Solid State Fuse

Your description of the application implies a simple fixed current limit with duty cycle monitor for pass element over-heat protection. And this duty monitor will consist of, and get this, a three component analog computer, aka R-C network driving a comparator with hysteresis, that simulates the constant rate of heat dissipation into the pass element thermal mass of fixed capacity. Said monitor then trips the pass element OFF to ON or LIMIT and back to OFF, as dictated by operating conditions, attempting to maintain the pass element junction temperature at Tj,max. This approach will be then be optimal in the sense that it is minimally adaptive to an otherwise unpredictable intermittent fault set. If battery charge consumption is a consideration then there are any number of ways to work this into an analog computation also, say by short term averaging of the fault events and charge gauging combined with a simple time-into-flight prioritization of the instantaneous current limit, which need not remain constant as either a threshold or limiting cycle parameter. But if all you can think of is "fuse" and "no moving parts" ( damned joke that one)- go back to reading your popular mechanics magazines.

John...

yes, I think you are correct, the polyswitch in the active region tries to maintain a constant temperature therefore constant power, so as the voltage across it goes up, the current through it must go down which indicates it is a negative resistance (over a long time constant) . So I agree, even if the load is a straight constant resistance, after an overload is removed, the polyfuse can latch and hang and fail to reset until the power is removed i.e. the system IS bi-stable. It's the same as foldback current limiting. Thank you for pointing this out about polyfuses..., I never realized it before.

Mark

hehe.. well, no moving parts meaning I can't have relays in a rocket because the vibration possibilities are wide, also I was thinking someone would tell me to use an automotive flasher and I didnt want to see that one.

For the record, I can take about 5 seconds of popular mechanics before I put it down.. but I'm not sure why you would offer some good suggestions and then in following suggest I read a low-grade magazine, to each their own I suppose there is some general rule in force to take part in online hazing or be subject to escalation by the more brainwashed marginals addicted to it.

My conclusion is I'll have suggestions for the guy most likely to implement the DC-DC converters and approach to distribution, and he will do what he thinks is best.

Correction..

I should have said negative INCREMENTAL resistance... Mark

Yes, the LTC1153. Some chips have the "C" others don't, at random. The LTC4213 is specified to 6V only. I wonder if it could be lifted with a Zehner underneath or such.

Rene

Not random. LTC's scheme is this: The LTCxxx parts are all CMOS, whereas the LTxxx parts are bipolar. But LTC doesn't reuse numbers between their LT and LTC parts, so in a sense both prefixes are extra characters.

Thanks that was informing.

Rene

ON Semi electronic fuse