Fan Speed Control - PWM
At ebmpapst we make lots of different fans just like every other fan manufacture. However we all use common speed control interfaces which are either PWM or 0-10volt. In this blog I’ll explain what PWM is and how to interface to fan with it.
PWM or Pulse Width Modulation is used on our compact range of fans and most you will have come across compact fans inside you PC or laptop. PWM fans allow you to speed control these fans via a digital rectangle waveform. Have you ever noticed that you laptop or PC fan changes speed as the Processor works harder – well that a PWM fan in action.
First the hardware...
PWM fan have not quite reached the high tech digital world of low voltage digital signals like 3.3v or 2.5v. They internally use 5volt digital electronics. Basically they don’t need to work lower and it helps nose immunity – we are inside a big spinning magnetic field after all! So the interface used is called an Open Collector Input. This means you can control the fan via a Open Collector Output.
It’s highly recommended to not use a 5volt digital output as your systems 5volts might not match the 5volts inside the fan. This can lead to currents flowing in your signal wire that can lead to damage in your electronics or that of the fan. Therefore the best way to do this is to use a Transistor or in my case my favourite is a FET, something like a 2N7002 is very low cost and can be driven from a logic circuit.
Connect your emitter or source to your system 0 volts and the collector or drain to the fans PWM control line. You don’t need any other components as these are all inside the fan and your now ready to go.
The PWM signal you generate will I guess come from a PIC micro or something like an arduino. These have build in PWM drive circuits that once configured will generate the required rectangle wave form. However its important to note that your PWM frequency (that speed it switches on and off) is above 2kHz. Lower frequencies can lead to the fan miss reading the input and causing undesirable results.
When you look at the signal you will see something like this...
The PWM signal is measured as a duty and we often refer to this as a percentage (ie 50%). The period (time from going positive to positive or cycle time) will remain constant as this is your PWM frequency. However as you adjust your duty you will see the period of ON time changes. If you have 10% duty then the positive ON time only appears for 10% of the time and when at 50% its equal to the off time.
So if you have your fan connected and have started to play with the duty you will notice some odd things.! The fan will not start at very low duties, typically less than 20%. At this point a fan will run at a speed I call the “Anti Cogging Speed”. This is the slowest the fan can fun without it stalling or stopping or making big chunky steps as it turns. Remember modern fans are brushless and have permanent magnets in them – like stepper motors as at low speed you see these off effects. Our fans are designed not to run at such silly speeds so have a default minimum speed build into them. This will differ from fan to fan but is around 2,000 rpm.
As you now increase your duty the fan will increase in speed until you give it 100% duty and the fan is running at full speed. Some people think that the duty you apply is equal to the percentage of the speed the fan runs at. I’d say that’s only true at 100%! The reason is because different fans all have different top speeds. Example would be a bottom range ran that runs at 4,000 rpm at top speed. As already stated a duty of 20% makes the fan run at 2,000 rpm, that 50% of the speed. If your top end of the range fan runs at 19,000rpm (and yes some do!!) then 2,000 rpm is around 10% of speed. So check it first.
One last thing about working with any fan. Keep safe and keep your fingers away from them! Compact fans may only be plastic but have sharp edges to the blade tips. These will take chunks out your fingers or anything else you put in there – believe me I know!
And if it just all too much and you want simple speed control from temperature then check this out Our temperature controller for DCP fans on the RS website.