thermistor

[larryg]

to prevent arching of power switch,in-rush current, where does one place the thermistor?

[johnnyx]

In series with the power switch.

I don't suppose it really matters where exactly, but they run hot, so that may determine the placement for you.

[larryg]

thanks johnnyx for your reply!

[AndrewT]

Hi,
I tried a thermistor in the mains supply to a toroid (625VA and 2*40,000uF). It blew up. Then tried same again with a power resistor in series with thermistor. Again it blew up. I've since learned that hi power versions are available. But my final solution was a relay shorting out an in line power resistor, The switch on for the relay came from the rising voltage on the capacitors, I used a bit of trial & error until it switched over after about a 1 second delay. Previously 6.3amp slow blow would fail at switch on, now 3 amp slow blow survives about 20 to 30 switch ons but I changed to 5amp slow blow & it seems to be reliable.
regards Andrew T.

[larryg]

thanks AndrewT, the thermistor i have is rated 5 ohm and 6 amps.
the ps has 2 10,000 caps. the toroidal is 6 amps and 80vct.
do you think this combo will work?

[AndrewT]

Hi Larry,
That thermistor is a lot bigger than mine.
Without a calculator to hand, some sums. Resistance of primary winding say 3r +5r of thermistor. pk current about 240/8 say 30a for a few msec. dropping as the transformer establishes flux. If it survives the first switch on will the standing current heat it too much? 5r & 6A comes to 180w the thermistor will not survive this. even 1a gives 5w & that will be very hot. Do you know what resistance it will have at hi temp?. personally I would try it but make sure the lid is on. We would be interested to hear the result ( maybe from a mile away).
regards Andrew T.

[larryg]

hi AndrewT, thanks for your reply! the data info of this thermistor
5 ohms cold and 6 amps does not show high temp resistance.
what it gives and i quote ' these NTC thermistors effectively control surge currents because the thermal time constant of the current limiter is longer than the electrical time constant [RC] of the thermistor and the capacitor.'
well! i hope to power up the ps this weekend. maybe i should check my home owner insurance first!
i will post the results of this event!

[BobEllis]

Originally posted by larryg
thanks AndrewT, the thermistor i have is rated 5 ohm and 6 amps.
the ps has 2 10,000 caps. the toroidal is 6 amps and 80vct.
do you think this combo will work?

Sounds like a CL-30. According to the digikey catalog, full current resistance is .11R, so it's going to be too hot to touch, but shouldn't blow up. http://dkc3.digikey.com/PDF/T043/1047.pdf

I used a CL-30 in my A-75 and my Leach amp. Both are have 84VCT/11 amp toroids and 180,000 uf and 78,000 uf respectively. I burned my finger touching the Leach amps' thermistor so I left the A-75s alone. I'd like a little more initial resistance, but the lights only dim slightly on turn on.

[larryg]

thanks BobEllis for your reply! i found in my parts bin a CL 100.
i did not see on the chart from digikey. it is much larger than the
CL 30. do you think this would be a betterchoice?
note: this is for a leach amp

[BobEllis]

I couldn't find a CL-100 in the data sheets either. http://www.thermometrics.com/assets/images/cl.pdf

A CL-30 is plenty for a Leach - you want it to get hot so that the resistance goes down, but still be able to handle the maximum current your amp will draw on a continuous basis. I have a bigger transformer and more capacitance than you propose, but the lights barely dim.

You Leach will probably draw more than 6 amps momentarily, but steady state it should be less than 2, unless you really push the volume. The CL-100 might not get hot enough to drop the resitance significantly. try it and see...

Don't forget a slow blow fuse in the mains line. I was fairly regularly popping 10 amp regular fuses, but a more reasonable 6 amp slow blow has lasted over a year.