Mixed voltage fan setup
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First of all, hello everyone
I'm still waiting for my Duet Wifi to come in, but I'm already planning my setup more precisely. I will be using a 24v MeanWell PSU, but I have good quality 12v Sunon fans. The question I have, will a mixed voltage fan setup work with PWM fans too? I know it says in the documentation that the Duet switches fans connections to ground, but I'm not sure exactly what that means for me? -
You should be able to use your 12V fans but you need some extra components. You can't just connect two 12V fans in series, that does not work (with the current brushless fans) and not recommended for any older fan either as the load is rarely equal.
The simplest thing that you can do is to connect a 12V Zener diode on the plus side of the fan with the ring on the diode facing towards the positive terminal on the Duet PWM connector and the negative terminal to the negative side on the Duet PWM.
That way the Zener will drop the voltage by 12V but it will also dissipate the same amount of power as the fan, so if you have a 1W fan, I would recommend maybe a 2W Zener for some margin. You should check how warm your Zener gets and place it accordingly.
You may need to play a little with the PWM parameters in config.g as not all the fans like to play ball with PWM right away. You may start with M106 S0 B0.5 F25000 to check if the fan likes a high (25kHz) PWM frequency or M106 S0 B0.5 F150 to try a low 150Hz (or even lower as David suggests in another message). You may also try adding a small (e.g. 220nF) capacitor across the fan terminals and set the frequency to 25kHz, has worked for my 40mm Sunon MB40101V2-000U-A99 very well.
You can also use a series resistor and the Zener as a shunt across the fan. In this case you can use a lesser wattage zener if you dimension the resistor right but in this case the resistor is dissipating the power so it needs to be e.g. 2W for a 1W fan but the first option is simpler.
The switching to ground means that on the duet PWM the positive terminal is always at the system voltage and the negative terminal is modulated i.e. it is either floating or at ground potential depending on the phase of the PWM signal. -
The switching to ground means that on the Duet PWM the positive terminal is always at the system voltage and the negative terminal is modulated i.e. it is either floating or at ground potential depending on the phase of the PWM signal.
A small correction, on Duet Wifi the fan PWM can be powered from the system voltage or from the 5V rail or from an external supply. So you might also consider installing a small 12V supply for the fans.
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I think something like this:
http://www.ebay.co.uk/itm/LM2596-DC-DC-Buck-Converter-Adjustable-Power-Supply-Step-Down-Module-Multi-/263022838841?var=&epid=943190096&hash=item3d3d620439
mK__JmcyNATYa-QgQqJ45LQ
is better. You can power all fans from the adjusted 12V.
The negative from the fans goes to the Duet PWM output. -
If I'm looking at the Duet schematic correctly (I might not be, I'm not very good with this), I can connect a +12V power to the middle pin of fan voltage selection header, and that would make the fans run at 12V independent of the main input voltage or the boards +5V power. Am I correct?
How did you choose a 220nF capacity? Is there a formula of some sorts? I just found today that Sunon fans are picky about PWM, I bought 3 fans, and 1 of them is not working with PWM power.
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If I'm looking at the Duet schematic correctly (I might not be, I'm not very good with this), I can connect a +12V power to the middle pin of fan voltage selection header, and that would make the fans run at 12V independent of the main input voltage or the boards +5V power. Am I correct?
Yes, we designed it that way. The constraint is that with the revision 1.02 PCB, the voltage you provide to that pin must never be greater than VIN, otherwise the flyback diodes will conduct. So if you want to supply 12V to the fans, use a voltage regulator driven from your +24V VIN supply, not a separate 12V supply.
Personally I prefer to use 24V fans when using 24V VIN because it is a simpler solution and fans are not expensive. I recently bought two 40mm ball bearing 24V fans from China for just £6.33 and they are working well in my delta.
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If the fan is picky with PWM, first try the F-parameter to change the PWM frequency. Often it works.
There is no formula for the capacitor. It's try and error and depends on the fan. A smaller value may be ok too.Edit reason; David was faster
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I'd go for 24v but the fan selection wasn't as good for that voltage in the store where I was buying.
Thanks for the help, I'll be using a voltage regulator, and once my Duet comes in I'll fiddle with the PWM frequency.
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How did you choose a 220nF capacity? Is there a formula of some sorts? I just found today that Sunon fans are picky about PWM, I bought 3 fans, and 1 of them is not working with PWM power.For sure there are formulas, look for e.g. RC circuit or for Low-pass filter in Wikipedia.
I just did some simulations in the free LTSpice program to get a rough idea about where I would be with the currents and voltages and it seemed to work fine for the 220nF capacitor I found in my junk boxSince I did not add a series resistor I wanted to keep the capacitor relatively small in order not to increase the peak currents too much (the wires have some resistance as well as the switching FET. The downside of this is a very high ripple in the resulting DC but seems to work fine for at least the Sunon I'm using. With larger capacitors you get a less ripply signal, but then you should also add a series resistor (a few ohms should be ok) to protect the switching FET.
