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
What was missing in previous calculations was actual ac line and converter ac input resistance. In my case, it's about 1.7 ohm. My test setup and ac power source is about 10 feet away from the main breaker. That means I don't have a long ac cable run to my test station. In the OP's case, I think we can add 1.5 ohm resistance to a peak current calculation.
That means if the converter is turned on when the ac is at it's peak, max current is:
With 2 ohm thermistor: I = 170V / (1.5 + 2 ohm) = 49 A
With 5 ohm thermistor: I = 170V / (1.5 + 5 ohm) = 26 A
The 2nd cycle is 78% of the first, so 38A for 2 ohm and 20A for 5 ohm.
Since the diode bridge has a 300A surge rating, it is well protected with the 2 ohm thermistor.
Sounds like PowerMax doesn't understand the failure mechanism. I don't believe the issue is a fast on/off/on sequence. You don't see large surge currents when the cap bank is partially charged. In my testing, I had to wait 10 min between turn-ons to ensure the caps are discharged to less than 1 V.
Let's compare a hot turn-on to a cold one.
The thermistor instantaneous power consumption of a cold turn-on is:
P_cold = I^2 * R = 26A^2 * 5 ohm = 3,380 W
The thermistor instantaneous power consumption of a hot turn-on is (note: cap bank is at 75V & thermistor at 0.03ohm):
P_hot = ((170V - 75V)/1.53ohm)^2 * 0.03 ohm = 116 W
The thermistor is not getting stressed that much during a hot turn-on.
That is not the problem.
Sal
That means if the converter is turned on when the ac is at it's peak, max current is:
With 2 ohm thermistor: I = 170V / (1.5 + 2 ohm) = 49 A
With 5 ohm thermistor: I = 170V / (1.5 + 5 ohm) = 26 A
The 2nd cycle is 78% of the first, so 38A for 2 ohm and 20A for 5 ohm.
Since the diode bridge has a 300A surge rating, it is well protected with the 2 ohm thermistor.
Sounds like PowerMax doesn't understand the failure mechanism. I don't believe the issue is a fast on/off/on sequence. You don't see large surge currents when the cap bank is partially charged. In my testing, I had to wait 10 min between turn-ons to ensure the caps are discharged to less than 1 V.
Let's compare a hot turn-on to a cold one.
The thermistor instantaneous power consumption of a cold turn-on is:
P_cold = I^2 * R = 26A^2 * 5 ohm = 3,380 W
The thermistor instantaneous power consumption of a hot turn-on is (note: cap bank is at 75V & thermistor at 0.03ohm):
P_hot = ((170V - 75V)/1.53ohm)^2 * 0.03 ohm = 116 W
The thermistor is not getting stressed that much during a hot turn-on.
That is not the problem.
Sal
