Hi
I want to design a simple circuit that will measure temp down to -20c
using a k type thermocouple . When the temp falls below -20c turn on a +24 volt 100 watt heating element I am thinking of using the AD8495
Any suggestion please ?
e mail gsharman100@hotmail.com

 

This could be purely analog system, without the use of microcontroller.

If I am reading datasheet right:
At -20 °C the output voltage of AD8495 will be -0.1 V. You apply this to inverting amplifier, so -0.1 V goes in, 1 or 1.5 or 10 or 15 or whatever volts comes out. This is then goes to the transistor that is used as a switch.

If you do it the way I described above, I am kinda concerned about the current. The 24 V 100 W load will have I=100/24=4.166 A current through it, lets play it safe and make it 4.2 A. Generally, at least in BJT transistors, you will want current going to the Base of BJT to be 1/10 of the current going into the Collector of BJT, so 4.2/10=0.42 A=420 mA. I don't know off hand of op amp chips that can source 420 mA.

Let us wait for more experience and wiser head to chime in.

 

For a 24v DC 4A load I would use a NFET.

Something like a 60v 50A N-channel FET in TO-220 package and "logic level compatible" are very commonly available now and can be driven direct from the opamp or comparator output pin. That solves the base current problem.

 

For a 24v DC 4A load I would use a NFET.

Something like a 60v 50A N-channel FET in TO-220 package and "logic level compatible" are very commonly available now and can be driven direct from the opamp or comparator output pin. That solves the base current problem.

See what I mean about more experience and wiser!

 

any why a thermocouple?
There are easier/less complicated ways of getting temps..

 

A basic thermostat works with a coil spring and a mercury switch:

http://home.howstuffworks.com/home-thermostat2.htm

 

A basic thermostat works with a coil spring and a mercury switch:

http://home.howstuffworks.com/home-thermostat2.htm

But... but... but... It is not sexy!
7

 

But... but... but... It is not sexy!
7

You think not?

Then go build me one accurate to +/-3 degrees F.

No cheating, you can't copy one. You have to start from basic raw materials.

Ready... set... GO !!!