How many thermistors?

[Duck-Twacy]

Maybe this has been asked before...

I have a double mono power supply with two 625 toroids in one enclosure (so one IEC inlet) for my Aleph5. Is it sufficient to use one pair of CL60 thermistors. Or should I apply them after spliting the AC inlet, thus having one pair of CL60s for each toroid?

I prefer to use one pair for both toroids, because of the heat build up (working temp 60 is degrees C I believe) and extra space they will require. But is it sufficient?

I do want one to use one fuse for each toroid.

[Duck-Twacy]

Btw the resistance of one of these CL-60 is about 10-12 ohm (so using 2 in serie gives about 20 ohms).

Doesnt look that much to limit current?? Is it just AC/20=240/20=12A max?

[kilowattski]

Use one thermistor per toroidal transformer. The only reason you need a thermistor is to limit the current surge on turn on. Once the thermistor gets hot, it's resistance approached zero ohms and is virtually out of the picture. So you see, there is really no reason to use multiples unless parameters require it. For the steady state current required for an Aleph 5, again, only one per toroidal.

[jackinnj]

Originally posted by kilowattski
Use one thermistor per toroidal transformer. The only reason you need a thermistor is to limit the current surge on turn on. Once the thermistor gets hot, it's resistance approached zero ohms and is virtually out of the picture. So you see, there is really no reason to use multiples unless parameters require it. For the steady state current required for an Aleph 5, again, only one per toroidal.

not quite right -- the "on" resistance is given in the Thermometrics or GE datasheets (or can be calculated if you have a programmable calculator) --- it can be significant enough that there will be an IR voltage drop

you can use one thermistor for the positive and one for the negative rail of a bridged power supply, or you can use on one the primary side of the trannie alone -- it just has to fit the design parameters.

[kilowattski]

you can use one thermistor for the positive and one for the negative rail of a bridged power supply

Jack,

Not quite Jack, but a nice try. If he uses a CL-60 on the primary the "hot" resistance is 0.18 ohms. His Aleph 5 according to Pass Labs will draw 2.5 amps. If he does use two transformers, each will draw 1.25 amps. That will lead to a voltage drop across the thermistor of 0.225 volts. I would hardly say that is a significant voltage drop when the power company has difficulties maintaining even 5% regulation on the power coming into your home. Of course if his primary is 240 volts the drop will be even less. Putting thermistors after the rectifier on the secondary is not much help in supressing inrush current since most of the inrush is caused by the toroidal transformer itself.

[jackinnj]

Originally posted by kilowattski


Jack,

Not quite Jack, but a nice try. If he uses a CL-60 on the primary the "hot" resistance is 0.18 ohms. His Aleph 5 according to Pass Labs will draw 2.5 amps. If he does use two transformers, each will draw 1.25 amps. That will lead to a voltage drop across the thermistor of 0.225 volts. I would hardly say that is a significant voltage drop when the power company has difficulties maintaining even 5% regulation on the power coming into your home. Of course if his primary is 240 volts the drop will be even less. Putting thermistors after the rectifier on the secondary is not much help in supressing inrush current since most of the inrush is caused by the toroidal transformer itself.

here's the problem in your analysis -- these current limiters have higher resistance when operated below their maximum rated capacity, so the resistance for the CL60 at 1.25 amps is closer to an ohm, and at 5 amps it is 0.18 ohms. furthermore, the device doesn't respond instantaneously to a change in demand -- unless you are using a regulated power supply you will see the effects -- at least in an amplifier within poor PSRR.

i had experienced the problem of blowing MUR860's and MBR20100's -- putting the inrush limiter in the diodes in the circuit as described cured the problem -- but one was needed on each rail -- this was for a 2 x 120 watt amplifier. when i found that i could cut the size of the filter caps and not hurt its performance i was able to put the limiter ahead of the transformer.

[jacco vermeulen]

Jack,

can you explain why the thermistors can be placed on the secondary rails ?

Thanks,
Jacco

[kilowattski]

Ok jack ya got me there..Good job.

[jackinnj]

Originally posted by jacco vermeulen
Jack,

can you explain why the thermistors can be placed on the secondary rails ?

Thanks,
Jacco

the inrush limiter is commonly used for "off line" switch mode power supplies, and as its name implies it limits the current into the filter/reservoir capacitor -- which in this case is on the primary side of the transformer --

for our linear supplies using a torroid there is a wallop of current into the transformer when the supply is turned on, and the filter caps also look like a dead short -- the limiter can protect "both" or "either".

[Duck-Twacy]

Thanks for your all your replies. I was planning to use 2 in the primairy side. If I understand it correctly with one Alpeh5 channel (1.25 A) the thermistors won't get hot enough to reach the 0.18 ohm, but will settle around 1 ohm. When I use 2 of them per side the voltage drop becomes twice as large: 2 * 1 * 1.25 = 2.5V. Still not that much of a problem. Especially since my secondairy AC is 30V, which is a bit above normal Aleph5 specs.

On the other hand if one is enough... (I have 10 lying around btw)

Do I understand it correctly that with one 10 Ohm Thermistor (cold) per toroid the maximal inrush current wil be 24A (with 240 V AC). Still seems rather high to me, but it is only for a short period of time of course.

(BTW there will be 220 mF caps per channel, with CLC)

Second question. How long do these Thermistors last and what happens when they break down?