Circuits designed by David Johnson, P.E.
Last Updated on:
Monday, December 25, 2017 02:05 PM
List of Dave's Circuit Designs
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12V Cooling Fan
Thermostat - January
12V Cooling Fan Thermostat
This circuit will turn
on a 12v DC powered cooling fan when the air temperature reaches a certain high
temperature and will keep the fan turned on until the temperature drops below a second
lower level. Both the high and low temperatures are both adjustable. For
ease of use, the temperatures are proportional to either degree F or degrees C, using
an accurate electronic sensor.
A LM34 is used for those wishing to measure the temperature in degrees F.
A LM35 is used for those wishing to measure the temperature in degrees C. The output
of the LM34 is scaled for 10mv per degree F so 100 degrees will produce a voltage of
1.00v. The output of the LM35 is scaled for 10mv per degree C so 20 degree C will
produce a voltage of 0.2v. This voltage can be measured at test point #1. At
test point #2, both the high and low switch points can be measured and adjusted. By
pressing the small pushbutton switch S1, while measuring the test point 2, the circuit’s
low switch point can be adjusted. Be sure to adjust the high temperature first, then
the lower one, since the first temperature setting will affect the second.
One half of a LM393 dual voltage
comparator is used to decide when to turn on the fan motor. A p-channel MOSFET
Q2 routes power to the fan when the voltage comparator changes state. Q1
switches in a second resistor network to change the temperature set point when the
comparator A3 changes state and turns on the fan motor.
on Drawing Below to view PDF version of Schematic