Electronics > Beginners
Can a variable thermal shut off circuit be build?
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paulca:

--- Quote from: Zero999 on August 01, 2019, 10:32:38 am ---An automotive relay certainly will not be able to break 100V at 75A.

--- End quote ---

I 'believe' an automotive starter motor using a much smaller/normal 12V relay to drive a solenoid which slams a conductor pin against the starter motor contact.  It is that contact which takes the 100-400+Amps of cranking current.  Obviously that solenoid is able to break the contact as well.  I would expect it has a seriously beefy spring on it.

I also expect this is normal in switching high currents.  IE.  a small relay closes to drive a solenoid/motor for a mechanical connection that takes the high current.
coppercone2:

--- Quote from: FriedMule on August 05, 2019, 10:05:17 am ---The contactor looks like a great unit and it looks like they have no problem handling high power!
I do just need some way to make the max temp variable and latch, and that should my circuit almost be able to, I just need to find out the problem as showen in my circuit. :-)

--- End quote ---

i recommend then making a gross overload safety with a thermostat that is way up there but fire safe, and a secondary adjustable safety that uses electronics. If you want to be really safe put them on separate contactors. or with more thought and analysis you can make a or-gate type circuit that triggers the contactor (but don't actually use transistors with the fire safety)

the main problem I see is that your temperature sensor is running in parallel with wires carrying 100 amps. i have no idea what that can do to a RTD, thermistor, thermocouple or other signal, it sounds kinda bad and like it requires shielding at least, if you can have the signal for the temperature wire come in at a right angle it would be better, but I assume its basically strapped to the main cable via zip ties for the sake of not being ridiculous. The thermostat kind of gets around alot of these problems but its not adjustable or precise. Someone with high current industrial experience can tell you what to expect, I never worked with a crazy circuit that required a signal line routed in parallel with a giant conductor.

But in terms of reliable function I think the contactor is basically a requirement for safety.

Keep in mind the electronics control system connected to the temperature sensor can get a malfunction from spikes possibly, or just get damanged from bad manufacturing or defects or age or whatever (i.e. arcing, surges), so its highly recommended to make it a one way trip that needs a manual reset if triggered so it can't turn it self back on. Another way to do this is to trip a breaker with a contactor that shorts out the supply, like a crowbar circuit, and blows a fuse.

I think the automotive stuff is more dealing with inrush survival from batteries , it sounds like you have a regulated supply that you just need to disconnect reliably.
Rick Law:

--- Quote from: coppercone2 on August 05, 2019, 04:35:10 pm ---
--- Quote from: FriedMule on August 05, 2019, 10:05:17 am ---The contactor looks like a great unit and it looks like they have no problem handling high power!
I do just need some way to make the max temp variable and latch, and that should my circuit almost be able to, I just need to find out the problem as showen in my circuit. :-)

--- End quote ---

i recommend then making a gross overload safety with a thermostat that is way up there but fire safe, and a secondary adjustable safety that uses electronics.
...
...

--- End quote ---

Actually, for real safety, you are best off using a smaller power source for smaller needs.  If there is some failure, 75Amp even at mere 10V can cause quite a bit of damage.  You may never get to the stage of "too hot".  Way before getting hot, something else got vaporized, melt, or sparked or...  750 Watt of power waiting to hurt something...

Power can be use for good or evil.  So give it enough power to do its job.  Anything more is just increasing the potential damage.
coppercone2:
oh man I assumed the load can take it and its hooked up to something like a glo-bar light source or something

Rick Law:

--- Quote from: coppercone2 on August 05, 2019, 07:36:06 pm ---oh man I assumed the load can take it and its hooked up to something like a glo-bar light source or something

--- End quote ---

The OP left an earlier reply that it is just "nice to have" instead of for any specific application.  Nothing wrong with that, I like to have something powerful to play with too.

Being a cautious person that I am (some would call me a pessimist), I worry about the price of failure before I contemplate the disposition of the fruits of success.  So I rang the alarm bell to make sure the magnitude of potential damage is not overlooked.  Not much there, just me having a typical pessimistic day.

I sure like to have a flame thrower to play with -  something like that could be kind of fun.  But I surely would not use it to light my cigarette - may be once for fun but for sure not as daily use lighter.
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