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) ...
Forum Discussion
Interesting photos. First they had the 2 emi inductors up front and then removed one. I guess their thought was to let emi go to pot. It also appears the wire in those inductors are too small. Does that look like a 10A carrying conductor? Perhaps they removed one of the inductors because they were getting too large of a voltage drop. They didn't want to incur the expense of winding a bigger wire. They probably would have needed a larger toroid.
I don't know why they moved the thermistor closer to the ac input. Everything is in series (between ac input and the rectifier diodes), so it shouldn't matter. My old Parallax converter has it placed right before the diodes. The body of the thermistor is then tied to the diode heatsink.
I think the first failure was due to the thermistor current rating was too low. You like to have 50% derating between max allowable current and typical current. The 20A thermistor gives that derating. They also went from a 2 ohm part to 5 ohm. My thought is that the 5 ohm part remains hotter during normal operation. This heat stress eventually causes it to fail. There's no gain in using the 5 ohm part. A 1 or 2 ohm part will work just as well. They will have significantly lower on resistance.
Anyways, if you get several thermistors, experiment with one. Install it and cycle converter. If my theory is correct the new part should work fine for a while; toggling and all. No amount of babying the converter will prevent another failure if it's caused by cumulative heat stress. Be sure to use an infrared thermometer and measure thermistor temperature.
Sal
I don't know why they moved the thermistor closer to the ac input. Everything is in series (between ac input and the rectifier diodes), so it shouldn't matter. My old Parallax converter has it placed right before the diodes. The body of the thermistor is then tied to the diode heatsink.
I think the first failure was due to the thermistor current rating was too low. You like to have 50% derating between max allowable current and typical current. The 20A thermistor gives that derating. They also went from a 2 ohm part to 5 ohm. My thought is that the 5 ohm part remains hotter during normal operation. This heat stress eventually causes it to fail. There's no gain in using the 5 ohm part. A 1 or 2 ohm part will work just as well. They will have significantly lower on resistance.
Anyways, if you get several thermistors, experiment with one. Install it and cycle converter. If my theory is correct the new part should work fine for a while; toggling and all. No amount of babying the converter will prevent another failure if it's caused by cumulative heat stress. Be sure to use an infrared thermometer and measure thermistor temperature.
Sal
