Appendix B

Shunt resistor example calculation

See below for an example calculation of a shunt resistor for a 10K3A1 thermistor sensor.

The 10K3A1 thermistor's maximum resistance rating is 335686.23 Ohm. The maximum rated input resistance for IEC's S1 port is 43 kOhm.

Working with the formula:

\[R_h = Shunt Resistance\]

\[R_{shunt} = Temperature Sensor Resistance\]

\[R_m = IEC Max Resistance Rating\]

\[R_{shunt} = 335686.23 \Omega\]

\[R_m = 43000 \Omega\]

\[R_h = ?\]

\[R_h = { R_{shunt} * R_m \over R_{shunt} - R_m }\]

\[R_h = { 335686.23 * 43000 \over 335686.23 - 43000 }\]

\[R_h = { 14434507890 \over 292686.23 }\]

\[R_h = { 49317.345 \Omega }\]

In this case, a 49 kOhm resistor would be perfect. However, since there are no standard resistors with 49 kOhm the closest we can get is 47 kOhm with a single resistor.

A 47 kOhm resistor in parallel with a resistance of 350 kOhm results in a resistance of about 41.k kOhm, well within the measurement range.

Resistor value (shunt) = 47 kOhm

See below for confirming that the resulting resistance is within the measurement range:

\[R_{shunt} = 335686.23 \Omega\]

\[R_m = ?\]

\[R_h = 47000 \Omega\]

From the formula earlier:

\[{1 \over R_m } = { 1 \over R_{shunt} } + { 1 \over R_{h} }\]

We have:

\[{1 \over R_m } = { 1 \over 335686.23 } + { 1 \over 47000 }\]

Which becomes:

\[{1 \over R_m } = 0.000002979 + 0.000021277\]

Result:

\[R_m = 41226.9 \Omega\]

So:

IEC maximum resistor value in S1 port = 41227 Ohm

This resistance is within the IEC S1 port resistor range, as mentioned earlier.