MCP3428 I2C with thermocouples?

Hardware
1

Good afternoon,

i recently bought two 12-Channel I2C 0-10V Analog to Digital Converters with I2C Interface (MCP3428) and plan to use them with a raspberry pi. Now I wonder if it is possible to read a thermocouple type K with the MCP3428 without additional hardware.

To try it, I connected a type K thermocouple to one input of the ADC and tried to read the value with 16 bit resolution and gain 8.
To read the data I use Java and pi4j (the code looks like this)

I2CBus bus = I2CFactory.getInstance(I2CBus.BUS_1);
// TODO - I2CDevice mul = bus.getDevice(0x77);
I2CDevice adc1 = bus.getDevice(0x68);
while (true) {
    // channel: 0 (1), res: 16, gain: 8 = 00011011 = 0x1B
    adc1.write((byte)0x1B);
    Thread.sleep(1000);
    byte[] output = new byte[2];
    adc1.read(output, 0, 2);
    int h = b[0] & 0xff;
    int l = b[1] & 0xff;
    long step = h << 8 |  l;
    if (step >= 32768)
        step = 65536L - step;
    // print output...
}

The problem is that the readings hardly change at all. I checked the thermocouples and they’re fine.

values200
values200.PNG915×570 155 KB

But if I understood the datasheet of the MCP3428 correctly, it should give about 5 steps per degree Celsius

Another curiosity is that if you touch the thermocouple, which should not result in a big jump in temperature at room temperature, the value jumps 1000 steps upwards immediately.

Now my question: is this behaviour normal for the MCP3428 and if so, what additional hardware would be needed to get good readings.

Tanhk you!

2

Hi,
This board wont work with thermocouple. The thermocouple outputs very low voltage and generally it between -10mV to 50mV. you will need additional opamp between ADC input and thermocouple.

Its possible to use MCP3428 read thermcoouple but you will need this kind of board

Although i would recommend looking into a board like this one

Thanks

1 Like
3

Hello,
first of all, sorry for the late reply but I was on vacation.

Thanks for the quick answer. Now I have two more questions.

What is the exact difference between the two boards with the MCP3428. I suppose the 4-channel one has a bigger pre-amplification? (Because both use the MCP3428?)

And a second question I have: what are the disadvantages of using the MCP3428 4-channel for reading thermocouples compared to the MCP9600? (Because there is a price difference)

Thanks again!

4

the 0-10V input has wide input range that means the smallest voltage change it can bread will be quite high in comparison to regular MCP3428.

The MCP9600 is a dedicated IC for thermocouples. it supports all kinds of thermocouples and does not require any external electronics to read theromcouples.