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
Salvo wrote:The time constant of 194 seconds does not seem to be right, as least from my minimal understanding. Maybe I'm reading this wrong.:?
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. The converter will be on after a fraction of a second.
If the converter is connected to a (low) battery, the device is now consuming power: P = I^2 * R = 13A^2 * 5 ohm = 845W (that's huge!)
We know that's not possible. Due to the thermistor's voltage drop, the converter will operate at reduced current and somewhat lower power dissipation. Still, the problem is that the thermistor is too large. It's thermal time constant is too big. That's the reason it's getting destroyed.
Parallex converter solved this problem by connecting the thermistor to the diode bridge heatsink. The device absolutely needs a heatsink.
Sal
