[rcheshire (Rowland)]

Nothing new about this but cheap to build and accurate - a little temperature controller made up of parts from Jaycar and a TMP36 temperature sensor. No doubt room for improvement.

It's just an op amp voltage comparator circuit switching a transistor on and off to drive the gate of a mosfet or whatever might be driving a high current load.

The TMP36 goes to the + (non-inverting) pin with the temp setting (mv) potentiometer hooked up to the - (inverting) pin.

The 8v supply is nominal and is followed by a two stage voltage divider, which sets ~5v into the second voltage divider used for temperature setting - room for a range of power supply voltages.

The TMP36 does not require calibration and has a linear output from 100mv (-40C) @ 10mv/1C up to 125C. It has an offset of 500mv which corresponds to 0C. Very easy to set a target temperature for cooling or heating; e.g., 300mv = -20C.

The opamp is an LM324 (a single channel opamp would do) quad channel so it's possible to drive 4 circuits - dew heating, cooling and so on.

Fixed - (On the test bench it does very well but needs a -40 to -50mv offset when setting target temperature, as it tends to sit above. I think this is due to the stepping up of the output voltage with each mv of difference from the target. The correction is quite small and not adequate to overcome the thermal imposition. So aim lower for the target. Otherwise it ticks over +/- 1mv (0.1C) of the 'target' setting.)

Schematic shows it driving a Peltier (TEC) device.

EDIT: the humble test bench - target temp and holding about 38mv above - need to preset lower. The difference reduces with a decrease in temperature.

EDIT: modelling the simulator opamp to approximate the LM324 as closely as possible and then stepping the sensor input in mV increments shows a gap of 30mV between the target and actual temperature (mV). The mosfet/TEC requires all the opamp output voltage to maintain temperature.

EDIT: updated this simple fix to the over temperature problem. Another voltage divider transparently reduces the opamp input by 50mv so that the operator gets and sees the target temp.