Hello guys,
I have 12V fan for my car's cooling system, which I control using a PWM at approx. 80% (when the temperature gets high). Now I want to connect the car A/C's PWM to the same fan at approx 40% (which is controled from the A/C control module). My question is - Can the two PWMs work together and what will happen when the first one supplies power to the fan (80%) and the second one delivers a 40% signal at the same time? Is there any danger for the 40% PWM when the other is engaged?
Thank you!

 

Hard to say if either will be damaged without knowing the circuit design.

But why you want to mash two PWM signals together?
It seems pointless.

 

I don't want to, but its inevitable to happen time to time. I have two different fans now and want to use only one and remove the other. There is no one-way diodes in the circuit, will there be a problem or should I add some of them?

 

Do the two PWM signals have the same repetition frequency?

 

I haven't tested them on oscilloscope, but probably not the same ...

 

How do you want the fan to respond to a combination of the two PWM signals? Should it obey the signal with the greater duty cycle, or an 'average' of the two, or the 'sum' of the two, or ....?

 

It should obey the greater duty cycle signal

 

Is the fan speed PWM controlled by a factory system or something is built being you post is not clear here.

I have 12V fan for my car's cooling system, which I control using a PWM at approx. 80% (when the temperature gets high).

and if it is factory what does the PWM control use for an input signal and where does it come from?

Also,

I have two different fans now and want to use only one and remove the other.

Why do you want to eliminate one fan? What do you plan/expect to gain from it?

 

It should obey the greater duty cycle signal

Well that would require some added logic and timing circuits with possibly a microprocessor.
If you just OR them together with two diodes, then you would get a duty-cycle somewhat higher than the highest, more or less.

Here's an LTspice simulation of that for two inputs with different duty-cycles and periods.

 

Is the fan speed PWM controlled by a factory system or something is built being you post is not clear here.



and if it is factory what does the PWM control use for an input signal and where does it come from?

Also,


Why do you want to eliminate one fan? What do you plan/expect to gain from it?

The A/C fan uses factory OEM module, controled by the engine control module, I don't know any specs for it. My module is a cheap ebay one 40A 12khz. I want to eliminate the fan, because it is in front of the radiator and since the main one now is in the back, the other only restrics air.

crutschow, thank you for the data, should I understand that diodes are a must?

 

crutschow, thank you for the data, should I understand that diodes are a must?

The are unless you are sure that not having them won't cause one of the modules to blow.
You can use Schottky types to minimize voltage drop and power dissipation.

 

I read that Schottky diodes tend to have a high reverse leakage current, so what voltage and amperage schottky diode should I use (I have 40 amps fuse on the circuit), to have as small leakage as possible? Also I should clamp the ground wire, right?

 

I read that Schottky diodes tend to have a high reverse leakage current, so what voltage and amperage schottky diode should I use (I have 40 amps fuse on the circuit), to have as small leakage as possible? Also I should clamp the ground wire, right?

It should be rated for at least double what the fan draws.

I don't think reverse leakage is a concern in this application.

What do you mean "clamp the ground wire"?

 

If you have two sources both generating heat, then how will a single fan provide sufficient cooling?

 

It should be rated for at least double what the fan draws.

I don't think reverse leakage is a concern in this application.

What do you mean "clamp the ground wire"?

Oh, excuse me for that, I mean where should I put the diode, on the ground wire?

 

Oh, excuse me for that, I mean where should I put the diode, on the ground wire?

The ground wire needs no diode.
Why do you think it does?

 

Okay, so only one diode on each positive wire, and what about the diode's voltage rating, what should it be?

 

Okay, so only one diode on each positive wire, and what about the diode's voltage rating, what should it be?

I would go for at least 25V, 50V if available.

 

I would go for at least 25V, 50V if available.

Got it, thank you so much for your help!