The microwave packed up last night. Before dumping it I thought I might as 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
See
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
Zen83237 formulated on Friday :
They are simply designed to allow an initial current surge without blowing. Your fuse obviously had worn out by the switch on surges.
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:
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:
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
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... ? [g]
george [dicegeorge] wibbled on Friday 12 March 2010 21:42
Yes - a big capacitor with many kV.
Although on the last microwave I disassembled, the capacitor did have secondary shielding, caution is always advised when removing the case of an unknown microwave oven.
It happens that george [dicegeorge] formulated :
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.
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.
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.
In message , Andrew Gabriel wrote
And if this was the scenario the replacement fuse would blow immediately for the same reason.
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.
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.
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
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.
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.
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?
If the fuse is replaced the now open circuit mysterious resistor in series with it will mean nothing at all will happen.
Prayer has nothing to do with it. Microwave ovens are sufficiently well designed that divine intervention is never needed.
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.
no, as I explained. It now operates on one interlock switch, and one of the pair of switches is faulty. Not a good scenario.
They havent worked like that since the 70s. The safety scare in the
70s ended that method of working.look at a circuit diagram for any modern non-electronic oven and it will be evident how it works.
see above
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
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
My two appear to.
The microwave oven is probably the safest appliance in a kitchen - a saucepan causes more injuries.
yours also has interlock monitoring.
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
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