DC step Down & duet 0.85

[Gaou]

Guys i need some advice .

I am running two fans on my printer but one of them ( hotend one) is 12v while my whole system runs on 24v.
So i thought to connect a dc step down converter to get 12v out of 24v. My thinking is to get this close to the mainboard in series with the fan, but i am not sure if this is a good idea as the whole delta eefector seems to get power from that always on fan pins.

What do you think should i procced and the only problem will be the leds or apart from that i am gonna run into much problems.

Thanks in advanced,
Pavlos

[rjenkinsgb]

The simplest thing is just connect a resistor in series with the fan.

Look at the fan rated current and use a resistor that drops 12V at that current.
eg. If it takes 40mA, the ideal resistance would be 12(V) / 0.04 (A) = 300 Ohms.
Minimum power rating = current x voltage so 12 x 0.04 = 0.48W
Use at least double the power rating so it's not running excessively hot; eg. 300 Ohms (or 330 Ohms) 1W.

If yours works out to something other than a standard resistor value, use the next higher one.

[Gaou]

@rjenkinsgb said in DC step Down & duet 0.85:

The simplest thing is just connect a resistor in series with the fan.

Look at the fan rated current and use a resistor that drops 12V at that current.
eg. If it takes 40mA, the ideal resistance would be 12(V) / 0.04 (A) = 300 Ohms.
Minimum power rating = current x voltage so 12 x 0.04 = 0.48W
Use at least double the power rating so it's not running excessively hot; eg. 300 Ohms (or 330 Ohms) 1W.

If yours works out to something other than a standard resistor value, use the next higher one.

the fan doesn't give me the watts of amps. so i measured the resistance and i am getting 8.20Mohm.

that makes me 12v / 1.5A = 8ohms and 18watt power rating ... am i doing something wrong here ?

[dc42]

@gaou you should connect the step down converter to VIN. Then connect the + side of the fan to the converter 12V output, and the - side of the fan to the negative pin of the Duet fan output connector.

The only other part of the Smart Effector that gets power from the fan pins is the LED lighting, which is connected in parallel with the hot end fan.

[rjenkinsgb]

@gaou said in DC step Down & duet 0.85:

am i doing something wrong here

A brushed motor will read near dead short, an electronic / brushless fan would likely give an extremely high reading, neither of which relates to the normal running current.

Without specifications, the only way to know the operating current is measure it while the fan is running on a 12V supply.

[Gaou]

@dc42 said in DC step Down & duet 0.85:
you should connect the step down ...

thanks David . thats what i did and it worked fine...

@ rjenkinsgb
24 Nov 2021, 18:17
that is what i would do i just didn't have a ps at that moment. But what David advised work pretty well so i am moving on.

thanks a lot for ur reply.