How many thermistors?

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pro said:
10 Hom thermistor in an AB class amplifier

I think that the 10 ohm is the rated zero-power resistance at 25 deg C. In my opinion, it could be used on the primary side because the initial resistance would be reduced after turning on the switch. At any value, the resistance would become less than 10 ohms. Even if it keeps almost 10 ohms, it does not matter because it means that there is very little current flow, and accordingly the voltage drop across the thermistor will be very small.

I would use the 10 ohm thermistor and steady state 5A rating.

I would like to invite other opinions if any.
 
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jh6you said:
Even if it keeps almost 10 ohms, it does not matter because it means that there is very little current flow, and accordingly the voltage drop across the thermistor will be very small.

Furthermore, if the thermistor resistance is maintained high (near to 10 ohms) at operation temperature, it will still act as a inrush current limiter when I immediately turn on the switch after turn it off, without waiting till the themistor temperature is cooled down.

For the time being, I am using varisters for my power amps. I am thinking to add the thermistors, and I am wondering if I should use thermistor only or both. Which way shold I go?
 
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Hmm¡¦ regarding the energy and thermistor¡¯s capacity, I would simplify my thinking as follows: When the switch is turned on, the filter capacitors will be charged in very short time and its energy will be stored in the thermistor as heat. This implies that we need to prefer the thermistor having big enough capacity for the inrushing energy. Otherwise, the thermistor will be often overloaded and accordingly its life may be somewhat reduced. For example, if I decide to use the thermistor of 10 ohms at 25 deg C and steady-state current rating 5A, I would choose the biggest thermistor available among its class. I mean the largest one in physical size. Then, I could smash headsucking mathematics¡¦ :)
 
I think jh6you has the simplest solution, and one best suited to the Fearless DIYer: Get the biggest one you can! :)

The energy storage of your capacitors is always dependant on the voltage across them. Unless you meant that you were putting all of those caps directly across the the power line (and I wouldn't recommend it!) you want to stick with the voltage after the rectifiers for that particular formula.

There is something else, though... if you have one of those big toroids, it potentially draws a humongous magnetizing current of its own. I only wish I knew how that was calculated. :xeye:


EDIT: I think I see where you got the mains voltage in there. I just looked at the sample on their website. That schematic doesn't include a transformer... so the voltage would indeed be whatever the input voltage was (anywhere from 90 to 265V in their example).

You are using a transformer, aren't you?
 
eLarson said:
You are using a transformer, aren't you?

Yes, but not too big - only 250 VA
The big as possible sounds OK but I would like to be sure what's minimum and then take something above that value...

EDIT
just one more question:
in primary coil of transformer the current will be ~1.15A so It would be the better to use thermistor rated closer to that current (ie. 2A NTC is better than 5A NTC?) ?
 
eLarson said:


There is something else, though... if you have one of those big toroids, it potentially draws a humongous magnetizing current of its own. I only wish I knew how that was calculated. :xeye:

There is some formula in this link: http://www.tortech.com.au/article.html

The absence of a gap in the toroidal core means that the maximum possible remanence (residual magne­tization of the core in a particular direction and mag­nitude) can be substantially more pronounced in a toroid when compared to an E-I laminate. This residual magnetism is the mechanism by which old computer cores memories functioned. The core "stores" the static magnetic bias when the power is switched off. If the removal of power occurs at an unfavorable time, the strongest magnetic remanence will be stored in the core. When power is again applied to the primary, the peak inrush current may be as great as Vp-pk divided by Rp, where Vp-pk is the peak primary winding, depending on the power capability of the transformer, and on how strongly the core was magnetized. This inrush current peak occurs for a short time during the first or second half of the power sine wave.

Btw if I use the Elektors inrush circuit with two 10 ohm themistors in series instead of the four 10 ohm power resistors? Maybe also one additional 5 ohm/10 watt resistor for safety (if the thermistors are still hot). Will that work for an Aleph5 (with a time delay of 3-5 sec)??
 
I was planning two 10 ohm thermistor + one 5 ohm resistor is serie (25 ohm total cold). The resistor is there only as a safety for when the power is switched of/on short time after the relay closes (so when the Thermisters are still hot).
Maybe its overkill, but I have plenty of stuff lying around.
 
I'd think thermistors change resistance kinda fast, you'd have to be trigger happy.
Switched off, the high bias will make voltage drop fast on the caps, but with dropping voltage bias drops too.
Only with connected capacitors of a reasonable size the secondaries react as a shortcut for a fraction, the peak current of a toroid lasts only milliseconds.
With still a reasonable voltage on the caps, the toroid secondaries will not see a zero impedance, outgoing currents cant be very high, so inrush is limited too.
Only when the interval out/off is long enough the caps will run empty, but then the thermistors had enough time to cool.

Theoretical ******** of course, anyone should be familiar with coming from me, to be on the safe side the 5-watter is a cheap way of making sure i think.
 
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