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
No, you're changing the conditions of operation. Initially you were suggesting a generator surge, while the converter is in operation, can cause this failure. You thought if ac voltage goes up, so will input current. This is wrong, wrong, wrong! As long as the converter is in regulation, it is a constant power device: P_in = V_out * I_out / efficiency. If V_in goes up, I_in has to go down. This is elementary. *If* I_in goes up during a voltage surge then the converter isn't doing it's job.
Now you're saying if nothing is attached to the caps, you would see a current surge. This is true. During power-up, the converter is disabled until the 170V cap is more or less fully charged. This is the only time when an increase in voltage also increases current.
Sal
Now you're saying if nothing is attached to the caps, you would see a current surge. This is true. During power-up, the converter is disabled until the 170V cap is more or less fully charged. This is the only time when an increase in voltage also increases current.
Sal
lorelec wrote:Salvo wrote:
The caps are feeding the constant power converter. As soon as input voltage goes up, current demand is reduced. There is no current surge.
On the other hand, during power-up, the converter does not turn on until the cap voltage (I'm guessing) is at least 120V. The requirement for current turn-on surge protection is still there.
Sal
Charging of the input filter caps is totally separate from the converter that is attached to them downstream. You could have *nothing* attached beyond the bridge and caps, and you would still get a large surge at turn on (assuming no inrush protection), just from the filter caps charging. Surely you know this, so I'm not sure why we're debating it. To quote you from your later post:
"Once you turn on the ac power there's a huge inrush of current into the capacitor as it charges up to about 170Vdc. That current needs to be tamed. That's the function of the NTC thermistor."
