Thermal Fuses

[Puffin]

I have a switch mode PS which has a failed thermal fuse. I cannot find any information about the PS and so do not know exacly what to replace it with. Any advice?

[!]

A bit more info... anything at all, could be a good starting point. You must have some ideas about things like manufacturer, or what product it was in, or wattage, voltage current, where the fuse is in the PSU (subcircuit), etc.

If all else fails, providing high resolution top-down and bottom-up pictures of the PCB might help. The other issue is whether the fuse just wore out (thermal fuses can do that eventually) or there was another problem causing excessive heat.

If the PSU is passively cooled it might run pretty hot but if actively cooled then you'd guesstimate what temperature would be considered excessive and pick based on that, and on peak current expected in that portion of the circuit if the PSU were working properly then add a little margin. For example you might seek about 70C active or 100C passively cooled, or a bit higher for active if it's 'sunk to another component's heatsink.

[sofaspud]

+1
+ Are you referring to a thermal fuse that looks similar to a diode, but has one tapered end? Can you see any ID numbers on the part?

[Puffin]

Hi guys. The PS comes from a frinds hot tub. I believe that this only powers the motor to raise and lower the TV screen.

I have assumed that it is a thermal fuse. The PS board shows "Fuse 1" although there is only one on the board. The fuse had self destructed and the only bits left were a fragment (which seemed to be glass) attached to one leg, and a heatshrink band. All that remained of the other leg was bare wire.

The board provides two 12v feeds.

Oddly the "fuse" was soldered at both ends, which seems to be a no-no when fixing thermal fuses. However the board has been modified at some stage from it's original state (perhaps to accomodate this application)

I can take pics if it helps (just need to get some batteries)

Rob.

[yajnaS]

Heat shrink tube on a thermal fuse ?

[Puffin]

Well it was there.

[!]

Thermal fuses (usually, none I have ever seen) aren't glass. It is curious that it is so severely physically damaged, whether it were a thermal fuse or standard current-trip type, normally only the element inside would break.

I would inspect the slide and gearing for the TV screen, perhaps the mechanical portion was in a bind and this caused an excessive stall current on the motor. I'd also check to see whether the motor seems to spin freely if not attached to the gearing in case the bearings failed.

If the mechanism seems to operate well and the motor is ok (you could apply 12V from an alternate source and see if it spins ok with no load) then see if there are any markings on the motor to indicate the power requirements if the PSU has no indication on it and there is no other way to find the info.

Where is the fuse? Primary side before a transformer, or secondary side after one? This will determine the (ratio) average current it has to handle. However you wrote it provides two 12V feeds which is odd, to power a single motor, unless this is some kind of retrofit installation and the PSU was just some random power supply lying around for a one-off hacked installation since it is less expensive to use a PSU with only the outputs needed.

Some guesses about the current might be made if there is no other way to figure it out, based on PSU size, the switching transistor spec or transformer although motors aren't my forte, I vaguely recall they may need a significantly higher start up current compared to steady run current. If you have a few spare fuses lying around and the time you could split the difference and put roughly 6A in and see if it works without blowing.

[Puffin]

Some pics.

[Puffin]

I inserted a thermal fuse and powered up with a safety light bulb tester to limit current and it lit immediately, so a straight replacement hasn't worked. Of course I guessed at the rating (but with no load not critical?)

[AndrewT]

Is that 2PIN connector @ the corner intended for 240Vac input?