Do Polyswitches ever go bad?

[spamtr...@gmail.com]

The discussion of the device in series with the Cannon XLR jogged
my memories of the Polyswitch, a protective device whose resistance
jumps up when sufficiently heated (via current), but goes back down
when it has cooled off.

http://www.te.com/usa-en/products/circuit-protection/overcurrent-devices/polyswitch-resettable-devices.html

We ended up not using them, but now I am curious if they show up in
damaged equipment.

[Gareth Magennis]

wrote in message
news:4ab48115-270e-465d...@googlegroups.com...

I know I've bought at least one, so the answer must be yes.


Gareth.

[Mike Tomlinson]

En el artículo <4ab48115-270e-465d...@googlegroups.com>,
spamtr...@gmail.com escribió:

>The discussion of the device in series with the Cannon XLR jogged
>my memories of the Polyswitch, a protective device whose resistance
>jumps up when sufficiently heated (via current), but goes back down
>when it has cooled off.

I thought they were called polyfuses.

--

[N_Cook]

I recently had a pair of polyswitches, same size and markings, in the
same kit, with both cold one was 0.5 ohm or so greater ohmage than the
other. Not that that was the problem.

[whit3rd]

On Friday, May 22, 2015 at 9:27:21 AM UTC-7, spamtr...@gmail.com wrote:
> The discussion ...jogged

> my memories of the Polyswitch, a protective device whose resistance
> jumps up when sufficiently heated (via current), but goes back down
> when it has cooled off.
>
> http://www.te.com/usa-en/products/circuit-protection/overcurrent-devices/polyswitch-resettable-devices.html
>
> We ended up not using them, but now I am curious if they show up in
> damaged equipment.

Yes, there were quite a few damaged polyswitches in the 1990's vintage
SCSI (computer bus) systems; these were used in powering the required
resistor arrays (called terminators), and were a big improvement over
the previous generation (which used fuses).

The problem, is that you couldn't power the bus without taking precautions
because other bus-resident boxes could have different +5V levels, or could
be powered off, or have unstable voltages, and were ALSO connected to
that same wires on the bus.

The issue I saw was usually brittle fracture, and mainly on the surface-mount
types. Before polyswitches, there were lots of fuses (one inside each CD or
hard drive mechanism) and when a fuse went out, the whole bus could lose
its logic power (or not, depending on which particular devices were plugged in).
A surge could pop two fuses, and leave the third device (like, a scanner) as the
only bus power source. When you unplugged that scanner, the hard drive
became unusable.

[Phil Allison]

** Polyswitches are damaged ( blown apart) by overvoltage when in the high resistance mode. The maximum ratings vary from 16V to 250V AC for different devices.

Polyswitches are in fact *temperate operated* so will switch at lower current when the ambient temp is higher than rated, usually 20C.

Another issue is that returning to the original low resistance value after cooling can take some time, days even - meanwhile the trip current will be lower than before.

Good practice is to make the trip current occur at double or more the normal running current and also allow for the effect of a hot environment.



... Phil

[Phil Allison]

Mike Tomlinson wrote:

>
> I thought they were called polyfuses.
>

** Yeah, that is one of the names they go by, but is actually a trade name registered to Littelfuse.



.... Phil

[Cydrome Leader]

whit3rd <whi...@gmail.com> wrote:
> On Friday, May 22, 2015 at 9:27:21 AM UTC-7, spamtr...@gmail.com wrote:
>> The discussion ...jogged
>> my memories of the Polyswitch, a protective device whose resistance
>> jumps up when sufficiently heated (via current), but goes back down
>> when it has cooled off.
>>
>> http://www.te.com/usa-en/products/circuit-protection/overcurrent-devices/polyswitch-resettable-devices.html
>>
>> We ended up not using them, but now I am curious if they show up in
>> damaged equipment.
>
> Yes, there were quite a few damaged polyswitches in the 1990's vintage
> SCSI (computer bus) systems; these were used in powering the required
> resistor arrays (called terminators), and were a big improvement over
> the previous generation (which used fuses).

Eek. I forgot about resistor packs and termination until you brought it
up. ESDI drives needed proper termination as well. That stuff was a pain.

[gghe...@gmail.com]

The trip current tends to decrease slightly with each fault.
And if left in the tripped (heated) state for extended time,
(like hours.) then again the trip current tends to decrease.
Which is exactly how you'd like a fault detector to fail.

George H.