Microwave Blown Fuse Symptoms

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Like all electrical appliances, microwaves rely on fuses to operate safely. Often referred to as ‘sacrificial’ components, fuses are meant to blow when the operating conditions become unsafe, thereby protecting the microwave and whoever might be using the appliance. But why does this happen?

The reason that a microwave fuse blows depends mainly on when it happens. Suppose the fuse blows when you press Start; that means the high-voltage capacitor inside may have shorted out. But if the fuse blows when you open or close the microwave door, the cause could be a faulty door switch instead.

In this article, we’re going to explore everything you need to know about these symptoms when microwave fuses blow. We’ll also explore a few critical questions surrounding microwave fuses.

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Microwave Blowing Fuse When Press Start

Suppose your microwave fuse blows precisely when you press Start. If that’s the case, the cause may be a shorted high-voltage capacitor.

Shorted High-Voltage Capacitor

What it is: Simply put, a capacitor is an electrical component that stores or condenses electricity before releasing it into the microwave’s electrical circuit. 

In a way, a capacitor is somewhat like a battery. Still, there is one significant difference: a capacitor is designed to charge and discharge its stored power very quickly for immediate use.

A microwave typically uses a high-voltage capacitor, which, as the name suggests, operates at a much higher voltage than capacitors in other electrical appliances.

Because they store electricity, capacitors are very dangerous. Even if the microwave is unplugged from its power supply, a capacitor can still cause severe electrocution injuries.

So, be sure to safely discharge the power before doing anything or hire a qualified professional to conduct the repair for you.

Why it fails: There are several reasons a high-voltage capacitor in your microwave may short out and blow your microwave’s fuse

If the microwave is still new, the capacitor itself may be faulty. Physical damage can also cause the same, such as if the microwave has suffered a strong impact that damaged the capacitor.

In older microwaves, the capacitor may have simply worn out after many years of continued use.

When you press Start on your microwave, the high-voltage capacitor is supposed to start working. That’s not possible if it’s shorted out, which is why it causes the microwave’s fuse to blow.

How to fix: Again, please be aware that replacing a microwave’s high-voltage capacitor is very dangerous, as the capacitor holds an electrical charge even when you disconnect the power supply.

It is critical that the capacitor be discharged safely before working on any internal part of the microwave.

Suppose you hire someone to replace the capacitor for you. In that case, they’ll begin by removing any panels on the microwave to access the components inside.

They will then discharge the capacitor before unscrewing the mount that holds it in place. After disconnecting the electrical connectors, a new capacitor goes in.

The steps are reversed; the electrical connectors are reconnected, and the mount is screwed back in place.

Microwave Fuse Blows When Opening Door

In some cases, you may find that your microwave fuse blows precisely while you’re opening the appliance’s door. That’s a tell-tale sign that the door switch may have something to do with it.

Stuck Or Faulty Door Switch

What it is: The door switch that you find on a microwave isn’t the same as those on other appliances. Microwave switches serve several purposes, each of them incredibly crucial to the safe operation of the appliance.

Firstly, the door switch ensures that the microwave only functions when the door is closed. That prevents anyone from being exposed to any dangerous microwaves when they’re near the appliance.

Besides that, the door switch also delivers power to electrical components in the microwave, which heat up the food that you’ve placed inside the appliance.

Why it fails: As with many other microwave components, there are several reasons why the door switch may fail. For starters, the door switch may have just gotten stuck. 

Remember: switches open to allow power to flow through the circuit, and they also close to stop power from flowing through.

A door switch that gets stuck will fail to do what it’s supposed to do, leading to a fuse being blown when the door is opened.

The same may also happen with a faulty or worn out door switch. A switch in this condition will not be able to do what it’s supposed to do, leading to a fuse blowing as a safety measure to protect the appliance and its users.

Lastly, a problem with the microwave door switch may also happen if you’ve just fixed or replaced them.

If that’s the case, you or the technician may have accidentally wired the switches wrongly, causing them to malfunction whenever you open the microwave door.

How to fix: Microwave door switches are fairly straightforward to replace. However, microwaves are still quite dangerous because they rely on high-voltage capacitors, as mentioned earlier.

So, if you or a technician removes the microwave’s panels to gain access to its components, bear in mind that there is a high risk of injury from electrocution.

It’s always best to refer to the microwave’s instruction manual to understand the inner layout of the appliance. That way, you’ll know what to look for once you’ve gained access to the components inside.

Replacing the microwave door switch will usually involve removing a front panel.

You may need to unscrew the housing that holds the door switch in place and remove its electrical connectors.

Once the new switch has been put in, the connectors must go in the correct way, or else it will cause another blown fuse.

To ensure that you install the replacement switch and reattach the electrical connectors correctly, here’s a useful tip.

Take a photo of the faulty door switch and its connectors before removing it.

By doing that, you’ll have a picture that you can use as a reference to ensure that you install the replacement correctly, just the same way as before.

Microwave Fuse Blows When Closing Door

Suppose your microwave fuse blows when you’re closing the door instead of opening it.

If that’s the case, then the cause is the same: you may have a stuck or faulty door switch.

You see, whether you’re opening or closing the microwave door, the component that reacts to that action is the microwave door switch. So, that is most likely the cause of the problem.

How Many Fuses Are In A Microwave?

Microwaves typically have several fuses inside, and they also tend to be of different types.

Depending on the make and model of your particular microwave, you may find fuses placed on top, behind, or even by the side parts of the unit.

The best way to identify the quantity and locations of the fuses is to refer to your user manual and any technical documents related to the model you have.

Typically, these documents will map out the entire electrical circuit of the microwave, so you know where to find each fuse.

Microwaves use two types of fuses: ceramic and glass fuses. They’re sacrificial components that are meant to get destroyed as a way of cutting the circuit to protect the appliance and its users.

Some microwave fuses called ‘thermal fuses’ are triggered by heat. So, if the microwave overheats, the fuse will blow and shut the power off to the microwave.

Other fuses are triggered by excessive electricity flow through the circuit. A short circuit, for instance, will cause that fuse to blow.

How To Test Microwave Thermal Fuse

Remember: a thermal fuse will blow if the microwave overheats. If you’d like to see if the fuse is still working, you’ll need a multimeter to measure for continuity in that fuse.

Start by making sure that the multimeter is calibrated and set it to the lowest ohm rating, regardless of whether it’s a digital or analogue multimeter.

Remove the fuse from the microwave and place one of the multimeter’s probes on each terminal of the fuse.

If the meter reading shows no resistance, that means the fuse has continuity (i.e. it still works).

But if the meter shows that there is no continuity, that means the thermal fuse has blown. You’ll need to find an appropriate replacement for that thermal fuse.

Can I Use Any 20 Amp Fuse For My Microwave?

No, you cannot simply use any 20 Amp fuse to replace a blown fuse inside your microwave. The key here is to pay attention to the type of 20 Amp fuse that needs replacing. 

For the microwave you need to use 20A SLOW BLOW Fuse, not Fast Blow.

For example, you can use a ceramic 20 Amp fuse to replace a glass one, but NOT vice-versa.

If you’re unsure, it’s always best to play it safe. In this case, that means referring to the user manual for your microwave or contacting the manufacturer.

By using the specific type of fuse that the manufacturer recommends, you’ll avoid any uncertainty about whether or not the fuse is safe to use for your microwave.

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