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Repair guides and troubleshooting information for microwave ovens.

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Which type of fuse for a microwave?

I have a 1980s Samsung microwave with a blown internal fuse. Trouble is, by looks alone it could be a slow-blow or quick-blow type. Obviously, I can try both, as fuses are cheap. However, does anyone know which type it should be?

(I can imagine the stop-start nature of low power settings could result in current surges that might cause a quick-blow fuse to rupture - hence a slow-blow is specified. However, IF the circuit demands a quick-blow, I do not want to compromise our safety by using the wrong type.)

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Hi @cycleboy1957,

What is the model number of the oven?

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RE-553T, Jan 1985 model

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Thanks for the tip about the fuse code labels. I didn't know that before.

The label next to the fuse holder and the circuit diagram just say '7A'. Neither makes any mention of quick or slow blow.

The fuse has '250 VOLT' on one cap and 'BUSSMD^7' on the other, though the'^' might be an incomplete capital A. It sounds as though an ordinary 7A fuse ought to be acceptable.

The circuit diagram shows a shunt resistor across the capacitor. It's been disconnected for months now, so is surely fully discharged by now. I'm not sure if the shunt is internal to the capacitor or an external component, but I will check and be careful.

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Thanks for the tip about the fuse code labels. I didn't know that before. And thanks for searching your sources.

The label next to the fuse holder and the circuit diagram just say '7A'. Neither makes any mention of quick or slow blow.

The fuse has '250 VOLT' on one cap and 'BUSSMD^7' on the other, though the'^' might be an incomplete capital A. It sounds as though an ordinary 7A fuse ought to be acceptable.

The circuit diagram shows a shunt resistor across the capacitor. It's been disconnected for months now, so is surely fully discharged by now. I'm not sure if the shunt is internal to the capacitor or an external component, but I will check and be careful.

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Hi @cycleboy1957

It is an old model and I cannot find any information on it.

Usually most microwave oven fuses are a 20A 250V standard type fuse i.e. fast acting.

The fuse rating should be stamped on the end caps of the fuse.

For example, if it shows T20A 250V or S20A 250V then it is a slow blow fuse. T=Timed and S=Slow (depends what the manufacturer decided to use) The two designations mean the same thing.

If it just shows 20A 250V then it's a standard fuse.

Regardless of this, if the fuse has blown there's a reason for this to occur as usually fuses don't fail of their own accord.

Here's a link that may be useful, if the new fuse blows as well.

Be safety aware when working in microwave ovens. The HV capacitor can store up to 5000V DC for months even if the power has been disconnected for this length of time. This amount of voltage can seriously injure you. The HV capacitor needs to be correctly discharged as soon as it can be accessed after the oven's cover has been removed and before any further work is carried out in the oven.

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2 comentários:

Thanks for the tip about the fuse code labels. I didn't know that before. And thanks for searching your sources.

The label next to the fuse holder and the circuit diagram just say '7A'. Neither makes any mention of quick or slow blow.

The fuse has '250 VOLT' on one cap and 'BUSSMD^7' on the other, though the'^' might be an incomplete capital A. It sounds as though an ordinary 7A fuse ought to be acceptable.

The circuit diagram shows a shunt resistor across the capacitor. It's been disconnected for months now, so is surely fully discharged by now. I'm not sure if the shunt is internal to the capacitor or an external component, but I will check and be careful.

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@cycleboy1957

Looks like is it a standard 250V 7A fuse made by Bussman, so really any 250V 7A fuse having the same dimensions so as to fit in the fuse holder would do.

Normally shunts are external to the capacitor. Even so short out the cap terminals with a heavy set of pliers or a thick bladed screwdriver, just to be sure. The shunt could be open circuit, who knows?

Not a pretty sight when using too small a screwdriver. I've seen a few examples of them being welded to the cap terminals ;-)

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