12V fan on 24V printer

I have a 24V printer and I added a new fan mount for a 5015 radial fan. I only have 12V ones. I was going to order a new one, but I thought, “what’s the harm?”

Anyone try this? I am thinking I will put a max fan speed of 50% in the firmware and try it out. I did some smoke testing up to 60% for a minute and didn’t see anything complaining.

It is the part fan, so if it breaks, it is not the end of the world. I hope it just doesn’t break in a dangerous way.

Heck you should be worried about skewing the print when that fan causes things to launch upward into the sky…

Let it run a while I think they tend to get very hot if something is wrong. I also have a feeling a lot of them are probably 24V fans being sold as 12V.

Ah, the comment where I set it to 50% makes it sound like this is a good idea:

 * PWM Fan Scaling
 * Define the min/max speeds for PWM fans (as set with M106).
 * With these options the M106 0-255 value range is scaled to a subset
 * to ensure that the fan has enough power to spin, or to run lower
 * current fans with higher current. (e.g., 5V/12V fans with 12V/24V)
 * Value 0 always turns off the fan.
 * Define one or both of these to override the default 0-255 range.
//#define FAN_MIN_PWM 50
#define FAN_MAX_PWM 128

Now I just need to make sure that doesn’t affect the hot end fan.

Hmm. It looks like it will. I am just going to leave setting off, and change the max to 50% in the slicer.

Is it not safer to just attach your hotendfan to the 24v powerrail?

Yeah, it’s safer, but I print at most 10% of the time, so I let Marlin turn it on whenever the hot end is more than 50C. This fan is pretty noisy, and I work in here.

Fair point, I just turn off the whole printer when not printing :wink:

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Fan voltage is a funny thing.

I use 24V for my printer and have several cases where I use 12V fans.

In most cases, I wire 2 identical fans in series. While this is perfect in theory, the reality is that one of the fans burns out before its time, since they aren’'t identical enough.

For a single fan, 50% duty cycle is 200% power dissipation in the motor coils. To have about the same power, a 24V fan has approximately 4X the DC resistance of a 12V fan. Because power = volts X amps, if you double the voltage, given the same resistance, you also double the amperage. This results in 4X the disipated power.

If you double the resistance, you keep the same current, but at double voltage, you’re still double power. Power dissipation is what burns out those motors.

So, for a single fan, it’s a cap of 25% duty cycle that will give the intended fan life at double voltage. It also gives the intended motor power.

This gets more complicated if you use an RC network to buffer the motor, since a 50% duty cycle can then be used to produce a constant half voltage. The fact that the motor is an inductive load also changes this a little. In short… It’s complicated.

In reality, you’ll have to figure out where the actual power setting is for any given model of fan. It’s tricky without using a scope to measure actual power in the circuit. Expect that you’ll go through fans much quicker than normal. I would suggest that you try it with a max duty cycle of 25% (safe) but expect that by about 35% you’re going to start losing fans.

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Ah, that’s a good point. I hadn’t thought of that.

I don’t really care if the fan just dies. I didn’t have a use for this fan anyway, and if it tanks, I will replace it with a rated 24V fan. It is definitely humming pretty good on 24V at 50% PWM. I will do some over hang tests later and see if I can get something good from it at 35% of max.

That is a good choice too. I should probably do that.