Temperature controlled fan with opamp and Lm35

dannyf

Joined Sep 13, 2015
2,197
Please help
Lots of possible ways to do it. Forr example, if on/off type of control is on, you can use a comparator, hook it to your sensor and compare the ssensor output to a present value to turn on or off the motor.

If you want pwm type controls (the fan runs faster at higher temperature), use a pmw controller, a 555 for example could do the job.

.....

All depends on what you want.
 

Thread Starter

joerack

Joined Jun 2, 2016
30
Thanks you for the answers, but nobody has still checked if my circuit corresponds to the schematic.
I need to bring it to school for a mark saturday. I'm counting on your approval
 

dannyf

Joined Sep 13, 2015
2,197
Your circuit will work. Whether it works as you expected is a different story.

It amplifiers the signal from ln35 and compares it against a reference. If the temperature is sufficiently high, it turns on thee motor. A little bit of positive feedback there is good.

I would add a diode across the motor.
 

MrAl

Joined Jun 17, 2014
11,389
Hello there,

Here is a reference design. I did this back in 1999 and cant believe it's been almost 18 years ago now.

This design is more complicated because i wanted strictly independent adjustment of both pots to adjust both the turn on temperature and the turn off temperature with individual pots. It also had to control a triac, which you can leave out. The DC power source came from an isolated wall wart.

The main idea is that as the temperature rises, Vd gets lower, and eventually it trips one comparator which then reduces the output of the comparator to about 1/2 of Vcc, but that's not enough yet to trigger the next stage into changing state, so nothing happens yet. Once the temperature goes higher yet and Vd comes down more, the second comparator output changes state and that makes the output go to nearly zero, and that is enough to trip the next stage into changing state and thus the triac turns on
As the temperature goes back down, the second comparator above changes state but that only allows the output to go to 1/2 Vcc so the next stage does not change state yet (hysteresis). When the temperature goes down more the first comparator changes state and that brings the output to around Vcc and that is enough to trigger the next stage into changing state, and that turns the triac off.
The actual voltages on the output of the first two comparators is not 0, 1/2 Vcc, and Vcc though, it's always less than that so we have voltages that are high, medium, and low. Only the high and low voltage states can change the traic state because of the built in hysteresis of the second stage.
The diagram has some graphs on it to show the operation too.

In a more simplified view, the second stage is a latch which does not change state until ether both comparator outputs are on or both off This means we get completely independent operation of the two pots that set the turn on and turn off temperatures without any interaction.

Of course a single comparator with hysteresis is the simplest, but there is then some interaction between pot settings so it is harder to adjust. If you need a simpler circuit though then you may have to do it that way instead.
.
 

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Jony130

Joined Feb 17, 2009
5,487
In this case, the diode junction is used as a temperature sensor. If the constant current is flowing through the diode the diode voltage will drop (-2.2mV/C° or more) as temperature increases.
 
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