Question
I have a question related to the use of the Analog Input of an MPT10000 temperature controller with an RTD. The sensor we are using is a PT100, with the sensor bias current set for 1 mA.
We want to change the set point temperature using an external voltage applied to the Analog Input (Pin 6 on the MPT Series Temperature Controllers). We would like to set the set point to 20°C, and apply an analog input that will change the set point up to 40°C or down to 0°C.
Answer
Stability with a PT100 and a bias current of 1 mA may prove to be an issue. The RTD at 25°C will exhibit a resistance of about 109 Ω. With a 1 mA bias current this would yield a voltage across the temperature sensor of 0.109 Volts. The recommended range for sensor voltages with the Wavelength temperature controllers is about 0.25 V to 2.5 V, and always make sure the sensor voltage remains below the supply voltage or damage to the controller may occur. If you are using a Revision E or later MPT you have the option of using a 10 mA sensor bias configuration. The 10 mA sensor bias current will bring the voltage across the sensor to 1.09 V at 25°C, so this will be in the middle of the recommended sensor voltage range and should offer better control stability.
Please remember that any external control voltage applied to the Analog Input sums with any voltage configured with the on-board set point potentiometer to achieve the final set point voltage. Also there is a transfer function (Vset = 1/2 * V input) associated with the Analog Input concerning set point temperature. So if you wanted to operate the MPT at a set point of 0°C using a 100 Ω RTD then you would want a final setpoint voltage of 1 V when using the 10 mA bias current (100 Ohms * 10 mA = 1.0 V). To achieve the final setpoint of 1.0 V, based entirely on the external control voltage you would turn the on-board potentiometer fully counter clockwise (at least 12 full turns, for zero Volts) and apply 2 V to the Analog Input. The final set point voltage can be monitored using the Temp Setpoint Monitor (Pin 11) of the MPT Controller.
The setpoint for voltage for a 100 Ω RTD using a 10 mA sensor bias current for an operating temperature of +40°C would be 1.155 V (RTD resistance at 40°C * sensor bias current = 115.5 Ohms * 10 mA = 1.155 V).