
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george [dicegeorge]
if you disassemble it and touch it, can it be lethal?
I half remember reading about it somewhere...
?
[g]
Harry Bloomfield
> Isnt there something dangerous inside a microwave,
> if you disassemble it and touch it, can it be lethal?
> I half remember reading about it somewhere...
There are some quite high DC voltages (and lethal) stored on the
capacitor. An high value resistor is placed across it to discharge it
quite quickly after the supply goes off, but these resistor often go
O/C - so always best to short out its terminals before venturing
fingers into the electronics.
Andrew Gabriel
I haven't looked in any modern microwaves, but old ones sometimes
use multiple interlocks. If one microswitch thinks door is open
and the other thinks it's closed, they effectively short out
to deliberately blow the fuse, as the interlock is no longer
trustworthy, and the microwave is therefore no longer safe.
You really need to know why the fuse blew. I have heard that
slamming microwave doors was a not uncommon cause, but it might
be that the door interlock really is no longer safe in yours.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
ransley
Or like mine I ran it for 10 minutes, then in about 5 seconds
restarted it, that was 10 years ago its still fine with the new fuse,
now I just wait 30 seconds between cycling it on. It depends on how
he used it, dont forgot line voltage could have been The issue. So
check line voltage if it just blew on its own, if you quicky did
another cook cycle, wait a minute next time between cycles to let it
cool, surge-startup load is greater than run load and on my hot fuse
it blew.
Alan
<and...@cucumber.demon.co.uk> wrote
>
>I haven't looked in any modern microwaves, but old ones sometimes
>use multiple interlocks. If one microswitch thinks door is open
>and the other thinks it's closed, they effectively short out
>to deliberately blow the fuse, as the interlock is no longer
>trustworthy, and the microwave is therefore no longer safe.
And if this was the scenario the replacement fuse would blow immediately
for the same reason.
--
Alan
news2009 {at} admac {dot} myzen {dot} co {dot} uk
Zen83237
"ransley" <Mark_R...@Yahoo.com> wrote in message
news:be26333d-a67e-4cf8...@q23g2000yqd.googlegroups.com...
Zen83237
"ransley" <Mark_R...@Yahoo.com> wrote in message
news:be26333d-a67e-4cf8...@q23g2000yqd.googlegroups.com...
It had been used twice in succession. It actually tripped the mcb on the
consumer unit the when I put the second item in but didn't realise that at
the time.
Kevin
Jon Fairbairn
> It happens that george [dicegeorge] formulated :
>> Isnt there something dangerous inside a microwave,
>> if you disassemble it and touch it, can it be lethal?
>> I half remember reading about it somewhere...
>
> There are some quite high DC voltages (and lethal) stored on
> the capacitor. An high value resistor is placed across it to
> discharge it quite quickly after the supply goes off, but
> these resistor often go O/C - so always best to short out
> its terminals before venturing fingers into the electronics.
But (a) don't short it out, use a resistor and (b) make sure you
actually make contact when you do this. I once had a big old
flash gun apart, and poked a resistor across the terminals, and
then a smaller one, then a wire. Then, to be absolutely sure, I
shoved a screwdriver across the terminals. That must have been
the point when I actually cut through the oxide film on the
terminals, because BANG! and about 3mm of screwdriver tip
vapourised. It gave me rather a fright, but probably better than
experiencing the current through my skin.
--
Jón Fairbairn Jon.Fa...@cl.cam.ac.uk
http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2009-01-31)
Peter Parry
wrote:
>Beware of microwaves like this. The fuse in question is heavy duty
>enough that it doesnt normally fail without good reason.
They frequently fail with time alone. They are quite highly stressed
by the high switch-on surge and the vibration caused by the door being
closed.
>When it does fail, a common scenario goes like this:
>1. Interlock switches fail
>2. this results in a short across the supply, via the fuse and a short
>current limiting resistor.
How? The interlocks are all open circuit and close when the various
safety conditions are met (such as the door being closed). The
interlocks all have to be short circuits for the oven to work so how
will one failing blow a fuse?
>3. Both fuse and resistor blow
>4. User replaces fuse
>5. You now have a microwave with a dangerous interlock, plus no
>working interlock failure protection system.
If the fuse is replaced the now open circuit mysterious resistor in
series with it will mean nothing at all will happen.
>6. Pray you dont get cooked.
Prayer has nothing to do with it. Microwave ovens are sufficiently
well designed that divine intervention is never needed.
NT
> In message <hnem0p$4d...@news.eternal-september.org>, Andrew Gabriel
> <and...@cucumber.demon.co.uk> wrote
>
>
>
> >I haven't looked in any modern microwaves, but old ones sometimes
> >use multiple interlocks. If one microswitch thinks door is open
> >and the other thinks it's closed, they effectively short out
> >to deliberately blow the fuse, as the interlock is no longer
> >trustworthy, and the microwave is therefore no longer safe.
Since the early 80s they've all used the same system, pairs of 2 way
switches that effectively cause a power line short if either one fails
to behave. Unfortunately the sc current limiting resistor seems to be
in this alternate path rather than in the main live input. Godo for
energy efficiency, but not ideal for safety, as a shorting event
normally kills this resistor.
> And if this was the scenario the replacement fuse would blow immediately
> for the same reason.
no, as I explained. It now operates on one interlock switch, and one
of the pair of switches is faulty. Not a good scenario.
On Mar 13, 11:48 am, Peter Parry <pe...@wpp.ltd.uk> wrote:
> On Fri, 12 Mar 2010 13:19:59 -0800 (PST), NT <meow2...@care2.com>
> wrote:
>
> >Beware of microwaves like this. The fuse in question is heavy duty
> >enough that it doesnt normally fail without good reason.
>
> They frequently fail with time alone. They are quite highly stressed
> by the high switch-on surge and the vibration caused by the door being
> closed.
>
> >When it does fail, a common scenario goes like this:
> >1. Interlock switches fail
> >2. this results in a short across the supply, via the fuse and a short
> >current limiting resistor.
>
> How? The interlocks are all open circuit and close when the various
> safety conditions are met (such as the door being closed).
They havent worked like that since the 70s. The safety scare in the
70s ended that method of working.
> The
> interlocks all have to be short circuits for the oven to work so how
> will one failing blow a fuse?
look at a circuit diagram for any modern non-electronic oven and it
will be evident how it works.
> >3. Both fuse and resistor blow
> >4. User replaces fuse
> >5. You now have a microwave with a dangerous interlock, plus no
> >working interlock failure protection system.
>
> If the fuse is replaced the now open circuit mysterious resistor in
> series with it will mean nothing at all will happen.
see above
> >6. Pray you dont get cooked.
>
> Prayer has nothing to do with it. Microwave ovens are sufficiently
> well designed that divine intervention is never needed.
thats true but for one weakpoint - fuse replacement. Fuse should be
replaced by someone that understands why they need to check the
resistor, and NOT fit a fuse if the resistor's gone o/c. Microwaves
are one appliance where failure can be nasty.
NT
Peter Parry
wrote:
>Since the early 80s they've all used the same system, pairs of 2 way
>switches that effectively cause a power line short if either one fails
>to behave. Unfortunately the sc current limiting resistor seems to be
>in this alternate path rather than in the main live input. Godo for
>energy efficiency, but not ideal for safety, as a shorting event
>normally kills this resistor.
I have a Sharp and Panasonic Microwave, both about 5 years old. Both
use two single pole door interlock switches both open circuit when the
door is open and one interlock monitor switch which is a normally
closed and across the mains supply when the door is open. There is no
series resistor in circuit with the interlock switch. Only if both
primary door interlocks fail closed and the oven operated with the
door open will the monitor switch cause the fuse to blow.
The monitor switch is designed to hard fail - the contacts will weld
closed if it ever operates. This ensures that in the case of failure
the door interlocks must be serviced - simply replacing the fuse will
not cause the fault to go away (remember the plug fuse may fail before
the internal fuse).
Replacing the fuse if this failure occurs will simply cause it to blow
again
>On Mar 13, 11:48 am, Peter Parry <pe...@wpp.ltd.uk> wrote:
>> How? The interlocks are all open circuit and close when the various
>> safety conditions are met (such as the door being closed).
>They havent worked like that since the 70s.
My two appear to.
>Microwaves are one appliance where failure can be nasty.
The microwave oven is probably the safest appliance in a kitchen - a
saucepan causes more injuries.
NT
> On Sat, 13 Mar 2010 04:39:37 -0800 (PST), NT <meow2...@care2.com>
> wrote:
yours also has interlock monitoring.
> >Microwaves are one appliance where failure can be nasty.
>
> The microwave oven is probably the safest appliance in a kitchen - a
> saucepan causes more injuries.
Agreed. Until an interlock switch faults and the resistor goes oc....
then the risks rise.
The OP's might be either type, like yours or like I mentioned, so we
dont know if its safe or unsafe after a fuse blow.
NT