On-Off Temperature Control
This circuit controls a load (in
this case a dc brushless fan) based on a temperature compared with
a setpoint. THe transduced is a diode in the forward polarization
regime. In fact when forward biased, the forward voltage drop
accross a diode has a temperature dependance, in particular has a
negative linear(ish) slope. This because of the boltzmann
distribuition, causing electrons to pass to the conduction band
thermically, lowering the voltage drop accross the diode.
Anyway this circuit comparates a precise voltage reference (zener)
with the forward voltage drop of the diode forward biased with
11mA of current.
The comparator is simply a LM158/258/358 working in open-loop
mode, the inverting input is connected to the diode sensor, and
the noninverting to the reference voltage. Se when the temperature
rises above the setpoint, the forward voltage drops under the
voltage reference and the comparator output is vccturning on the
transistor and so the fan.
Higher power transistor can be substituted for bigger fans, or you
can substitute a relay, IGBT, mosfet etc to control higher loads
(and higher voltages).
The setpoint is adjusted with the potentiometer, and you can use a
LM3914 led driver to make a temperature setpoint indicator (needs
careful calibrations and the use of excel to calculate slope and
Many modifications can be done, but the circuit works very well in
its basic form.
THe comparator can distinguish 10uV differences so approx 0.01°C
differences (carefully adjusting the potentiometer can allow to
feel body heat from 1/2 cm from the sensor, or feel ambient heat,
making to turn the fan on and off continuosly)
You can control temperatures up to 140°C (150 max diode
temperature), but linearity is not ensured
Possible uses? Heatsink cooling, computer emergency cooling (but i
thint that a linear device would be better than a on-off)
metal cooling when drilling etc...
Ah! One note: you can even heat with this circuit but you need the
reverse comparator inputs and substitute the fan with a relay
controlling the heater.
Also check the conversation about this project at the community.
Post you questions here.