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
Wasn't the first thermistor 2 ohm? My Parallax converter also has 2 ohms. I've shown by analysis that you need a minimum of 0.6 ohms to meet max rating on diodes.
I did some turn-on tests on the Parallax. The 170V powers-up (un-loaded) in about 4 60Hz half cycles. The first half cycle stresses the parts the greatest. The capacitor bank (2400 uF, which is the same as the OP's) charges to 75V in about 5 ms during the first half cycle. The current is calculated:
I = C * dV/dt = 2400 uF * 75V/5ms
I = 36A
That's not bad at all. I believe the diodes can take 300A peak surge. This thermistor is tie wrapped to the diode bridge heatsink. I've removed the heatsink connection for these tests.
Edit:
Given the calculated 36 A, we can also calculate the ac input resistance:
R = V / A = 120Vrms / 36A = 3.3 ohms
So we have about 2 ohms from the thermistor and 1.3 ohms from, converter input inductors and ac house line resistance.
Sal
I did some turn-on tests on the Parallax. The 170V powers-up (un-loaded) in about 4 60Hz half cycles. The first half cycle stresses the parts the greatest. The capacitor bank (2400 uF, which is the same as the OP's) charges to 75V in about 5 ms during the first half cycle. The current is calculated:
I = C * dV/dt = 2400 uF * 75V/5ms
I = 36A
That's not bad at all. I believe the diodes can take 300A peak surge. This thermistor is tie wrapped to the diode bridge heatsink. I've removed the heatsink connection for these tests.
Edit:
Given the calculated 36 A, we can also calculate the ac input resistance:
R = V / A = 120Vrms / 36A = 3.3 ohms
So we have about 2 ohms from the thermistor and 1.3 ohms from, converter input inductors and ac house line resistance.
Sal
