Miniature temperature controller

This is used to heat small aluminum blocks to keep them at fixed temperatures.

I use a LM35DT (U3, now obsolete) as the temperature sensor. It outputs 10mV/C. The heating element is a darlington transistor (Q1, a TIP101G, also now obsolete). A 2.5V voltage reference (U2, a MAX6225BCSA) is used with a potentiometer (R4) to set the desired temperature (again, 10mV/C). Both the measured temperature and the set-point voltage are buffered with voltage followers (U1B & U1D). U1A is a difference amplifier with a gain of 100 (set by the resistor ratio 100K/1K). If the desired temperature is greater than the set-point the the output voltage increases. The larger the voltage difference the larger the output voltage. R9 & D2 cap the max voltage at pin 10 of U1C at about 0.6V. U1C, Q1, & R11-R14 are configured as a voltage to current converter. Whatever voltage shows up on pin 10 of U1C will also appear on pin 9 of U1C. Therefore, whatever voltage is on pin 10 will also appear across the five 10 ohm resistors (in parallel). This sets the maximum current at about 0.3A (0.6V/2 ohms). With a 12V power supply this will limit the power output to about four watts.

Note: The metal tab on the temperature sensor is tied to its ground pin. The metal tab on the transistor is tied to its collector (12V in this case). This would cause a short if both were tied to an aluminum block so a thermally conductive insulator is placed between the transistor tab and the aluminum block. A thin layer of thermal grease is used on the temp sensor, transistor, and insulator to help with heat transfer.

On version two the pot is backwards (i.e. turning clockwise decreases the set-point). On version three it should be fixed (but I haven't built that one yet).

Schematic and Board Layouts:
  • Schematic
  • silkscreen
  • tracks
  • solder
  • solder mirrored (for printing transparencies)

    Express PCB file (schematic not redrawn in express PCB):
  • tempmini 2 with lots of other stuff
  • tempmini PCB version 2 & 3
  • tempmini 3 without version 2

    Pictures:
  • tempmini 2 solder side with everything soldered in
  • tempmini 2 without through hole components soldered
  • tempmini 2 component side without through hole components
  • tempmini 2 component side with through hole components
  • angled view of PCB before through hole components soldered
  • side view of PCB with through hole components soldered
  • Testing setup - screwed to block of metal
  • Bare PCB showing scratch off errors and solder side jumper locations in lower left