# Low temperature measurement

Discussion in 'Embedded Systems and Microcontrollers' started by gsharman100, Jun 17, 2014.

1. ### gsharman100 Thread Starter New Member

Jun 4, 2014
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0
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
e mail gsharman100@hotmail.com

2. ### shteii01 AAC Fanatic!

Feb 19, 2010
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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.

3. ### THE_RB AAC Fanatic!

Feb 11, 2008
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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.

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
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See what I mean about more experience and wiser!

5. ### mcgyvr AAC Fanatic!

Oct 15, 2009
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any why a thermocouple?
There are easier/less complicated ways of getting temps..

Oct 2, 2009
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Feb 19, 2010
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8. ### ErnieM AAC Fanatic!

Apr 24, 2011
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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.