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
The problem now seems to be well defined. The thermistor is blowing when the battery is connected after the converter has been on for a while and not connected to the battery. Is this correct?
This tells me a few things. The converter inrush current at turn on is not the problem. The thermistor will handle the initial inrush current.
So what is happening?
The converter is turned on and the initial inrush current charges the input capacitors. The thermistor has heated up somewhat during this time and the resistance is going down. The thermistor is rated to charge this input capacitor. Now the converter is sitting there with no load on it. This means there is very little current through the thermistor and it is now cooling off. The resistance is going up toward the cold value. Then the battery is connected. This is the same as turning on the converter with no load on it. There is a large inrush current that the thermistor moderates. The problem now is that instead of charging the input capacitors you are trying to charge the battery. This is the same as having a huge input capacitor that the thermistor is tying to charge. It is not rated for that and is being blown. This is a design defect!
What to do?
There may be nothing you can do without a redesign. You need a much bigger thermistor, don’t know if they make such a thing. One thing I would try is connecting several 5 ohm thermistors in parallel. I know you should not parallel thermistors. They may share current at startup enough to provide the energy absorption need at battery connection. I’m not going to try to calculate the best configuration of thermistors but maybe someone else will.
Hope this helps.
This tells me a few things. The converter inrush current at turn on is not the problem. The thermistor will handle the initial inrush current.
So what is happening?
The converter is turned on and the initial inrush current charges the input capacitors. The thermistor has heated up somewhat during this time and the resistance is going down. The thermistor is rated to charge this input capacitor. Now the converter is sitting there with no load on it. This means there is very little current through the thermistor and it is now cooling off. The resistance is going up toward the cold value. Then the battery is connected. This is the same as turning on the converter with no load on it. There is a large inrush current that the thermistor moderates. The problem now is that instead of charging the input capacitors you are trying to charge the battery. This is the same as having a huge input capacitor that the thermistor is tying to charge. It is not rated for that and is being blown. This is a design defect!
What to do?
There may be nothing you can do without a redesign. You need a much bigger thermistor, don’t know if they make such a thing. One thing I would try is connecting several 5 ohm thermistors in parallel. I know you should not parallel thermistors. They may share current at startup enough to provide the energy absorption need at battery connection. I’m not going to try to calculate the best configuration of thermistors but maybe someone else will.
Hope this helps.
