Power supply 250V Ceramic Fuse Replacement

Well, my laptop power supply died. There was no voltage at the computer tip at all. So, I cracked it open and started testing around. I came across a fuse, not the glass kind, though. The center, where the glass would normally be, was white. I figure this is a ceramic fuse, from pictures online.

Anyway, I ran a continuity test across it, but got nothing. I'm assuming that I should be able to put voltage through this fuse, unless it's blown.

I'm hoping this is my problem, since I could probably find a replacement..


It's marked ET4H-250V. Is that sufficient to get a replacement from, say, radioshack? Just pick up any ceramic 250V fuse? What about amperage?

Look arond the PCB does it say any where the fuse rating?

Keep in mind that fuses rarely just blow randomly. You may have a short somewhere.

Originally posted by Format _C:
Look arond the PCB does it say any where the fuse rating?

Click to expand...

That's what both the fuse itself and the PCB are labeled as... "T4AH 250V"

...As for fuses not randomly blowing, I'm not really sure how old this power supply is. There's a good chance it's over four years old, I got it as a refurbished component, so...

...regardless, I'd be willing to replace the fuse for the little trouble it would be, in hopes of having it work again, and not spend god knows how much on a replacement. IBM isn't known for their low prices on these things...

Would amperage come into play here, or is it just a 250V fuse that I'm looking for?

When I worked in an electronics shop for a couple years when a fuse blows it's usually so something more expensive doesn't. Something is probably not working right on the rest of the board.

If you replace it. It will probably blow again. I'd still try replacing it but just keep this thought in mind when you do.

Would you have suggestions about what else it could be?

It's a pretty small device, but still simple...very few components. One large cap, a transformer, a few resistors and some transistors. I'm certainly not a repair technician, though. I only look for things that are melted, burnt, or I've replaced before (fuses, for example).

If I could repair this thing, I'd really like to.


It stopped working when I plugged it in to an outlet at school. I'd have no idea whether this was a good or a bad outlet, as I've never used it before. At the time, my laptop was suspended, so I don't think there was much coming back into the power supply from there, maybe a few millivolts...

T4AH 250V

i would gess means its a 4 amp 250v fuse

If you plug the AC Adapter to your computer, THEN into the wall, their are instances it could short out the laptop. Have actually seen this happen!

I can think of zero good reasons not to replace it and see what happens.

Fuses are easy to get, but take the one that's blown with you when you go shopping. I wouldn't worry about it being ceramic vs. clear.

The fuse is located inside the monitor chassis, mounted on the mainboard in front of the IEC320 power connector. To access the fuse, you must remove the rear cover of the unit. Replace the fuse as required with a 5X20mm , T4AH 250V fuse.
http://www.ab.com/manuals/ic/monitors/6157-um001a-en-p.pdf
page 39 2nd 2 last paragraph at least you know how big the fuse its
http://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=+++++++5X20mm+250v+ceramic+fuse
might want to call one of these companys and ask for help
if this helps good if it dont eh

If it's that simple.
1. Buy a fuse and try it. It's possible something just went awry when you plugged it in.
2. Take it to a electronics shop. They could probably fix it. They will basically just test the cap and resistors. They can check IC's if they are standard to.

Originally posted by matoch
If it's that simple.
1. Buy a fuse and try it. It's possible something just went awry when you plugged it in.
2. Take it to a electronics shop. They could probably fix it. They will basically just test the cap and resistors. They can check IC's if they are standard to.

Click to expand...

But be sure to get the same amperage rating- if possible the same exact model fuse. If not, underestimate it. By using a fuse that's larger capacity than the old one, you're defeating the purpose of the fuse altogether- I wouldn't risk using a too-big fuse with a laptop at stake. As I said above- it has to have a reason to blow, which could very likely be a bad component with a mean streak, one that wants to fry your expensive hardware.

Okay, just a quick update and another question before this is solved...


I went to radio shack and picked up a 4amp 250V fuse, but they didn't have the ceramic kind, just the glass.

Also, the fuse is marked "slow blow"... now, does this have any bearing on the functionality? Usually "slow" in electronics is at most, a second or so.. Is this something I should worry about?


I got home, replaced the original with the new kind, and plugged in the power supply to the UPS I have (this, I know, is a safe power source, no spikes)

Now, I get 12V on the output, which is correct...

but...

Second Question:


The power supply, when plugged in, makes a very, very, very faint audible "hissing" noise. It's more of a whine, really. I have to be within a few inches to actually hear it, at all. Now, this is with the casing removed for repairs, so perhaps this noise was there all along?

I was just wondering if anyone has heard this from a power supply before, or if it's reason for me to worry. The output, as read by a multimeter, is exactly what the laptop needs, but I'm afraid to plug it in before I get the "ok" about this apparently new noise it makes. The noise dies down slowly after I unplug the power supply, but not immediately. It sort of winds down to a stop.

Thanks again guys, you may have saved me again!

Er, a 4 amp fuse? Something about that seems a little high. Might not be tho, to allow for inrush currents. As far as the noise is concerned, can't tell ya. I've had new destop supplies that have made some noise, and others that are silent. In any case, if there's magnetic elements involved, it wouldn't be unheard of to get some noise created by AC magnetic effects.

I guess the big question i'd have would be why the thing blew in the first place. Folks are right in saying that they don't die without a reason. (BTW, the fuse is generally included to protect more expensive things like houses... Semiconductors more often than not end up protecting the fuse...=) If you accidentally shorted the thing and it blew, it's understandable. I guess you gotta weigh the risk involved and choose what you think is best.

On the underside of the laptop, it'll usually say something to the likening of...

18V 4A. Lets say your power brick puts out 18V. how about go buy yourself a fat pack of power resistors, and making yourself a load. Load test the thing at 4A and see if it'll handle it. If you're curious how much current is really going through that fuse, put your 'load bank' in series with your fuse, then run a voltmeter across your load. V=IR, I=V/R will tell you how much current (being that you know the voltage and the resistance of your load).

Well, hope my speil was of some help.

Best of luck to you!

All switching power supplies 'hiss' to some extent. Sometimes you can hear it, sometimes you can't.

It's caused by windings 'vibrating' in the power supply's magnetic bits. If a transformer or inductor inside has any kind of core loss, then it will act like a solenoid and the windings will "pull" on the core.

We build high power switchers here at work that push upwards of 4KW through a single ETD49 core. The production transformers are potted with varnish to keep them quiet and reliable, but the prototype supplies don't exactly hiss - they put out more of a high frequency shriek...

High frequency, that's definitely what I'd call this sound. It's higher pitched than the sound a TV makes when it's on (CRT type), if you can hear that kind of noise.

I'm going to take the plunge and plug in my laptop after I reassemble the power supply. The output is good and clean according to my multimeter, and well within operating range of the laptop.

I'm suspecting that the fuse was blown when I plugged the brick in at the university. The room I have Astronomy class in has lights that flicker on and off constantly. I had never used these outlets before, and never got it working in that room, since that's the room where I noticed "hey, my laptop isn't charging..."

So, let's hope that's the reason here. I'll leave the power supply plugged in for an hour or so before I test the laptop, to see if the fuse blows.

Thanks to everyone!