# Thermocouple voltage boost circuit needed.

#### Mully911

Joined Oct 21, 2020
12
Hello all
I am new here and am searching for a circuit to boost the 5mV output of a thermocouple to a level that can drive a transistor that in turn will drive a normal red LED. Will someone help with circuit design?
Mully

#### ericgibbs

Joined Jan 29, 2010
14,693
hi Mully,
Welcome to AAC.
Do you have a datasheet or part number for the thermocouple you could post.?
E

#### Mully911

Joined Oct 21, 2020
12
hi Mully,
Welcome to AAC.
Do you have a datasheet or part number for the thermocouple you could post.?
E
Hi Eric thanks!
I do not have the data sheet. I measured the voltage across it's load at 5mV to 8mVdc

#### jpanhalt

Joined Jan 18, 2008
11,088

#### Mully911

Joined Oct 21, 2020
12
Thanks, jpanhalt
i am aware of dedicated ic solutions but Im not looking to measure temperature and need a low cost solution with some discrete components. Maybe an opamp circuit

#### jpanhalt

Joined Jan 18, 2008
11,088
You are not measuring temperature, you would be setting threshold(s).

Of course, there are designs using discrete components for a TC amplifier. If you want to deal with the noise problems, that's your choice. Using something professionally engineered and tested for that purpose is simplier, more likely to give success, and probably much cheaper considering the time it will take.

Let me ask what is the difference between making your own amplifier that gives a voltage proportional to the temperature and buying one that does that too? If you don't want the voltage to be proportional to the temperature, just toss the TC and use a resistor from your power supply.

#### drc_567

Joined Dec 29, 2008
1,156
... The non-inverting circuit in this link may be a way to do what you want.
op--amp comparator
However, your mention of 5 mv seems too small to effectively use as an input to the comparator.
You might try to multiply the 5 mv signal by 100, giving you 0.5 volts as a switching threshold.

#### Ian0

Joined Aug 7, 2020
4,110
You just need 120 junctions in series.
360 junctions in series will light the LED.

#### sghioto

Joined Dec 31, 2017
2,703
Maybe an op amp circuit
Op Amp amplifier with comparator. Some component values dependent on the supply voltage

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#### Mully911

Joined Oct 21, 2020
12
You are not measuring temperature, you would be setting threshold(s).

Of course, there are designs using discrete components for a TC amplifier. If you want to deal with the noise problems, that's your choice. Using something professionally engineered and tested for that purpose is simplier, more likely to give success, and probably much cheaper considering the time it will take.

Let me ask what is the difference between making your own amplifier that gives a voltage proportional to the temperature and buying one that does that too? If you don't want the voltage to be proportional to the temperature, just toss the TC and use a resistor from your power supply.
I do not require a voltage proportional to the temperature, I only need to increase the TC voltage enough to be able to bias a transistor or FET to drive an led.

#### Mully911

Joined Oct 21, 2020
12
Thanks sghioto
This is the sort of circuit I was looking for, I will give it a try, Thanks to all!!

#### sghioto

Joined Dec 31, 2017
2,703
What is going to be your supply voltage or Vcc?

#### Mully911

Joined Oct 21, 2020
12
What is going to be your supply voltage or Vcc?
Hi I will use 5Vdc

#### Mully911

Joined Oct 21, 2020
12
Hi I will use 5Vdc
I am not sure what the values for the unmarked resistors should be...

#### sghioto

Joined Dec 31, 2017
2,703
See post #9. Added diode D1 and final component values.

#### Ian0

Joined Aug 7, 2020
4,110
What temperature are you measuring? A type-K or type-T thermocouple needs over 200 degrees C to reach 8mV output.
And don't forget the offset voltage of the LM358, which might be as much as 3mV. Whilst it's perfectly acceptable to use an op-amp as a thermocouple amplifier I'd choose one rather better than an LM358! For instance, OP07 is ten times better.
Anything less than 90 degrees C, and the offset voltage may be giving a bigger input than the thermocouple.

What's wrong with a thermistor?

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#### ericgibbs

Joined Jan 29, 2010
14,693
hi Mully,
Could you post more details of your project, its application and purpose.
We may be able to suggest alternative solutions.
E

#### Mully911

Joined Oct 21, 2020
12
Thanks for the added info Sghioto!! I will try this out.

My application involves an oxygen depletion sensor that uses a pilot burner calibrated for the flame to tear off the surface at a certain concentration of O2 in the air at a given elevation. I have a single wire welded to the pilot which creates my TC junction and outputs outputs 5mV to 7mV when the pilot sensor is lit. I want to turn on the LED when the pilot is lit as an indication it is functioning. The current produced by the TC is used for other circuits.

#### Ian0

Joined Aug 7, 2020
4,110
In that case, go for sghioto’s circuit, but with a better op-amp. With a LM358 you will have a random error of about 90 degrees, due to the offset voltage.
TLV07 will give 100uV offset and work off a 5V supply.

For an even simpler solution, just use one op-amp with a gain of 360, so that the output will be just enough to light the LED when it is hot enough, and brighness will vary with temperature.