[SOLVED] Temperature cut off?

[Englewood]

I have a change of design.

Our safety system outputs 24V to a door switch or a flow switch, when the switch is broken the system cuts off.

I want a LM35 to run off a 24V supply and when the LM35 reaches 60C i want the system to shut down.

Now i know the LM35 works by 1c = 10mv, so max is 1.5v (150c)

So when the LM35 gets to 60C, there is a voltage of 0.6v on the output.

How can i keep a 24v output below 60C and at 60C or above the output is 0v and cuts the system

Is this possible?

 

[D.A.(Tony)Stewart]

Analog Comparator gives logic level out to drive logic level High side switch. But if thermal sensor is just for regulator, does it have have thermal shutdown?

I would recommend a smart high-side switch with thermal shutdown used in automotive. Tons avail from D-K.

https://www.digikey.com/product-sea...tches-load-drivers/2556406?k=High side switch

eg. https://www.digikey.com/product-detail/en/VN7050ASTR-E/497-14300-6-ND/4552533

 

[Englewood]

Hi Sunny,

So the high side switch will shut down at a certain temperature?

My electronics is quite basic as you can proberbly tell.

Looking at the data sheet it says operating temperatures between -25 to +85c.

 

[D.A.(Tony)Stewart]

Yes , many protection circuits such as short circuit, over temp, over voltage, depending on your needs.

 

[d123]

Yes it is possible, there are lots of ways, more than I know to say the least, I'd listen to the SUnnySkyguy.

Another approach is to use an inverter. The 7404 is a ready-made low voltage device (not good for what you now plan to do). It makes a high signal low and a low signal high. You can achieve the same effect with a TTL or a CMOS inverter using discrete components, or even easier is one power transistor (NPN or NMOS) configured as an inverter. Here is a schematic:

https://en.wikipedia.org/wiki/Inverter_(logic_gate)

And this is a simple explanation with schematics too:

https://www.cl.cam.ac.uk/teaching/1112/Hardware/Workshop5_2011.pdf

If you can, get a breadboard, a trimpot, a transistor, the LM35, an LED, a DVM or a DMM which can measure temperature, put them together (I'm happy to do a simple schematic for you if it helps, but I'd need up to a day or two), and with a hairdryer, cigarette lighter or other heating device (if you really need it, which you would to get 60ºC), actually test in the real world when the signal goes low - the LED would go off. Better to test starting off with a lower voltage, and then increasing the voltage to the 24V when the basic version works.
MOSFETs don't seem to do well with a gate voltage over 20V max, they fry I think, they can pass much higher voltages but the gate is like a transistor base, so you would need a device to boost the input signal to the gate/base from the LM35, which could just be a smaller BJT or NMOS/NFET, or an OpAmp set up as a comparator.

 

[Englewood]

I have tested a LM35 on a breadboard with a LED with a 9V input all work fine.

Yes that could be another approach using the inverter.

With the LM35 i would need to make the output 24v below 60C and 0V above 60C.
So increasing -15v - 0.6v (-15C - 60C) to 24v, then 0.61 - 1.5v (61C - 150C) to OV seems complicated.

Logic 1 (24V) Logic 0 (0V)

Myabe sunny is right and the high side switch is the way forward.

 

[d123]

Whatever is easiest and least parts count to implement is best, and a device with thermal shutdown sounds a good way to go.