Electronics > Beginners

Can a variable thermal shut off circuit be build?

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FriedMule:
Sometimes when I do test of circuit I would like to be able to shut off the PSU / Load leads if my circuit gets to hot.
Imagine a thermal sensor that are measures a components temperature and if it exceeds the temperature I select, it shall shut of, not the unit but the test leads.
Like if I am testing a DIY circuit and it i.e. gets above 120F after longer time test 3-4 hours into the test.

It is important that the "cut off" part is as "invisible" for the test result as posible, so it do almost not change the power, resistance or anything like that.

Edit: forgot to say that we are talking about max 100V or 75A (max voltage one of my psu can deliver and max ampere an other psu unit can deliver)

Rick Law:

--- Quote from: FriedMule on July 31, 2019, 07:33:37 pm ---Sometimes when I do test of circuit I would like to be able to shut off the PSU / Load leads if my circuit gets to hot.
Imagine a thermal sensor that are measures a components temperature and if it exceeds the temperature I select, it shall shut of, not the unit but the test leads.
Like if I am testing a DIY circuit and it i.e. gets above 120F after longer time test 3-4 hours into the test.

It is important that the "cut off" part is as "invisible" for the test result as posible, so it do almost not change the power, resistance or anything like that.

Edit: forgot to say that we are talking about max 100V or 75A (max voltage one of my psu can deliver and max ampere an other psu unit can deliver)

--- End quote ---

Until someone with more experience comes along and give you a better option, let me share my hobbyist experience...

(Beside, I have to jump in to see what you want to do with 75Amp!!!)

I do what you wrote about all the time but at 12V 2A.  The same can be done by using a better relay rated 100V 75A.  75A is a hell of a lot of current and that is going to be your road block.  Whether you can do it or not would be decided upon if you can find a relay that size.  My full size fridge takes 6.5A (@100V) only, 75A is more than 10 full size refrigerators like mine!?  75Amp (at any voltage) is still a lot for any relay to handle.

You should consider controlling the PSU's power-in instead of power-out.  See the last paragraph.


I use a simple Arduino NANO with LM35z as temperature probe to drive a relay.

- The LM35z is able to measure −55°C to 150°C.  When you are doing just +C temperature, the connection is a lot easier.  You can also offset the LM35z with a couple of diodes so you don't need -Vin as shown in the datasheet (page 16).  The LM35z will give you 10mV/degree C for the Arduino to read.
   Link: http://www.ti.com/lit/ds/symlink/lm35.pdf 

- To "connect" the LM35z as probe, I just tape the LM35z onto what I want to measure.  Typically, I am measuring a 18650 battery, so getting a good thermo-contact is not hard.  You will have to work that out properly.
If what you want to probe is smaller than the LM35z, you are out of luck with this method.

- The Arduino NANO reads the analog port connected to the LM35z and make decisions based on input voltage.  In my case, I also display the temperature on a 20x4 LCD and some times I output it back to the PC for logging.

- I use the regular cheap Arduino relay module like the one discussed in this thread:
   Link:https://www.eevblog.com/forum/beginners/chinese-relay-module-reliability/ 
  The module I use has relays rated for 250V @ 10A, I am only doing 12V@2A only.   Arduino output port alone doesn't have enough power to drive even those small relay modules, the module has its own +5Vin which is necessary.  You will need way way way better relay modules if you are going to do 75A!  You can also use the cheap module to drive another (bigger) module with the 75Amp relay.  I suspect the 75 Amp relay would be your difficulty.
Make sure you read that other thread and understand the reliability and issues with this cheap relay module if you end up using that to drive power-in.

- Alternative is:  Just relay the input side of the PSU -- if the PSU power-in doesn't exceed the "easy to obtain" relay's spec, let the relay control the power-in for the PSU rather than having the relay module handle power-out at 75Amp...

Good luck there...

EDIT: Forgot the links - added now.  (Also added the last section suggesting relaying the PSU input instead of output)

GerryR:
An automotive starter relay can handle the current (12 volt coil) or a motor contactor used in industrial applications (different control coil voltages available).  Thinking about the current, sounds like your getting into the welding arena, so a welder control relay may also be a possibility.  :-//

Zero999:

--- Quote from: GerryR on August 01, 2019, 10:25:12 am ---An automotive starter relay can handle the current (12 volt coil) or a motor contactor used in industrial applications (different control coil voltages available).  Thinking about the current, sounds like your getting into the welding arena, so a welder control relay may also be a possibility.  :-//

--- End quote ---
An automotive relay certainly will not be able to break 100V at 75A.

One thing to note is that the original poster might not be running at 75A and 100V, as they mention two power supplies, but neglect to mention the VA of them both.


--- Quote from: FriedMule ---we are talking about max 100V or 75A (max voltage one of my psu can deliver and max ampere an other psu unit can deliver)
--- End quote ---

FriedMule:
Yes 75A is an insane amount og current, but nice to have, because, hmm, just nice to have. :-)
The 75A is a 10V unit and I do newer think I'll use that many amps, but why not build so it can be used?

I imagine some sort of turn button that when turned, changes the max temperature on a display and if the real temperature on the probe is going above, then off!
My reason for wanting to "cut" the leads and not the PSU is that I have heard so many times, that: "if you want a reliable and precise result, keep the unit on for at least xx hours"

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