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


> 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


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 

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