Thermistor / Op-amp / Fan circuit.

Thread Starter

NMC

Joined Nov 16, 2018
13
Hi,

I am looking to use a LM675 op-amp to drive a fan motor, the device can supply up to 3A.
My fan is specified with max input voltage of 11V, Resistance 4 Ohms and Max power 30W.
I need to supply the op-amp with a split +/-12V and the output can swing 0.2v either way.

Using my thermistor circuit (voltage divider) I need to supply an appropriate voltage and current to power the fan so it delivers no power at 20 degrees and maximum power at 30 degrees. The load of the fan is 4 Ohms.
 

dendad

Joined Feb 20, 2016
3,796
A couple of questions...
Why did you pick the LM675?
Why do you need a dual +/- supply? Can you use a single supply?
Have you looked at comparator circuits?
Here is a thread you may want to look at. https://forum.allaboutcircuits.com/threads/thermistor-comparator-circuit.65113/

One problem I see is the specified minimum LM675 supply voltage of 16V. If it was a bit lower, you could drive the fan from it without any problem. But it may be worth trying the LM675 on 12V and see if it does run ok as you are not interested in low distortion, just switching.
 

Thread Starter

NMC

Joined Nov 16, 2018
13
Hi Dendad.

Its a project specifying the LM675 which I have to use a 12V +/- split supply.
I will have to research comparator circuits, tried following the link (thankyou) but need to do more research.

below is the circuit I put together but its giving me -12v output.

upload_2019-3-22_13-22-35.png
 

pmd34

Joined Feb 22, 2014
518
Hi NMC, I guess this is some sort of homework question! So we should not give the answer away to easily. Here is a useful data sheet with some very common uses of Op-Amps, it should give you a few ideas of how you can achieve what you want and give you a feel for how Op-Amps can be used:
 

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Thread Starter

NMC

Joined Nov 16, 2018
13
Hi pmd34, its a project related to designing circuits.
Apart from the basic inverting and non inverting circuits, I have little experience with them. I have tried and will continue to study/research them in order to complete this task.

Thanks.
 

pmd34

Joined Feb 22, 2014
518
Ah I see.. well it sounds like you need something like a non inverting amplifier.. you are halfway there with your schematic, but consider some feed-back (to ramp up between 20-30C), and also supplying "some other" - reference voltage to the negative terminal of your op-amp.
 

AnalogKid

Joined Aug 1, 2013
9,078
Between 20 deg and 30 deg, what do you want the driver output to do? Linear ramp? Jump all the way in one step somewhere between 20 and 30 deg? Other?

also, those are strange voltage and power numbers for a DC fan. Manufacturer? Model number? Datasheet (or a link)?

ak
 

Thread Starter

NMC

Joined Nov 16, 2018
13
Hi AnalogKid,

Yes some of the voltages and specifics are a tad random and also the fan hasn't been specified for model.
I have to use the LM675 op-amp along with the split supply, resistances, load etc.
I am looking closely into 'pmd34' feedback regarding some feedback and reference voltage to enhance my circuit to deliver no power at 20 degrees and maximum power at 30 degrees. I am not sure of the ramp up specific however, I would like it controlled sensibly for each degree change from 20 to 30.

NMC.
 
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