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Old 18th December 2004, 04:20 PM   #1
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Default Sizing thermistors

How should I go about choosing a suitable thermistor for an amplifier's power supply?

If I was leaving it permanently connected, and using linear regulators at the output of the supply, would I want to choose the thermistor so it's lowest resistance was achieved at the current continually flowing down the regulator's adj pin? For the case in question, that would be around 2mA of constant current, with the amplifier it's self capable of around 40mA peaks.

Or would it be worth choosing it's required current as somewhere close to the maximum the winding is rated for and then having it switched out of the circuit by a relay once the supply was charged to the operating voltage?
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Old 18th December 2004, 04:30 PM   #2
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First I don't understand completely the question. Secondly what is your goal really? Can you explain what you are after?

I have never heard or seen anybody who has thermistors at this place.
/Per-Anders (my first name) or P-A as my friends call me
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Old 18th December 2004, 05:28 PM   #3
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The surge current through the winding when switching the supply on is about two or three times higher than the winding's continuous DC rating.

Also, the surge produces a substantial voltage spike that rings as the capacitors charge to their operating voltage. I am already having to use quite high voltage, read expensive, components and this surge spike is likely to put a lot of stress on them.

So I would prefer for the supply to turn on slowly, perhaps over thirty or sixty seconds. The heaters have to warm up anyway, so it's not as though I need the supply instantly.
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Old 18th December 2004, 09:15 PM   #4
cpemma is offline cpemma  United Kingdom
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I don't quite see how you'll stop the switch-on surge with a thermistor in the regulator control. Too late. Isn't it more effective to use an NTC Inrush Limiter type in series with the transformer primary?

These are normally rated by the current carried, so the 1A type in my catalogue is 120 ohms cold, 2.29 ohms after warming up.

Putting a thermistor on a regulator to act as a sort of thermal control based on the heating effect of the adjustment current isn't feasible AFAICS, the elevated equilibrium temperature reached would be too sensitive to a myriad factors affecting rate of heat loss.
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Old 18th December 2004, 09:42 PM   #5
johnnyx is offline johnnyx  United Kingdom
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Thermistors go in series as suggested by cpemma above.

To stop voltage spikes, I use a varistor (MOV - metal oxide varistor) directly across the transformer primary. One rated at 275vac is good for 240v mains. This absorbs the spike energy, preventing it from doing any damage.
Hope this helps
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Old 18th December 2004, 09:58 PM   #6
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Sorry, I think I might have phrased my question confusingly, I should have really said NTC thermistor instead of just thermistor.

I mentioned the adjust current because this is the most constant current drain on the amplifier, but it's only around 2mA. At it's absolute maximum peak, the amplifier could draw up to about 40mA.

The idea is as you say, to put an NTC on the winding to limit the inrush of current when switching the supply on.

Are the currents the NTC's are catagorised by their maximum current capability or the current they need to get to their warm operating resistance?

The manufacturer I'm looking at quotes a 25 Celsius still air resistance, an R/T curve profile, a beta and an alpha value.

Would their wattage rating be under normal, warm, resistance conditions or throughout their entire cold to warm phase?
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Old 19th December 2004, 04:58 PM   #7
cpemma is offline cpemma  United Kingdom
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A bog-standard NTC thermistor is designed to be run with very low currents to avoid any self-heating, and responds to changes in ambient temperature.

The current-inrush limiter types (again NTC) are calculated to get hot due to the steady-state current through them, but the higher cold resistance limits start-up surges in motors, power supplies, etc.

This link may help.
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Old 19th December 2004, 07:03 PM   #8
jleaman is offline jleaman  Belgium
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How many thermisters do yo haev to use in a aleph 2?
Do you have to use any ?

And if you have to use them you parallel them on the ac side right ?
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