4-20mA Pt100 input for the generic heating / cooling controller

The generic heating / cooling controller (hcctl) is a flexible bang-bang controller based on an ATTiny25.

The controller will accept 4-20mA input if the input is shunted by a 50Ω 1% resistor (2 x 50Ω 1% resistors is a practical implementation) and the 4-20mA sensor is wired between nominal +24 and the input pin. Note that 4-20mA sensors may operate properly on less than 24V (the one used here is a DIN rail mount that works for 10-24V).


To demonstrate the capability, a Pt100 RTD is connected to a 4-20mA converter and then to a hcctl test module (see above). The test module is driving a small incandescent lamp which is clipped to the RTD to provide a cycling test.


The trial 4-20mA converter is specified for -50-100° input from a Pt100 RTD. For the test, the set point is chosen as 40°, so current I=(40–50)/(100–50)*16+4=13.6mA, input voltage developed across the 50Ω resistor is 0.0136*50=0.68V, the ADC count setpoint=0.68/1.1*1024=633. Differential is set to 5 for short cycles for testing purposes.

The concept of hcctl is flexible common code based with application constants stored in EEPROM. Above is a screenshot of the EEPROM structure for the application.

4-20mA converters are available for a wide range of inputs, eg temperature, pressure, humidity etc.

Above is an example Type K thermocouple converter, note two terminals for the thermocouple and two to the output connections.

Above is a Pt100 converter, three wire connection to the Pt100 is supported.

Above is a Chinese WZP Pt100 RTD, it is more than 10° high at 20°… Chinese junk. None of the WZP Pt100s I have measured approach Class B accuracy (Class B is the poorest accuracy at 0.4°). Pt100s are very accurate sensors… except if they come from China.

Can you work out what this one is?

This is Chinese Quality, it is advertised as a K themocouple converter 0-1000°, and it probably is judging from the two input screw terminals designed to take thermocouple wire, but the label shows a three terminal connection as would be used with an RTD for lead compensation; and the text is for a thermocouple. The Chinese do not understand what they are making and selling, and from experience, don't care.

Full documentation at Generic heating / cooling controller.