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:
No surprise. Nothing had changed from the previous thermistor failure. We already knew the initial power-up was not the problem. The problem is P = I^2 * R. Where I is the 10A to 13A when converter starts operation and R is the resistance of the thermistor. The resistance is too high! Use 1 or 2 ohms. It probably also helps if a heatsink is attached to the thermistor.
Sal
I disagree. the issue, and it relates to figuring out what is the failure mechanism, is that BFL is connecting the powered on converter to a very deeply discharged battery, when the likely design criteria assumed that the converter would be powered on with a battery or other load.
This subjects the converter to a near instantaneous high current demand from the converter, something it won't see if connected to a deeply discharged battery before powering on. If connected to a discharged battery before power on, the converter can ramp output voltage to around 11V before it sees a load, and then the current ramp up will be determined by and limited by the converter design.
Without schematics it's hard or impossible to determine exactly what is happening under BFL's load condition. all we know is that the thermistor blows, that's at this point a symptom of a problem, nothing else. It may be protecting something else, it may be that the converter goes open loop for a while, who knows. This is not a linear power supply, it is a switching supply and they can behave or misbehave in bad ways when subjected to load conditions they weren't designed to handle. And we don't know if it was designed to handle BFL's load condition.
doing something to keep the thermistor from blowing in this use case may hide or solve the exact failure mechanism, or it may end up causing something else to fail later.
all we see is the symptom, a blow thermistor, that's all we have to go on at this time.
What is unknown is the root cause(s) of the problem, and without a set of schematics, that likely isn't going to be found.
