Surface mount fuses

Thread Starter

geoffers

Joined Oct 25, 2010
322
Hi all,

I've a problem with a circuit board I've got and I think its one of what I assume to be a surface mount fuse?

I've attached a picture, the power input is 12v ac, both sides have a fuse if that's what it is? F3 and f4, f4 conducts f3 doesn't!

The third connection in the middle is to the ground plane.

First question is does anyone recognise the fuse?

Second, why would both sides be fused?

Third, a quick look on rs shows a few fuses to be auto reset? If this is the case I would assume it to conduct with no load?

Thanks GeoffIMG_20200316_132359.jpg
 

Thread Starter

geoffers

Joined Oct 25, 2010
322
Thanks,

I only mentioned resettable fuses as they popped up in the rs search for surface mount fuses.

I'm not sure what they are, I was concerned if they where resettable I have a bigger problem as one isn't resetting ?

I'm tempted to by pass that fuse, if it is? And see if the thing works, it should still be protected by the fuse on the other side of the ac input?

Or have I got the wrong end of the stick??

Cheers Geoff
 

dl324

Joined Mar 30, 2015
11,577
I only mentioned resettable fuses as they popped up in the rs search for surface mount fuses.
For user convenience, self resetting fuses are better. Most USB powered devices use them.
I'm not sure what they are, I was concerned if they where resettable I have a bigger problem as one isn't resetting ?
The things in your picture aren't PTC fuses. They're usually black in color and the aspect ratio wouldn't be so narrow.
I'm tempted to by pass that fuse, if it is? And see if the thing works, it should still be protected by the fuse on the other side of the ac input?
It's never a good idea to defeat protection included in a circuit. It was put there for a reason.
 

Thread Starter

geoffers

Joined Oct 25, 2010
322
That makes more sense, f for filter instead of fuse, so likely one of those has failed then, makes more sense as to why there is one on each side of the ac input.

Any thoughts on temporarily by pass it see if the rest of the thing works?

Cheers Geoff
 

AlbertHall

Joined Jun 4, 2014
10,417
That makes more sense, f for filter instead of fuse, so likely one of those has failed then, makes more sense as to why there is one on each side of the ac input.

Any thoughts on temporarily by pass it see if the rest of the thing works?

Cheers Geoff
If it failed because of excess current then bypassing it may do more damage. What is the current rating of the AC source? How much current does the board need?
 

peterdeco

Joined Oct 8, 2019
231
YES! Years ago I repaired stereo receivers. Many customers would open it up, see board mounted fuses blown and wrap them in aluminum foil. Of course now they see smoke.
 

Thread Starter

geoffers

Joined Oct 25, 2010
322
Thanks everyone!

Yep I think you've got it with the feed through capacitor :) Its a agricultural device and I plugged it in to a very old socket which crackled a bit..... I think this may have caused a surge? I'm hoping that's what did it, anything the other side maybe expensive.

It works around 420Mhz would anyone hazard a guess what value it should be?

Up until last night I'd never heard of a feed through capacitor, learn something every day!

Cheers Geoff
 

Thread Starter

geoffers

Joined Oct 25, 2010
322
Thanks to everyone!

I soldered in my new feed through cap, one happy farmer!

Can't quite make out why its made like it is? Rectifier is on the main circuit board, the transformer in a different enclosure, why bring ac onto the board?

Wouldn't it make more sense to rectify in the transformer enclosure?

The feed through caps seem to be first line of defense?!

Cheers Geoff
 
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