Converter In-Rush Thermistors etc, UPDATE -4

BFL13 wrote:
I don't care what R it is, I just want one that won't blow! :)

So remind me again - when does it blow?
1) During steady state operation? 2) During cold start? 3)Hot start?

I thought it was 2) - cold start. I thought I read that care was being taken to avoid hot starts, and it was never failing except at startup - which led me to think we were dealing with a 2) cold start issue.

Let's briefly discuss some of the options:

In general, changing the resistance (the "zero power" resistance, which is the cold start resistance) is how you limit the charging current. It protects diode bridges, transformer windings, etc. from too much inrush current. It limits inrush current

Changing the max steady state current limit is how you protect the thermistor against damage during steady state operation.

Changing the Joules rating (which is the same as the capacitance rating if you know the charge voltage since joules=1/2 CV**2) is how you prevent the device from being damaged during cold start.

(Above when I say "changing" I mean selecting a thermistor having a different rating.)

Hot start protection is handled a couple of ways. One is to choose one with a low thermal time constant. It will cool more quickly after shut down. None will cool instantly, so the most common way to deal with that issue is to add a relay. Once the thermistor has done its work at limited inrush current, it gets bypassed, no current flows through it, and it cools so it's instantly ready for another cold start. If it fails at hot start, this is the best option. You just bypass the thermistor with a time delay relay.

Now, why have some been recommending 5 ohms, while others do not? From the above, one would think that the proper response would be to select a thermistor having a higher Joules rating. The problem there is that the selected thermistor seems to have a more than sufficient Joules rating. All of those selected do, and all seem to have way more than required.

Nonetheless, the thermistor is still failing, and as I understood it, it was failing at startup. If that is the case, then what options are left? One can certainly select a higher resistance - after all the resistance calculation produces a minimum, and higher resistance is allowed. In general, the higher resistance reduces stress on the device at startup, all things being equal. Of course, "all things" aren't equal. You have to select another thermistor. It will have different specs. But as long as the Joules rating is the same or higher, cold startup is likely to produce less stress on the device. It also produces less stress on all parts of the circuit. Current flow is reduced. Rate of change of voltage is reduced, which may help out if the problem is inductively caused.

OTOH, as long as the device installed has specs sufficient for the circuit conditions it faces, it should work. A 5 ohm may have a longer time constant, which may result in more problems with hot start. It may have higher steady state resistance, which causes more power to be dissipated in it. So the selection between 5 ohms and 2 ohms is a tradeoff. That's always the case with electronics.

The problem here is we don't know why the device is failing, given that all those installed seem to have sufficiently high specs for what we know about the circuit. I still think that the simplest test is to go with higher resistance (and higher Joule rating). It should produce better performance with respect to the other circuit components, which remain unknown to us, by reducing inrush current.

Basically, a thermistor is a type of "soft start" circuit and the higher the resistance when cold, the "softer" the startup. I don't see any advantage to lower resistance except possibly with regard to thermal time constant/hot start issues.

There may be some steady state improvement by using a lower resistance, but in general, if the steady state current spec of the thermistor is met, then it's met and problems are unlikely. The steady state of the circuit is much easier to understand, even when we don't know the details of the other circuit components. I'm willing to accept that the other (unknown) circuit components could affect the dynamics of startup in a way that causes the thermistor to blow. I find it much harder to believe they would cause steady state thermistor failure when we know the steady state current and the steady state current rating of the thermistor.

If hot start is the problem, then greater care with respect to operation will help - wait longer after turn off. If it turns out that using more care to avoid hot starts works when a higher resistance/ higher Joule rating thermistor is installed, then adding a time delay relay might be the next step. It will ensure the device is cold when a restart is needed, without requiring any waiting period. There is a reason why such relays are often added to the circuit design to bypass the thermistor - even though they add expense.