Buck converter without load? (MP1584)

[Magnum]

Hello,

I want to make a 12V fan adjustable with a 3 way rocker switch: Off, low RPM, full RPM. I don't want to use PWM for the low RPM , instead I would like to use a cheap MP1584 Buck Converter, which is available from Amazon.
When I am in the Off or Full position, the Buck Converter will have no load. Is that a problem? I have read, that the output voltage will then rise up to the input voltage without load, but that would not be a problem for me, as the input voltage is the rated voltage of the fan. Might there other problems arise?

Many thanks,

Magnus

[mariush]

You can do what you want with plain resistors, no need for a voltage regulator.
Fans don't typically consume a lot of power, so you're not really making the circuit much more efficient by using a buck converter.

The downside is you need to pick the resistor values for each fan, the values will depend on the fan's current rating.

Let's make an example, let's say you have a fan which says  12v 0.25A on it.

You can use Ohm's law : voltage = current x resistance to figure out the fan's internal resistance at 12v  : R = voltage / current = 12v / 0.25A = 48 ohm

Now let's say we want to make the fan "see" 9v instead of 12v. In order to achieve that, we need to add a resistor in series with the fan that would drop 12v - 9v = 3v.

What current would we have at 9v?  Again using Ohm's law  : voltage = current x resistance  so  Current = Voltage / Resistance = 9v / 48 ohm  = 0.1875 A

Now we must pick a resistor that at 0.1875 A will drop 3v across it :  Voltage = Current x Resistance => Resistance = 3v / 0.1875A = 16 ohm

So I would put a 15 ohm resistor in series with the fan, and the resistor would drop around 2.8v, making the fan run at around 9.2v and lower rpm.

In the off position, you simply disconnect the voltage wire. 
In the low position, you have the resistor in series
In the high position, you short the two terminals of the resistor, making it a 0 ohm resistor

You can use a slide switch with two independent sides - on one side you have the voltage on/off , on the other side you have the resistor added or not 

example DP4T switch : https://www.digikey.com/en/products/detail/c-k/SS-24E06-TG-5-P/2747191

would allow you to use off, low , med , hi  - 2 resistors and no resistor value

gonna edit the picture it's not quite right


1a - 2a - 3a ---- 4a - 5a

1b - 2b - 3b ---- 4b - 5b


switch on 4-5 :   

ground - 3a --- nowhere
voltage - 3b --- nowhere 

switch on 3-4 :

ground - 3a - low resistor - 4a - to fan ground
voltage - 3b - 4b- fan

edit : sigh ...  it's not quite right still ... because the resistors are basically in parallel in the picture, figure out a way for one resistor to be disconnected when other is in circuit.
anyway, you get the idea.

You can pick the resistors so that   when you're on 3-4  you create a short between 3-4 therefore you short out that low resistor in picture, so the overall resistance decreases. when you slide to 2-3, you short out the med resistor so you have another resistance... and when you're on 1-2 you don't short either resistor, so you have both resistors in series therefore lowest rpm speed.

[Magnum]

Thank you for your reply.

The fans i want to use draw around 0,8A, so thats about 9,6 Watts at 12V. So I would have to use resistors with a high wattage rating. I also would like to keep the dissipated heat down, that's why I want to use a buck converter.

Does someone know what the drawback is if I use a buck converter with no load?

Many thanks,

Magnus

[RES]

pin2 is EN(able),  you could solder a tiny wire on the small SMD pin/pad - small tip solder iron.