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Old 22nd June 2004, 10:31 AM   #1
LBHajdu is offline LBHajdu  United States
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Default Thermistors in class AB amps ?

For inrush current limiting Mr. Pass uses a thermistor on all has amps. I think it’s the CL60. Could the CL60 be used to limit the inrush in a class A\B amp that idles between 80 – 100Watts? That’s the idle of the two channels together; each channel could put out a maximum of 250watts.

I guess the question is, is the idle sufficient to turn on (heat up) the thermistor and will the peek draw of 500w hurt the thermistor. And is the CL60 the thermistor I need. If this solution does not work then I will have to use a more complex time delay circuit with a high wattage resistor and a bypass relay.
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Old 22nd June 2004, 06:58 PM   #2
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Theoretically, Thermistors work best with power supplies
that have a constant or at least relatively high draw.

Nevertheless, I wouldn't be afraid to use one anyway. If
you don't like it, you're out 2 bucks.
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Old 22nd June 2004, 07:26 PM   #3
K-amps is offline K-amps  United States
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Default Re: Thermistors in class AB amps ?

Quote:
Originally posted by LBHajdu
For inrush current limiting Mr. Pass uses a thermistor on all has amps. I think it’s the CL60. Could the CL60 be used to limit the inrush in a class A\B amp that idles between 80 – 100Watts? That’s the idle of the two channels together; each channel could put out a maximum of 250watts.

I guess the question is, is the idle sufficient to turn on (heat up) the thermistor and will the peek draw of 500w hurt the thermistor. And is the CL60 the thermistor I need. If this solution does not work then I will have to use a more complex time delay circuit with a high wattage resistor and a bypass relay.
Didn't Nelson use the CL-101 in the Adcom GFA-5802 (300wpc) ? That should work.

K-
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Old 22nd June 2004, 08:55 PM   #4
joensd is offline joensd  Germany
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I use a 5 Ohm thermistor in my Crescendo Mosfet-amp and to be honest I couldn´t hear a difference= I still sleep well
(bias is around 300mA per channel)

(Unfortunately now I can´t hear any sparks from switching the amplifier anymore and the lights in the room stay almost constantly bright. )

In comparison I´d guess you lose more dynamics in your amp through power supply sag than such a thermistor as it reacts relatively slow but well that´s just my theory......
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Old 22nd June 2004, 09:35 PM   #5
K-amps is offline K-amps  United States
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The CL-101 is rated at 0.5 ohms and goes to near zero when heated (i.e. current passed through it).

So if the devices has sufficient load upon it, it should decrease in impedance and not act like a series resistor sapping current...
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Old 23rd June 2004, 03:59 AM   #6
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Actually, If you put the thermistor between the mains and transformer, It should work fine. An unloaded transformer acts like a choke, and while it isn't putting out but .5A (as in class ab) it still has a bit of current flowing through primary, but 90 deg out of phase with voltage. sSo it should still heat up and drop resistance.

What I did on an amp once is use the thermister to soft start, But put a relay to short thermisor when the rails reach running voltage.
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Old 23rd June 2004, 09:16 AM   #7
joensd is offline joensd  Germany
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Quote:
What I did on an amp once is use the thermister to soft start, But put a relay to short thermisor when the rails reach running voltage.
That certainly is a more elegant solution although you could take resistors then just as well.
Here you´ll find an example of how it can be implemented.
Click "Projects" "other projects" "delay circuit for toroids".
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Old 23rd June 2004, 06:09 PM   #8
djk is offline djk
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The CL-101 offers virtually no protection.

McIntosh uses values around 4R, and on their largest amplifiers shorts this out with a relay.

I use the CL-30, 2R5 cold, 0R06 at 8 amps. For amps with the transformer larger than 1KVA I use a relay. Under certain fault conditions the inrush limiter could burn up (blown outputs), BGW uses a fuse in series with the inrush limiter so if the relay doesn't pull in and there is a high current fault the inrush limiter will not blow.

I view this as a seconday failure as the amp is blown anyway so it is just another part to replace.
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Old 24th June 2004, 01:09 AM   #9
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Joensd
There is one problem with using a resistor, The amp was class A and loaded the PSU too heavily to reach a high enough voltage to engage the relay without the relay seeing too much voltage across coil after it shut. I added the relay because I didn't want to loose the volt or so with it in thew circuit. I though it a more clever aproach than a RC timed circuit and it was easily hardwired in after the fact without looking too assy.
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Old 24th June 2004, 01:20 PM   #10
LBHajdu is offline LBHajdu  United States
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I think my best bet is to try the thermistor first, and if the amp doesn’t draw enough amps to keep the thermistor hot them I need a time delay circuit. The best circuit would be the simplest one (less chance of failure) that’s why the thermistor is tried first. joensd circuit is nice, I like that besides the rectifier you don’t really need any semiconductor or a transformer. I found another version of it at:
http://www.reed-electronics.com/ednm...s/198898f1.pdf
It may be a little better. I think in the other circuit it depends on that exact relay being used to keep the voltage at 24v, where as in this one a zener diode is used. I have never seen one, but the best solution would be a thermistor with a heater resistor on it. That way no matter how small the current is flowing in it, it would always get hot.
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