Testing KTY10-6 on AnalogIn

Page last updated 22 Dec 2010, by Michel Jank. 0 replies AnalogIn, KTY10

The KTY10-6 is a low cost PTC that I found in my stock. So I thought, why not use it to control temperature of some rooms.

First some calculations:

In the Handbook of mbed, I found,that the resolution of  a analog/digital converter is 12Bit. This gives us  4095 possible results for one sample.

3,3V / 4096 = 0,81 mV/div. The function we use to get a sample is input.read_u16() and gives us a result of 16Bit. This means, that for every 0,81mV that we have more on the entry of the analog/digital converter, we will get a a 16 bit greater result.

By Datasheet, KTY10-6 has (around 20 degrees Celsius) about 15 Ohms per Kelvin and seems to be nearly linear between ~10 and 20 deg. Celsius, which will be enough for our needs. The resistances for 10, 20 and 30 degrees, I found in the datasheet of the KTY10-6. So I calculated a table with the resistances and the samples between.

The difference of the resistance between two samples:

Diff = (R20-R10) / n

Diff = (1922-1772) / 10 =15 (between 10 and 20 degrees).

When we want to show 0,5 degrees difference, we use n = 20.

Between 20 and 30, Diff = 15,8. (Remember, KTY10 is not exactly linear)

The voltage  at KTYcalculated:

U(KTY) = U(Ref) - U(Rv)

U(KTY) = U(Ref) - Rv * I(temp)

U(KTY) = U(Ref) - Rv * U(Ref) / (R(KTY)+Rv))

U(KTY)=3.3V-(Rv*3.3V/(Rv+R(KTY))

Here my first try:

In the forum, we can see, that there are problems with the analog inputs.

I tried to solved it by reading 255 samples and just showing the one that appears most.

/*

Testing low cost temperature sensor KTY10-6 on analog in.

*/


#include "mbed.h"

// Serial over USB (need driver for Windoes)
Serial pc(USBTX, USBRX); // tx, rx
// AnalogIn
AnalogIn input(p20);
DigitalOut led1(LED1);


int main() {
    unsigned short samples[255] , mostsample; // 16 bit 0-65535
    unsigned int appears, count;
    pc.printf("\r\n\r\n");
    pc.printf(" ==================================================\r\n"); // I need CR (\r) in HyperTerminal
    pc.printf(" = Michel`s KTY10-6 Temperature Measuring System. =\r\n");
    pc.printf(" ==================================================\r\n");
    pc.printf("\r\n\r\n");  
    while (1) {
        // wait for a certain time between measurements
        wait_ms(300000);
        
        // flash Led1
        led1 = 1;
        wait_ms(250);
        led1 = 0;

        // get analog in
        for(int i=0; i<255; i++) {
            samples[i] = input.read_u16();
            wait_ms(10);
        }
        // Find the most appearing sample
        mostsample=0;
        appears=0;
        for(int i=0; i<255; i++) {
            count = 0;
            for(int j=0; j<255; j++) {
                    if(samples[i] == samples[j]) { 
                        count++;
                    }    
            
            }
            if (count > appears) {
                mostsample = samples[i];
                appears = count;
            }
        } 
        
        
        // put to serial
        pc.printf("Analog In on Pin20: %d \r\n", mostsample);
       
    }
}

2. Using constant current for measuring:

Fixing current to 1mA with R1=1250.

R(Temp) =U(KTY10) / 1mA

U measured on AIN ; I = Const. we can calculate R, and then Temperature with the formula from datasheet:

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