0-12v Op Amp Fan Control With Potentiometer (Beginner)

Thread Starter

Agent Green

Joined Nov 13, 2015
11
Hi, I have very basic knowledge about all these electronic doodads.

I am looking to control a 12v fan with an op amp also using the same 12v power supply with potentiometer to control the input.
Here is my circuit, missing the power lines into op amp.

VariableFanControl.png

Is this a good way to solve the problem? I had been using something similar with a PWM from a raspberry pi to control a 0-10v actuator.

Thank you
 

wayneh

Joined Sep 9, 2010
17,496
Is this a good way to solve the problem?
It's probably not. For one thing, it requires a special "power" op-amp to supply enough current to the fan.

Can your fan be externally PWM controlled, or does it have an internal PWM? The details of your fan dictate the circuit you need to drive it.
 

AnalogKid

Joined Aug 1, 2013
10,990
What power supply voltages are available for the opamp? As shown, the common mode input range goes all the way down to 0 V, and many parts cannot do this without a negative supply.

As above, what is the peak current needed by the fan?

Also, what is the max output voltage you need? Again, most opamps cannot swing the output p[in to within 2 V or so of the positive rail, so a +12 V supply to the part means a max output of +10 V.

ak
 

Thread Starter

Agent Green

Joined Nov 13, 2015
11
I am using a LM324N op amp and supplying it with 12v from a cheapo 12v plug.

The fan seems to say Nominal Current 0.11A - Here is the data sheet for it. http://docs-europe.electrocomponents.com/webdocs/140f/0900766b8140f8d0.pdf

No pwm on the fan and I don't actually want to hook it up to the pi. Prefer to be able to run the fan and control of the same 12v line so I have no computers to worry about.

It is probably ok going to 10v, the 12v transformer actually seems a little off anyway and outputs 15v on the 12v setting and 12v on the 9v setting so I could supply 15v I think.
 

Thread Starter

Agent Green

Joined Nov 13, 2015
11
Thank you. Is there any documentation that goes with that DodgyDave to explain a little? The R8 says see text. Never seen the symbol next to that either. Looks great but I am lost from the start xD

Here is a circuit i used once for a fan speed control for a computer, it uses a pwm control.
 

ian field

Joined Oct 27, 2012
6,536
Thank you. Is there any documentation that goes with that DodgyDave to explain a little? The R8 says see text. Never seen the symbol next to that either. Looks great but I am lost from the start xD
Today; I'm too knackered to study DD's schematic properly, but it looks like the fairly standard method - U1a & U1b would be a ramp or triangle generator, U1c would be a comparator comparing the ramp amplitude to the voltage selected by the pot to vary the pulse width driving the MOSFET. U1d is a voltage follower/buffer for the pot voltage - it would probably work OK without it, but using the left over op-amp for something keeps everything neat & tidy.
 

Thread Starter

Agent Green

Joined Nov 13, 2015
11
I think I was over complicating things. I didn't know you can use transistors as amplifiers, had loads of NPN 0.8A ones knocking around and after a few false starts getting the pin outs the wrong way round it seems to be working. Not tested with the potentiometer but that should be here tomorrow but used resistors to get it working at a set speed and changing the values changes the speed. The fan is only 0.1A

Screenshot from 2017-05-03 22-00-55.png
 

ian field

Joined Oct 27, 2012
6,536
I think I was over complicating things. I didn't know you can use transistors as amplifiers, had loads of NPN 0.8A ones knocking around and after a few false starts getting the pin outs the wrong way round it seems to be working. Not tested with the potentiometer but that should be here tomorrow but used resistors to get it working at a set speed and changing the values changes the speed. The fan is only 0.1A

View attachment 126016
If the transistor gets too hot - use a bigger transistor or try the PWM method.
 

wayneh

Joined Sep 9, 2010
17,496
I think I was over complicating things. I didn't know you can use transistors as amplifiers, had loads of NPN 0.8A ones knocking around and after a few false starts getting the pin outs the wrong way round it seems to be working. Not tested with the potentiometer but that should be here tomorrow but used resistors to get it working at a set speed and changing the values changes the speed. The fan is only 0.1A
Consider adding another resistor to protect against the inevitable turning of the pot all the way up. The fan itself will limit total current in that case, but I think it would be better to limit the base current. Maybe it's not a big deal. Curious to hear other opinions.
 

Thread Starter

Agent Green

Joined Nov 13, 2015
11
Thank you everybody for the help. Potentiometer arrived this am so tested out and all works a treat. Only problem is it doesn't move enough air. Hopefully I can get something closer to 0.8A that is the transistors max cont. current I think.

It's a forced air burner for firing pottery.

IMG_0902.JPG
 

wayneh

Joined Sep 9, 2010
17,496
For the emitter-follower configuration the base current is dictated by the fan current, so is 'self-limited'.
I'm curious how the base current and C-E current self-distribute in that situation and whether it matters for heat dissipation in the transistor.
 
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