Converter In-Rush Thermistors etc, UPDATE -4
UPDATE on 17 Nov, Update 2 on 22 Nov, Update 3 on 16 Jan 14, Update 4 on 19 Oct 14. --------------- In a recent thread there was mention of how you can wreck your gizmo (in my case a converter) ...
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There's still way too much resistance in the device while the converter turns on. The failures are not caused due to the capacitor charging to 170V, but when the converter is in operation. The greater the converter load, the greater the stress on the thermistor.
That means you want to choose a thermistor resistance as low as possible. A 1 ohm thermistor consumes 5 times less power than a 5 ohm thermistor. If a 1 ohm device provides adequate current limit, then that's a far better choice than a 5 ohm device.
BTW, the 5 ohm part has an instantaneous failure at 300J. My previous power dissipation at 13A was 845W. The device just needs to hold 5 ohms for 355 ms for it to blow! The 1 ohm part has a much better chance of surviving.
Sal
That means you want to choose a thermistor resistance as low as possible. A 1 ohm thermistor consumes 5 times less power than a 5 ohm thermistor. If a 1 ohm device provides adequate current limit, then that's a far better choice than a 5 ohm device.
BTW, the 5 ohm part has an instantaneous failure at 300J. My previous power dissipation at 13A was 845W. The device just needs to hold 5 ohms for 355 ms for it to blow! The 1 ohm part has a much better chance of surviving.
Sal
ken white wrote:Salvo wrote:
Also to consider is the thermal time constant of the thermistor. The 20A, 5 ohm device has a time constant of 194s. That sounds like the device will hold 5 ohms for quite a while.
Sal
Just an FYI, the thermal time constant is calculated using values measured from the maximum hot temperature R to the ambient R.
It can't be used to calculate the negative change in R when operational current flows since I is not known and is a function of the value of C and series resistance of the transformer windings and diode bulk resistance...
