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Old 27th December 2012, 10:38 AM   #51
RCruz is offline RCruz  Switzerland
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So maybe I should use NTC resistors instead of normal power resistors.
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Old 27th December 2012, 10:48 AM   #52
AndrewT is offline AndrewT  Scotland
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NTCs are possibly the ideal form of resistor to use for the current limiting action, were it not for their cost.

I have said similar in many of my previous posts and I have stated why I sue resistors instead.

But most on this Forum will not read what is posted for them.
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Old 27th December 2012, 01:36 PM   #53
RCruz is offline RCruz  Switzerland
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I found 22ohm NTC resistors at 1.5€ in Conrad... Can I use two in series to get 44ohm ?
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Old 27th December 2012, 09:51 PM   #54
AJT is offline AJT  Philippines
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just one ntc is good enough...this ntc once heated up will be like a sub-ohm resistor and so will be invisible to the transformer....
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Old 28th December 2012, 08:14 PM   #55
RCruz is offline RCruz  Switzerland
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Quote:
Originally Posted by Tony View Post
just one ntc is good enough...this ntc once heated up will be like a sub-ohm resistor and so will be invisible to the transformer....
Thank you Tony

That is good to know, in fact Papa uses these in his A75 power amp....

I will experiment with a single ntc and if I feel it does someting nasty to the sound, I will build the relay thing with the timer
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Old 4th January 2013, 02:47 PM   #56
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Why does R14 a 1M resistor need to be 3W?

I = V / R = 250V/1000000 = 0,25mA
P = I * V = 0,00025A * 250V = 0,0625W

So even if it is directly applied to 250VAC a 0.25W resistor should still be fine.

I would imagine this has something to do with safety or the 250VAC.

I am just having some difficulty getting a 3W 1M resistor.

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Old 8th January 2013, 11:03 PM   #57
nrg2009 is offline nrg2009  United States
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Originally Posted by AndrewT View Post
Thermistors are probably the best style of resistor to use in both a soft start and in a slow charge.
But here in the UK Power Thermistors are very expensive.
Therefore I use multiple wirewounds as the current limiter.

In the mains circuit they must be switched out quickly, probably no more than half a dozen seconds. But for soft start that is not an issue the soft start limiting is redundant within 100ms and can be switched out long before the resistors get hot.

Thermistors don't have that problem, they adjust dissipation to suit the temperature they are running at. They get hot, they adjust the resistance down, they get less hot.

However in a slow charge circuit the capacitors never really fully charge. The charge voltage is asymptotic to the final voltage, They never quite get there.
This results in a current pulse when the resistance is switched out.
The sooner you switch out the resistance the bigger the current pulse.
For this reason you want a long duration on the timer. That long duration does not suit resistors, it does suit Thermistors. They don't burn out, if selected appropriately.

Initially they warm up and the resistance falls, as the charge current reduces they cool down and the resistance increases. But eventually they stabilise to the quiescent condition. But if the music starts the resistance will vary with the "beat" of the music. The supply rails jump around due to the high and variable source impedance. You must switch them out.
Andrew,

I read your interesting posts on thermistor use.
I am just getting started and want to learn more. You mention that thermistors are very effective at startup of an amp, even better than the softstart board offered by te diyaudio store.
However you also mention that they limit the amp from reaching its full current potential.
Also, you mention placing the thermistor on the secondary, whereas i see most people placing the thermistor on the primary side of the trafo, for us that is 110volt. In the guidelines from GE who manufactures the thermistor also shows it on the secondary winding.
Any suggestions on how i should use thermistors?

Thanks.
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Old 8th January 2013, 11:07 PM   #58
AJT is offline AJT  Philippines
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Quote:
Originally Posted by steelwil View Post
Why does R14 a 1M resistor need to be 3W?

I = V / R = 250V/1000000 = 0,25mA
P = I * V = 0,00025A * 250V = 0,0625W

So even if it is directly applied to 250VAC a 0.25W resistor should still be fine.

I would imagine this has something to do with safety or the 250VAC.

I am just having some difficulty getting a 3W 1M resistor.

Thanks
your call, you can do it if you wish.....when using 240volt mains the 1ufd capacitor must be rated 630volts dc or better....
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Old 8th January 2013, 11:15 PM   #59
AJT is offline AJT  Philippines
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Quote:
Originally Posted by nrg2009 View Post

However you also mention that they limit the amp from reaching its full current potential.
Also, you mention placing the thermistor on the secondary, whereas i see most people placing the thermistor on the primary side of the trafo, for us that is 110volt. In the guidelines from GE who manufactures the thermistor also shows it on the secondary winding.
Any suggestions on how i should use thermistors?

Thanks.
not Andrew, but i have had extensive experience with these NTC thermistors repairing hundreds of ATX psu's....

these thermistors have cold resistance values from say 4 ohms to about 20 ohms, but as soon as it gets warm, the resistance drops to less than an ohm so that it does not interfere with the working of the power traffo, the dc resistance of the primary of the power traffo will have more resistance....

you can verify what i say by connecting a thermistors to an ohmmeter and training a heat source to it like a hair dryer and see for yourself how the resistance drops....

use it in circuits where you think adding a series resistance to limit surges will help, doesn't matter primary or secondary....
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Old 9th January 2013, 12:22 AM   #60
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Just pointing at a tiny little case here Tony.

It does matter wether the thermistor is to be mounted on the primary or the secondary.
One should always mount them where the nominal current is the lowest.
This to secure the best (longest) service time for the cirquit.

But otherwise I totally agree with You. Although I always bypass them with a relay after a suitable inrush-time. Again to secure extended service-time.
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