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Old 16th August 2006, 04:30 AM   #1
jarthel is offline jarthel  Australia
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Default how to select an ntc thermistor?

I will be using them on the primary side of the transformer.

amperage: I assume this should be equal or greater than fuse rating?

resistance: no idea at all how this is chosen

I was looking at an Epcos thermistor datasheet and it seems for all resistance values, the "start-up" time is approximately 90secs.

Thank you for the help.
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Old 16th August 2006, 05:40 AM   #2
lgreen is offline lgreen  United States
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Default hi

Hi, good question.

I have used thermistor CL-60 successfully for startup, takes about 1 second to go to very low resistance. I have also used CL-30 which did not work out so well (only 3 ohms cold).

I would say that most people aroiund here simply use the CL-60 as it has a large current capability, reasonable resistance and is available from digikey.

In Class A amps its easy but in Class AB with primary current variable some think it could current starve your power supply, but its up to you.
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Old 16th August 2006, 10:08 AM   #3
AndrewT is offline AndrewT  Scotland
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some clever guys here could set up a simulation for us to enable selection, but I afraid to say we have two choices; copy someone elses successful circuit or experiment by trial and error until you find a combination that works.

The problems are three fold.
Cold resistance to limit the intial surge current.
Warm to hot resistance to minmise the voltage drop once upto operating currents/voltages.
Power/energy absoption/dissipation in changing from cold to the warm/hot state.

The big ones with high energy dissipation (joules) are expensive, tempting users to try smaller ones and this goes so far that the thermistor blows while trying to absorb the current spike.

Good luck.
regards Andrew T.
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Old 16th August 2006, 01:25 PM   #4
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one of the problems with NTC resistors is that if the power supply is turned on again (and at the wrong moment) , before the resistor has had a chance to cool, you will have the same inrush problem as before.

for safety's sake you should make sure that there is a bleeder resistor across the filter caps -- i use the rule of thumb that the bleeder should reduce the cap voltage by 90% within 30 seconds.

and even this residual voltage can give you quite a shock -- and a lot of other unexpected things happen with capacitors -- read what Bob Pease has to say on capacitor "soakage" on the National Semi website.
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Old 2nd April 2007, 05:54 PM   #5
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hi! I have a similar question as the OP... what's the relationship between a thermistor, fuse rating, toroid, filter capacitance, etc.? I am looking to see if this is necessary for my application... One of the designs for the Pass Ono uses thermistors, but I cannot understand why they are necessary.
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Old 2nd April 2007, 06:40 PM   #6
AndrewT is offline AndrewT  Scotland
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forget smoothing capacitance for the moment. That's a different design problem.

Considering just the mains voltage, toroid transformer and primary circuit fuse, The mains voltage at worst case sees just the transformer primary winding RESISTANCE and the mains side cabling resistances and tries to push a surge current into the primary. This can be many tens of amps with the bigger toroids.

Very quickly the transformer core builds up a magnetic circuit and within just a few mains cycles behaves as a true transformer.

The extra resistance provided by the power thermistor reduces the surge current. While it does this it absorbs considerable energy and heats up.
This heating reduces it's resistance and lets most of the mains voltage through to the toroid. I prefer to then switch in a bypass to allow the thermistor to cool and to ensure minimum source impedance for the transformer. I have seen recommendations for a bypass delay of just 100mS but I prefer 200 to 300mS. any longer is a waste.

Eliminating the surge allows a close rated fuse in the prinary circuit. a 240Vac 500VA transformer can probably run on a T2A fuse.

Now to smoothing capacitance.
The capacitor input filter after the rectifier starts at zero volts (=no charge) and absorbs current from the secondary through the rectifier. The capacitor and the rectifier can be damaged or their lifetime shortened by these initial high start up currents. Adding a power thermistor to the secondary circuit reduces the initial current peak and preserves these components. Again, bypassing is recommended, this time after about 5seconds rather than the 0.2seconds needed for the primary bypass delay. Fuses in the supply rails should be placed after the main smoothing caps.
regards Andrew T.
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Old 2nd April 2007, 09:57 PM   #7
cpemma is offline cpemma  United Kingdom
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Ametherm (their NTC Inrush Limiters are available from Rapid Electronics in the UK) have some info on their site, including a calculator for transformer applications (though it needs some way to measure the surge current).
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Old 2nd April 2007, 10:00 PM   #8
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Originally posted by cpemma
Ametherm (their NTC Inrush Limiters are available from Rapid Electronics in the UK) have some info on their site, including a calculator for transformer applications (though it needs some way to measure the surge current).
Easy to estimate with a 0R1 resistor in the neutral end and a differential pair of 'scope probes.
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