hey i need some help with connecting a thermocouple to an adc

Thread Starter

nnaselli

Joined Feb 12, 2011
5
hey i am trying to get a thermocouple to connect to ac adc to show temp on 7 segment displays and turn a heater on depending on the temperature. but i have no idea how to get the thermocouple to connect to the adc. anyone have any wiring diagrams to help me out.
 

wayneh

Joined Sep 9, 2010
16,384
hey i am trying to get a thermocouple to connect to ac adc to show temp on 7 segment displays and turn a heater on depending on the temperature. but i have no idea how to get the thermocouple to connect to the adc. anyone have any wiring diagrams to help me out.
You may find it easier to use a digital thermometer IC such as LM35, which outputs a voltage directly proportional to temperature. A thermocouple changes resistance with temperature, and you need circuitry to convert that to voltage, and making it linear can be a challenge. For simple thermostat applications, it's probably fine. But I really like the LM35!
 

bertus

Joined Apr 5, 2008
20,454
Hello,

What is the temperature range you want to measure?
The LM35 will work from - 55 to + 150 °C.
For the negative temperatures it needs a negative powersupply.

Bertus
 

wayneh

Joined Sep 9, 2010
16,384
i need to measure from 0°F to +80°F
Well, you could use the LM34 (Faranheit), although it's harder to find than the LM35 (Celsius). The LM34 reads 0V at 0°F and 0.8V at 80°F. It might not be accurate right down at 0° if you power it only with a single supply. Read the datasheet - it'll answer all.
 

Thread Starter

nnaselli

Joined Feb 12, 2011
5
i could just convert F to C and use the lm35. because i just need to know when it dips below 70°F which is about 21°C. that is no problem just that i need help trying to get it to an adc to an 8051 microcontroller
 

wayneh

Joined Sep 9, 2010
16,384
Well, if your equipment can use the voltages directly from the LM35, eg 15-25°C = 0.15-0.25V, then you're all set. You just need to power the LM35. Also, I've had better luck when I load its output with a 220Ω resistor, at the meter end. Others here have not needed that, so you may want to just experiment. I also recommend using shielded cable if you need the sensor to be very far from the meter, more than a few feet.
 
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