Is this a varistor?

[mcandmar]

Found this in a speaker crossover HF section, am i right in assuming its a varistor? And can anyone decipher its value from the markings?

The odd thing is only one speaker had it which has me curious. Kef were known for tightly matching part tolerences so i am wondering if it was added for a bit of resistance rather than for protection

 

[MarcelvdG]

It could also be a polyfuse, a PTC used as self-resetting fuse. That would make more sense in a loudspeaker. Is its resistance low?

 

[tomchr]

It could be a ceramic cap, though it's awfully large for a disc cap. Can you show a picture (or write down for us) any markings on the other side of it?

Tom

 

[Galu]

I agree with MarcelvdG.

It is probably a polyswitch (Positec) speaker protection device.

If so, its resistance will be around 0.5 ohm.

 

[Galu]

Yes, definitely a PolySwitch (Positec).

Here are two (one for bass, one for treble) on a Mordaunt Short crossover board.

I have worked with these devices, so there's no doubt.

 

[Galu]

And can anyone decipher its value from the markings?

I can't find these particular manufacturer's markings.

The cold resistance can range from a small fraction of an ohm to several ohms.

Here's a useful calculator for future reference: Loudspeaker Protection Components

 

[mcandmar]

Thanks guys, looks like a poly fuse it is. Measures ~.2ohms, still curious as to why its only in one speaker, and not listed in the crossover schematics. Its located between the 3.3uf and 4.2uf paralleled caps and the rest of the circuit.

 

[Galu]

It looks like (see attachment) KEF added the PolySwitch as a later revision to protect the tweeter.

It is not included in the earlier crossovers.

You may have an "in-between the revision" pair of crossovers!

Provided you are not going to overload your tweeters, I suggest the protection device would best be removed.

 

[Galu]

And can anyone decipher its value from the markings?

OK, the "135" marking would appear to represent 1.35A.

You will see an SE135, 1.35A, 0.19 ohm PTC in the calculator to which I linked earlier: Loudspeaker Protection Components

Versions are also available from Digi-Key: Blocked

 

[mcandmar]

Nice find on that photo, confirms it is 100% factory fit then. It's gone now, i won't be driving these anywhere near their limits so they are just not needed. I have just competed the fun job of taking the tweeters apart and replacing the ferro fluid so they should be good for another 30 years ;-)

Thanks for the help...

 

[Galu]

Happy listening!