Get rid of PWM noise in brushless fan

Hi!
I just got som TIP120 transistors today, that i'm going to PWN controll som old PC fans with. I hooked it up to two pc fans (in parallell). schematic is showed below. The fans works great, but I got some loud PWM noises, which is really annoying. how do I get rid of these?
Im using an external power supply with a diode bridge and two 100 uF caps i parallell for the power supply.

You don't provide the code you use for these tests. Maybe you have to change the PWM frequency for your kind of motors.

It's a huge code for an amplifier, but i'm using basic syntax, such as analogWrite(fan_pin,150);
Will a lage capacitor (lets say 1000uF) remove this noise if I connect I replace the two 100 uF caps?

hansibull:
It's a huge code for an amplifier, but i'm using basic syntax, such as analogWrite(fan_pin,150);
Will a lage capacitor (lets say 1000uF) remove this noise if I connect I replace the two 100 uF caps?

I was just about to suggest you add a capacitor, so yes.

I also have a digital potentiometer the controlls the input volume on the amplifier, hooked up to the SPI pins on the same Arduino. I also got a problem that the fan noise is interfering the digital potentiometer (mcp42010), and the result is the horrible fan-noise on the speakers too! will a large capacitor remove this problem too?

I strong suspect that adding more capacitance to your voltage source won't do a thing for the motor noise you are hearing. It's more likely a mechanical resonance noise based on the motor induction and the PWM switching speed being used by the analogWrite command. Changing the PWM switching frequency used is probably the only path to peace and quite, but I can't suggest how to go about that.

Lefty

Will a lage capacitor (lets say 1000uF) remove this noise if I connect I replace the two 100 uF caps?

Well no. All that will d is to make the supply smoother. The noise is coming from the pulsed current in the fan. The capacitors you have to add are between the base of the transistor and ground. I would start with 47uF first. This will convert the PWN output into DC, but will also drive the transistor into the linear mode. This means the transistor will run hotter, maybe too hot. Also you might find you do not have very good control over the fan's speed with there being a dead section at one end where the fan does not start and a dead section at the other end where you do not see any increase in fan speed for increasing PWM values.

But wouldn't a big capacitor work like an AC to DC converter, if you're excluding the diode bridge? wouldnt the capacitor make the PWM smooth, like a clean DC voltage?

EDIT:
Like this? (the FET transistor in the schematic is the TIP120 that I'm using)

Two points:-

  1. That capacitor in the last post is not where you said you were going to put it in the previous posts.

  2. Exactly how big do you think it would have to be to drive a 60V motor?

  3. no one expects the Spanish inquisition - that diode is in the wrong place.

So should I connect the cap like the schematic I draw in my last post, or should I place it at the "high voltage" input?
and what do you mean about the diode? is the diode at the wrong place? I've looked at some guides for TIP120 with arduinos, and many guides recommends hooking it up that way :slight_smile:

is the diode at the wrong place? I

Maybe you didn't spot that I said

that diode is in the wrong place.

I've looked at some guides for TIP120 with arduinos, and many guides recommends hooking it up that way

Yep they do they are wrong. How do you expect to protect from the reverse voltage generated by the motor by putting the diode across the transistor?

So should I connect the cap like the schematic I draw in my last post,

No.

or should I place it at the "high voltage" input?

No.

It constantly amasses me that some people ask for advice and then ignore it without even trying to understand it.

where should the diode go in this diagram to make it work? i’ve been trying to get this circuit to work, and the diagrams i’ve found show it in the same position, and i am so relieved to hear you say it is wrong. where should it go?

What kind of fans are you using.
Brushless DC fans commonly used in PCs cant be speed controlled, unless they are specifically designed to be .
In which case the fan will have 4 wires going into it.
Supply voltage + - and speed detection and speed control.
Trying to PWM the supply voltage will simply destroy the internal electronics in the fan over time.