How to convert 6V to 5V signal - AD8495 to use with high temperatures

Projects General Guidance
1

Hi,

I would like to create a thermocouple real time data viwer.

for now I would like to choose the thermocouple amplifier,and a good candidate is the AD8485 with its 5mV/ºC output. But I would like to use it with high temperates around 1050ºC but up to 1200ºC. This causes a problem, at 1200ºC the output voltage is 6V and the analog inputs in the arduino are 5V.

Any ideias?

2

Use a resistor attenuator (voltage divider) on the analog i/p.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/voldiv.html

3

If you use a thermocouple amplifier and read the output using the ADC, then your reading will be affected by the supply voltage (unless you use a precision voltage reference) and the accuracy of your voltage divider. Why not use a MAX31855 instead? It produces a digital output in Celsius that you can read via SPI.

4

(unless you use a precision voltage reference) and the accuracy of your voltage divider

You could use a good digital multi meter to measure the o/p from the divider, then calculate the conversion ratio and use this in the Sketch.

5

My enfasis is on accuracy.

The AD8495 offers a error of +-1ºC.

True that has many things I attach i will increase the error.

I'm not looking at the moment to the MAX31855 because the error is +-6ºC on those temperatures.

6

At 1200ºC range error is much bigger than ±2°C.

AD8495 Tech Sheet

AD8495

  • Thermocouple Type:K
  • Max Error:±2°C
  • Ambient Temperature Range:0°C to 50°C
  • Measurement Temperature Range:?25°C to +400°C

For temperature ranges outside those listed in Table 7 or for instructions on how to correct for thermocouple nonlinearity error with software,

English version: see the AN-1087 Application Note for additional details.

Chinese version: contact field support engineer.

AN-1087 Application Note

7

sonnyyu you have a good point that I haven't noticed. Thanks!

So it seems that the MAX31855 might be the best solution.

Now, I need to use 18 Thermocouples. I guess that 20 inputs in the arduino will be fine.

I haven't tested it yet, but I guess that I can use the analogs inputs of the arduino has digital to avoid using multiplexers. I'm tring to use the UNO for this project.

8

The MAX31855 uses a 3-wire SPI interface, so all of them share the MISO and SCLK pins but each needs its own CS pin. That uses up all 20 pins of the Arduino Uno, leaving no pins to do anything else with. I suggest you use a 74HC4515 or 2 x 74HC138 demultiplexing chips to generate 16 of the 18 CS signals from 5 pins.

9

Thanks for the reply.

Objective is just dump the values to a pc for visualization, so i shouldn't need any more inputs.

But thanks, I will have a look and consider adding them if needed.

10

fenix999:
Thanks for the reply.

Objective is just dump the values to a pc for visualization, so i shouldn't need any more inputs.

But thanks, I will have a look and consider adding them if needed.

Then you need a Leonardo, not a Uno. The Uno needs pins 0 and 1 to communicate with the PC.

11

dc42:

fenix999:
Thanks for the reply.

Objective is just dump the values to a pc for visualization, so i shouldn't need any more inputs.

But thanks, I will have a look and consider adding them if needed.

Then you need a Leonardo, not a Uno. The Uno needs pins 0 and 1 to communicate with the PC.

:fearful:

and any idea on how to improve the accuracy of the readings?

12

fenix999:
and any idea on how to improve the accuracy of the readings?

Calibrate your thermocouple/MAX31855 pairs against a digital thermometer with higher accuracy.