Part Identification: Fuse and Varistor from a PSU

Power Circuit Protection
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Hi TechForum/DigiKey,

On the weekends, I do repairs of the small electronics in my home among other tasks. During the week, I have a degree in physics and do scientific research at the local university. So, I have some background with this sort of work, but it is not my primary focus.

I am currently trying to fix a power supply. There is normally an LED that indicates the status of the power supply. Orange for standby, Green for ON, and Red for malfunction.

Currently, when plugged in, the LED does not turn on at all. I looked around online to see if anyone else had posted resources on this particular repair. From what I read, there is a fuse and a varistor that can go bad at the front end of the unit.

I found the fuse. It has no continuity. So, it definitely needs to be replaced. The varistor, I didn’t know how to test… However, I want to try replacing it too.

I thought about bridging the fuse to see if the power supply was fine otherwise. It seemed dangerous though. If something else is wrong with the circuit. I’d rather the fuse blow out again than something else…

Anyway, these are the two parts I want to try replacing.
The fuse product number is T6.3A 250V CQ MST (then a bunch of symbols I don’t understand)
The varistor part number is 14N511K (then a bunch of symbols I don’t understand).

I found two parts on the website which look similar to what I want…

The listing for the fuse looks very similar to my part. However, the listing for the varistor is a different color (teal vs. yellow). It is also named 14D vs. 14N. (I’m a bit inexperienced with this sort of thing and I don’t know what that means precisely. Are these letters code for something?)

I’m afraid of putting in the wrong part and then something sparks or smokes. Could someone please help me?

Here are some photos of the parts in question. You might need to zoom-in to read the labels. (Hotkey: control+mousewheel)

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20240413_153825
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Much appreciation,
Allison

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Don’t have time this long weekend to read this in detail (I’m in MA so it’s Patriots’ Day weekend).

However this tip may prove helpful.

Varistors almost always serve a single purpose, to clamp large voltage spikes to a safe level. It is virtually always safe to simply remove the varistor from the circuit and get everything working without it in place.

Just don’t put the device back into normal service where it will see large power spikes before you attend to the varistor.

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Thank you for taking the time to respond during your long weekend.

So, it sounds like the varistor acts almost like a secondary fuse. As long as I’m not anticipating there being any voltage spikes in my system. This should be okay for testing if the rest of the circuit is okay?

My main questions is this one? What is the difference between a varistor 14N511K and 14D511K? Does the letter come down to the manufacturing technique? From what I understand, varistors are made from sintering grains of ceramic together.

I think the video I saw was this one. The engineer bridges the two points connected by the fuse with a clamp. Then, the two 150V capicators exploded.

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Hello,
Welcome to the DigiKey tech forum. The fuse you selected, Digikey part 4598-MST6.3A250VCT-ND looks to be a good match. The varistor , DigiKey part 118-MOV-14D511K-ND also looks to be a good match, the markings usually break down as 14 (diameter in mm) , 511 (varistor voltage 51 with 1 as the multiplier so 51 with 1 zero added or 510) and K as the voltage tolerance of 10% . Color has no relevance to the functioning characteristics of the MOV.

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Hello Allison,

We do not carry the 14N511K part, so I searched for a datasheet on-line. The one I found said the N was for Different Internal Structure. When I checked the 14D511K part, it said the 14D was for the 14mm Diameter. Maybe someone else can add to this, that knows a lot more about them.

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Hi Allison,
Welcome to DigiKey tech forum.

I found a couple data sheets online that seem to somewhat depict what that N calls out.

and

They both seem to call out the same body composition difference that the N calls out, howeve neither has an option for a D call out.

The data sheet for the D call out we have is https://www.bourns.com/docs/Product-Datasheets/MOV14D.pdf.

It seems to be the composition differance if you review the data sheets. Those marked with N seem to be older generation types. The 14 on them also meant the diameter from what I found out.

Hope that helps.

PS(EDITED): Given that these marked with N are not from vendors we carry we cannot fully verify the meaning of the N marking. The data sheets I linked are as close as we can presume.

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Thank you all for the information. I’ve found it edifying.

It sounds like the D has a different material composition. However, it shouldn’t be a problem if it has the same voltage characteristics. If the technique is newer, maybe it was to address the problem which resulted in the Ns failure.

I will try ordering the parts and see how it goes. I might post here later to say if it worked or didn’t work and what happened.

Kind regards to everyone!

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I believe you got the gist of it.

However a varitor is almost nothing like a fuse, their only similarity is that they are both in the broad category of circuit protection devices that includes gas discharge tubes, and safety grounds.

A fuse is a series connected device that creates an open circuit when the current exceeds it’s rating.

A varistor is a parallel connected device that creates a short circuit when the voltage exceeds it’s rating.

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Oh, that makes sense. Thank you for that insight. I’m interested in learning everything I can about well everything I can. I love this sort of stuff.

So, when the varistor has not been tripped, it has a high resistance and not much current flows through that pathway. However, when it encounters a rapid voltage spike, it triggers and creates a short circuit that diverts the current down a path that won’t hurt the rest of the circuit.

It’s like a divertor channel that can be opened to pull water away from a dam. It removes water which has built up too much and could cause structural damage.

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Close, but not quite. It doesn’t create a true short circuit, but rather it tries to limit the voltage by shunting much of the current to ground during a surge. The voltage across the device during a voltage surge event will still rise, but not by nearly as much as if it were not present. The larger the body, the more energy it can handle without catastrophic damage.

There is a different type of voltage surge protection device based on silicon that does act more like a short circuit. It is a thyristor type structure (similar to a transistor, but with some important differences) which acts like a “crowbar device”. They go by various names including "Sidactor, “Transil”, “Bi-directional Thyristor”, amongst other names. In normal operation, these devices are in a high impedance state. Once the voltage exceeds a defined break-over voltage, they trigger and drop to a very low impedance state, essentially shorting out the circuit. An important difference between these and varistors is that they will remain in this shorted out state until power is removed. Once power is removed, they will reset itself back to the normal high impedance state.

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Once a varistor fails it will be very close to a dead short and that condition is non-reversible.

So in high power circuits you must have other protection in the circuit to prevent the failed varistor from catching fire.

The other protection is usually a fuse or circuit breaker between the varistor and high power source.

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Thank you for the clarification David. So, it sounds like the varistor goes to a low resistance state that diverts much or the current to ground. It doesn’t quite short the circuit and their still will be power going through the rest of the circuit, just less.

It makes sense why the varistor is rated by it’s body diameter then. It sounds like the amount of load the varistor can take is a function of its diameter.

Thank you for the information Hutch. It sounds like when the varistor fails, it then takes a permanantly shorted state… I think this power supply did fail during a surge event. I’d prefer to not start any files,

I’ll replace the parts and see if the power supply than powers on. It’s possible that it has other problems however.

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Hi @Allison You may enjoy reading Metal Oxide Varistor (MOV) Specifications & Selection Guide

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Thanks, I will.
I enjoy learning. This kind of stuff is fun for me.

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