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
I don't think this is an issue of power cycling. We had already gone through this after the previous failure. The OP is not cycling power.
Also, an overvoltage surge will REDUCE current, not increase current.
I took a look at my Parallax 45A converter. I have a 2 ohm, 18A thermistor. What's interesting is that the thermistor is tied to the heat sink of the input diode bridge. There are two possibilities for this:
1. It guarantees the device remains HOT while the converter is in operation. Hot means lower thermistor resistance and less heat dissipation.
2. The device gets too hot when standing along. It need a heat sink. As I mentioned before it could be the continuous 8W (or more) power dissipation that is stressing the part.
While doing battery resistance testing in the past, I've cycled this converter many times in succession with no failures.
Sal
edit, don't have Progressive converter, but Parallax.
Also, an overvoltage surge will REDUCE current, not increase current.
I took a look at my Parallax 45A converter. I have a 2 ohm, 18A thermistor. What's interesting is that the thermistor is tied to the heat sink of the input diode bridge. There are two possibilities for this:
1. It guarantees the device remains HOT while the converter is in operation. Hot means lower thermistor resistance and less heat dissipation.
2. The device gets too hot when standing along. It need a heat sink. As I mentioned before it could be the continuous 8W (or more) power dissipation that is stressing the part.
While doing battery resistance testing in the past, I've cycled this converter many times in succession with no failures.
Sal
edit, don't have Progressive converter, but Parallax.
lorelec wrote:Salvo wrote:
This is obviously a design flaw. I believe everybody in the converter business uses a NTC thermister to limit turn-on current.
Overvoltage has nothing to do with this failure. The device is not a transorb.
Sal
That's why I suggest trying a more gradual power cycling first to see if that helps. If not, then perhaps a design flaw. A hot thermistor will have very low resistance, and so will be susceptible to very large surge currents passing through it - which it will likely not be able to tolerate on a repeated basis. The caps may be discharging much faster than the thermistor is cooling, presenting a large surge next time the converter is turned on.
An overvoltage event will increase the peak surge current, and may overstress the thermistor.
