Polyswitch fuse for speaker protection


2010-07-22 1:58 am
I want to protect the 1" HF horn ( 40W/8 Ohms ) of my bass guitar cabinet with a Polyswitch resettable fuse. If I have calculated well the result for my HF is 2.23A. Do I have to match this result with the trip current of the Polyswitch? The RXEF- 110 shows a 2.2A trip current : Digi-Key - RXEF110-ND (Manufacturer - RXEF110) .Is it the right one or do I have to choose another value ?: http://dkc3.digikey.com/PDF/US2010/P2107.pdf
Many thanks,
Be advised...

I used these in a couple builds, and it actually added an ohm of resistance to the design with it engaged and not preventing the current. You may need to lessen your L-pad a smidge.

Also- I would recommend placing a 12-15 ohm resistor in parallel with the Polyswitch so that you know the driver is okay when it interrupts the circuit. It will just strongly limit its output when engaged. After the polyswitch cools, the output will return to normal.

Wolf, can you - or anyone having experience too - please
say a word or two about "audibility" of polyswitches ?

Are there any effects before reaching the "trip current" ?

Is there any ageing noticeable, especially when switched
more often ?

Kind Regards

I can't tell it's there, other than the reduced output. I have not tried it without on one of the 2 I've used them.

Don't know that either- I've never managed to trip one.

Thank you Wolf,

i found some information on characteristics
of PTC resetable fuses, unfortunately the
text is in german language only:


The rise in resistance between "normal" and "tripped"
state seems to be highly frequency dependent.

Especially in the tripped state, there is a capacitive

Nevertheless the elements seem to be usable quite
well for the purpose, see attached picture ...


  • PTC_frequency_dependency.JPG
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