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[General] Thermocouple voltage amplifier

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vrmidhun12

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I need a voltage amplifier for J type thermocouple, can anyone help me?
 

Hi.

What especially do you ask for?
*This is often discussed here in this forum --> do a forum search
* this is discussed many time in the internet --> do a internet search
* many semiconductor manufactureres have dedicated ICs, datasheets, schematics, application notes, calculation examples...

so what else do you need?

Klaus
 

Hello!

Just a first hint for you.
A thermocouple does not deliver a voltage but a current.
You should search for a current/voltage conversion circuit.

Dora.
 

Hi,

@doraemon:
Wikipedia says:
A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature.

Klaus
 

Hello Klaus!

Oops, sorry! You're right!
And sorry for the original poster for wrong info!

Dora.
 

To amplify a few uV from a low-impedance source, consider a transistor in common base mode.



This takes advantage of the transistor's full gain, although it needs a lot of adjusting to make it perform the way you wish. It may also require additional components. For instance, if you want zero output to result from zero input.

If the transistor is placed near the thermocouple, it may be exposed to heat which affects transistor response.
 

Hi,

Is this circuit really suitable for thermocouples?

A tyke K thermocouple has about 40uV/K.
But a bjt V-BE will drift with about -2100uV/K.

Therefore the change of 1°C at the bjt will cause an error of about 50°C in thermocouple temperature reading.
****

I recommend to use an Opamp with very low temperature drift of offset voltage. Maybe a chopper stabilized opamp.
Or use dedicated thermocouple ICs. Some of them have cold junction compensation inside, some have an ADC inside.

Klaus
 

How about AD595 ?
 

AD595 doesn't force you to use your brain! Just copy the schematics from datasheet and it will work. So boring! :laugh:
 

Boring but practical.

The little elves at Analog Devices know all the mysteries of the universe.
Seek and ye shall find.
 

Do not forget that the temp reported by the thermocouple is rather relative; unless of course you keep the reference junction at a fixed temperature. That can be a mess and hence you should not bother too much about the accuracy.
 

Hi,

I just design a temperature measurement with MAX31855. With digital output and cold junction compensation. One SO-8 IC.

Klaus
 

I need a voltage amplifier for J type thermocouple, can anyone help me?

You do not need any amplifier, you just need to buy a a quality J type thermocouple this having necessary cable length for your application, this be made by a reputable vendor.
 

Hi,

Is this circuit really suitable for thermocouples?

A tyke K thermocouple has about 40uV/K.
But a bjt V-BE will drift with about -2100uV/K.

Therefore the change of 1°C at the bjt will cause an error of about 50°C in thermocouple temperature reading.

Perhaps some kind of cancellation circuit can be added. My schematic is bare basics. A second bjt somewhere, influenced by ambient temperature but not influenced by the thermocouple, might serve to cancel out the effect of ambient temperature, leaving only the thermocouple's effect.

I suppose this is the same idea as a differential amplifier. I'm uncertain how to imagine it for using common base operation. The easier path is your op amp suggestion.
 

The whole thing depends on the intended purpose.

If its for accurate measurement and readout display, use a proper thermocouple amplifier chip from Maxim or Analog Devices.

If its some rough on/off temperature control circuit, maybe a soldering iron, hot water heater or similar, just use a chopper stabilised amplifier such as a 7650 and don't worry about cold junction correction.
https://www.intersil.com/content/dam/Intersil/documents/icl7/icl7650s.pdf
 

If its for accurate measurement and readout display, use a proper thermocouple amplifier chip from Maxim or Analog Devices.

Thermocouples are useful thermometers generally useful for measurement of high temperatures. Because of the cold junction compensation problem, they are often accurate to +/- 5 deg and for high temperatures, perhaps more. If you want to measure the body temperature of a patient, use a different thermometer.
 

If you use certified thermocouple wire, and a high end data logger for professional laboratory measurements its more like +/- 0.5 degrees Celsius accuracy.
Thermocouples for home use, obviously not that good.

Patient monitoring usually is by thermistor bead, because of the very limited required temperature range, and the high sensitivity of a thermistor.
 

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