K type

hai

Aim using K type thermocouple for temperature measurement its range is 0-1000degress.

0 Degree - 0.000milivolt
1 Degree - 0.039milivolt
500 Degree- 20.640milivolt
1000 Degree - 41.269milivolt


These values taken from K Type thermo couple data sheet.

How to convert this 0 – 41.269mv signal to 0 –5 v.

Please provide me signal condition circuit for K Type thermo couple.

Aim using 89C51 micro controller

Thank you

Regards

chandu

Hi chandu;

A good low noise instrumentation op amp with the correct gain could scale the value to your desired range. However be aware that thermocouples are slightly non-linear in response and require compensation to correct their response curve. This is often done in a look up table. Also be aware that thermocouples require cold reference junction compensation to offset the error created by the 'cold' end of the thermocouple, you basically need to know the temp at both ends of the TC. There are special IC chips designed for TC processing, which included all of the above functions, check out Analog Devices web site for one.

Good Luck

chandu13 said:

How to convert this 0 – 41.269mv signal to 0 –5 v.

Click to expand...

Use an op-amp with a gain of 121.156, that will make an input of 41.269mA = 5V output

**broken link removed**

For an ideal op-amp, the non-inverting amplifier gain (Vout / Vin) is given by
**broken link removed**

Use a 1M pot for Rf and a 8.2K for R1, that should give you a good range for the gain

**broken link removed**

hi chandu,

I would recommend that you avoid the use of the '-' symbol in your number values, it can be mis-read as a minus variable.

How to convert this 0 – 41.269mv signal to 0 –5 v.

I would suggest a modification to Gramo's 1Meg Resistor, make it two resistors, one fixed 910K and a 200K multiturn,
else the pot will be near end one for a gain of 121

Regards

gain

hai

Thanks for the reply

Here aim using LM324. Its maximum gain is 100

But we required gain more then 100.

So how to improve the gain of the amplifier.

How to design the amplifier to reduce noise in the signal.

Regards

chnadu

Ahh yea. maybe use a dual op-amp, and set the gain to 10 on one 12.187 on the other, the gains would multiply

eg 41.269mV into the 1st amp(with a gain of 10) would give 410.269mV

410.269mV into the 2nd amp (with a gain of 12.187) would have an output of 5V

Your LM324 has 4 amps built into it according to the datasheet

**broken link removed**

A thermocouple is not a battery. You cannot draw any current when taking the measurement. The only way to do this is to output a voltage until current flow is zero. The outputted voltage is your reading. Using an opamp as suggested will result in a very inaccurate reading (if any). Use a thermocouple amplifier IC such as the AD595.

Mike.

Actually, a thermocouple is very like a battery, it's impedance is just extremely high, so it would be better said that the current flow must be extremely low. Though settling time might be an issue simply using a capacitor as a buffer for a circuit which requires a current pulse for a reading might be acceptable, though it would require a very stable capacitor.

Sceadwian said:

Actually, a thermocouple is very like a battery, it's impedance is just extremely high, so it would be better said that the current flow must be extremely low. Though settling time might be an issue simply using a capacitor as a buffer for a circuit which requires a current pulse for a reading might be acceptable, though it would require a very stable capacitor.

Click to expand...

TC impedeanace is a spec I hardly ever recalled reading. However having troubleshoot and repaired hundreds in a refinery over the several decades I can say that with a ohmmeter a good TC seldom reads over 200 ohms, depending on lead length of course, so I always considered a TC a low impedeance device But I guess low and high are pretty subjective terms.

Have you taken into account the cold junction compensation of the thermocouple? A TC only measures the difference in temperature between two points so a voltage reading will only tell you the temperature difference between both ends. In order to get an absolute temperature reading you need to know the temperature at one end at the TC. THis is so called "cold-junction compensation" or you set one end to a known temperature.

Unless you are only interested in the temperature difference, you will need to use an absolute form of temperature measurement on your microcontroller to figure out the absolute temperature you are measuring. You could also use an IC like the max6675 that takes care of the amplification, cold junction compensation and look-up tables for you.