COMPLETE THE CIRCUIT

So far, I’ve covered only the measurement technique. But for the application to be of any practical use, you need to add some circuitry to apply power to the heating element when you want to heat it up. Simply connecting a voltage source across the heating element would have some undesirable side effects. The relatively high voltage would also appear at the output of the current source (through R1). As a result, the current source would have to be designed to be immune to it. More importantly, an extra current would start to flow through the rest of the bridge circuit. Because all of the resistors have a comparable resistance, the dissipated power would be considerable.

The solution to the problem is depicted in Figure 2. The high-side PMOS switch applies voltage to the heating element. Diode D1 protects the bridge circuit when the power switch is on. The voltage drop across D1 in one branch of the bridge is compensated by the addition of D2 into the other branch. The voltage drop across a diode is primarily dependent on the current flowing through the diode and its temperature. The bridge is always close to a balanced state during measurement. Therefore, currents through the diodes are similar.

A slight difference in the diode currents will contribute to the additional nonlinearity of the circuit. Placing the diodes close together on the PCB will help to keep them at the same temperature. This is more important because you can deal with nonlinearity, but the temperature drift would be hard to compensate for.

Now you can apply voltage to the heating element and raise its temperature. The bridge circuit is now protected when the high voltage is switched on. There is only one piece of the measurement circuit missing: a differential amplifier. A differential amplifier will tell you how far the bridge is from a balanced state during measurement. Thus, it will indicate the temperature of the heating element.

The differential amplifier depicted in Figure 3 is a standard differential circuit based on an op-amp. The Zener diode protects the op-amp from overvoltage when power is applied to the heating element. A Zener voltage of 4.7 V is suitable for op-amps powered by a 5-V supply. The ratio between resistors RA2 and RA1 determines the circuit’s amplification factor. This makes the circuit easy to trim so you can make the best use of the ADC’s available input voltage range.