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'[EE] Polyswitch?'
2009\06\14@084904 by Roger Weichert

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Hi Guys,

I'm attempting to repair a treadmill which has a faulty motor speed
controller.

One leg of the 240v ac goes onto the board, through a 20 amp fuse to one leg
of a bridge rectifier  ...  to supply the gutsy dc motor.

The other side of the ac in  ...  goes through a device which has two legs
... is 20 mm in diam and 4 mm thick  ..  to another leg of the bridge
rectifier.

This device is open circuit, has bulged a little and appears heat
distressed. Despite close inspection I can find no identifying markings at
all.

>From the fact that it has to pass many amps at 240v ac  ..  I assume it is
some sort of polyswitch  ...  but I have not been able to find specs on any
240v ones that can handle more than a couple of amps.

Can anyone point me in the direction of a device which may do the job
please.

I did bridge it to see if it would run ok (after close inspection of the
rest of the unit) and it ran quite happily. The machine had been clogged
with dust and fluff  ..  so I suspect that overheating was the main cause.

Regards,    Roger

2009\06\14@090332 by Mike Harrison

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On Sun, 14 Jun 2009 22:19:00 +0930, you wrote:

{Quote hidden}

I've never seen a polyswitch used on mains - I think there are probably power dissipation/arcing
issues.
My guess is it may be an inrush current limiter. e.g. http://www.ametherm.com/

2009\06\14@090613 by Djula Djarmati

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Roger Weichert wrote:
{Quote hidden}

It is probably a NTC inrush current limiter:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=656273&keywords=inrush%20current%20limiter

Djula

2009\06\14@094648 by Roger Weichert

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----- Original Message -----
From: "Mike Harrison" <spam_OUTmikeTakeThisOuTspamwhitewing.co.uk>
>
> I've never seen a polyswitch used on mains - I think there are probably
> power dissipation/arcing
> issues.
> My guess is it may be an inrush current limiter. e.g.
> http://www.ametherm.com/


Thanks Mike and Djula

>From reading through your links, I can see that is likely what it is  ...
an NTC.

All I have to do now is determine what value to use.

I appreciate your help, thank you.

Regards,   Roger

2009\06\14@133407 by Harold Hallikainen

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Someone said they've never seen Polyswitch PTC thermistors used for
over-current protection on mains. I've designed them in to several
products. I think they were from BC Components.

However, the fact that this circuit already has a fuse and that it's
driving a rectifier and, I suspect, a filter capacitor after the
rectifier, makes me think it is instead an NTC thermistor used to limit
inrush current as the capacitors charge when the device is first powered
up. This used to be pretty common in switching power supplies, and I had a
lot of trouble with them. If there is a short power interruption, the
thermistor does not cool off and stays at low resistance. The power comes
back on and the rectifier is blown out. In one product used at radio and
TV transmitter sites, I added a 3 ohm 20 watt resistor between the power
inlet and the power supply on each side of the line. I also put varistors
to ground on each side of the line after the resistors to help keep
lightning out of the box. That pretty much solved the problem.

So, I think you have an NTC inrush current limiter. You could probably use
a 2 or 3 ohm resistor instead and have it work.

Harold


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2009\06\14@175908 by Roger Weichert

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----- Original Message -----
From: "Harold Hallikainen" <.....haroldKILLspamspam@spam@hallikainen.org>


{Quote hidden}

Thanks Harold,

Thats why I like this list so much. Always someone out there with some
relevant experience who is willing to help out and provide advice and help.

I've been in this game over 35 years and have a lot of experience in one
main area, but there's always something new to learn.

Now that you've explained this, I recall seeing some gear with just what
you've described  ...  I just didn't understand why it had been built like
that. Most of the gear I see doesn't come with circuit diagrams, so it's
often difficult to understand what everything does.

You are right too, in that the rectifier does feed a couple of fairly hefty
filter caps  ... which then feed the large dc motor (via speed control of
course).

Thanks again Harold.

Regards, Roger

2009\06\14@192841 by Harold Hallikainen

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> Thanks again Harold.
>
> Regards, Roger


You're welcome!

Harold


--
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!

2009\06\15@102401 by Herbert Graf

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On Sun, 2009-06-14 at 22:19 +0930, Roger Weichert wrote:
> Hi Guys,
>
> I'm attempting to repair a treadmill which has a faulty motor speed
> controller.
>
> One leg of the 240v ac goes onto the board, through a 20 amp fuse to one leg
> of a bridge rectifier  ...  to supply the gutsy dc motor.
>
> The other side of the ac in  ...  goes through a device which has two legs
> ... is 20 mm in diam and 4 mm thick  ..  to another leg of the bridge
> rectifier.

This doesn't sound like any polyswitch I've ever seen, it sounds alot
more like a surge type device perhaps?

TTYL

2009\06\15@185739 by Roger Weichert

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Original Message -----
From: "Herbert Graf" <hkgrafspamKILLspamgmail.com>

>
> This doesn't sound like any polyswitch I've ever seen, it sounds alot
> more like a surge type device perhaps?
>
> TTYL


Thanks Herbert  ...  yeah, Mike, Djula and Harold all set me straight over
the last couple days, suggesting it is an inrush current limiting NTC
thermistor.

I'm still trying to decide exactly which type to replace it with.

Regards,   Roger

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