fans in series

Uneven airflow loading could burn out a fan...

This sounds like it would work. Should probably add a fuse to it.

Zeners and pass transistors would also work.

John :-#)#

On a sunny day (Wed, 12 Jan 2022 19:48:59 -0800) it happened snipped-for-privacy@highlandsniptechnology.com wrote in snipped-for-privacy@4ax.com:

My system using Meanwell 7.5 V supplies in series allows any voltage output in steps of about 6 to 8 V.

In your case and if customer has only 48 V, bolt an ebay 48 V to 12 V switcher in the housing, maybe something like this:

a fuse in primary..

You should use the switcher or buy 48V fans.

More fancy fans have things in them to intermittently attempt to restart after they stall without running the windings continuously, and other fancy things. There is no reason to believe that the fans would each want to draw the same current at all times. If you put zeners across each fan, it might work, but in the event that one fan doesn't draw current (e.g. stalls and goes into intermittent restart attempts) the corresponding zener might get very hot.

Why?

For some reason, 35mm fans only seem to come in 5 or 12 volts. They only need 50-90 mA. High voltage switchers are a nuisance.

This might be OK:

$7, if we can get them.

But we have forced air cooling!

Well, if you are sure that your fans don't have anything smart in them and always draw current, monotonically increasing with voltage, or if the power of the fans is small enough that you can dissipate the same amount of power in a parallel zener if you need to, then fine you could put them in series.

I have seen some fans that when you stall them, they go into some low-power mode and then every second or two, briefly have a go at restarting, so they are high-impedance most of the time then draw a big spike of current when they try to restart. I don't think these would work well in series without the zeners.

Don't do it. They will not share voltage and the losers will burn out on the first start. Been there, done that. Find 48 volt telecom fans or 24 volts ones and use individual series resistors. Fans are mechanical so don't increase your failure rate by using any sort of chain.

The 35mm fans I want to use seem to only come in 5v and 12v. And the supply is 48.

I suppose I can make a 48-to-12 switcher on this board. Still, a series string with shunt zeners ought to work.

søndag den 16. januar 2022 kl. 16.30.17 UTC+1 skrev snipped-for-privacy@highlandsniptechnology.com:

once you add a switcher you can also stop it sounding like jet engine when it doesn't need to

snipped-for-privacy@fonz.dk wrote: ===================

** Ever heard the *roar* of a 35mm, 1W fan ?

..... Phil

I expect you will have some size issue that prevents you from using a single fan, but you get a lot more CFM with less noise from a single larger fan than three or four small ones.

Looks like many are unobtanium, but sizes down to 40 mm with 60 mm being in stock.

Why the need for 35 mm exactly? Cooling different parts of the design?

Yes, one that moves any air is like a very large mosquito. Very annoying! Put three in the same box with slightly different speeds and it's quite annoying.

That guy is pretty bold with the stuff he does indoors.

For brushless fans, they just give up smoke and go open, and there's likely a fusible resistor in the larger ones. Never seen on fail short and consume more current than expected.

If you can go for a larger diameter fan, even mounted at an angle. They run slower, make less noise and can last longer. This also opens up the ability to get telecom fans. Fancy options allow for analog or PWM speed control as well.

If reliability is key, run two fans in series (air flow wise). There is no major performance difference otherwise. You can even get counterrotating double thick fan modules as used in servers. The major brand ones like Nidec are actually extremely reliable at high temps and speed, even with ball bearings. They really figured figured these things out.

==============================

** Liar.

** Bullshit.

Why do you f****ng LIE so much??

Rhetorical question .....

he's done all the proper safety assessments

We plan to control the speed of the two giant fans on the front panel of the 3U rackmount box. But the three little fans will be on a plug-in board, specifically a programmable ac/dc dummy load board. Those boards will be deep inside so not very audible.

We will know the heatsink temperature, and we have an fpga on each board, so we could get fans with a pwm control input and throttle. That would probably improve fan life.

Of course to pwm all three it would be rational to have all the fans grounded.

I guess we'll switch from 48 to 12.

I considered all sorts of ways to use one big fan, horizontal or angled, and couldn't make it work. It would need some sort of ducting, and the next board is 1.6" away so there's no way to get the air into and out of a big fan. We do want to shoot the hot air out the rear of the box, not stir it around inside, another constraint.

We're building a mockup for thermal testing. I have no analytical or simulation tools for a thing like this, and my instincts for air flow are all mediocre guesses.

It wouldn't be hard to make a rotating magnetic field that would spin a metal disk or sphere to, say, a million RPM, enough to tear anything apart.

søndag den 16. januar 2022 kl. 22.39.02 UTC+1 skrev snipped-for-privacy@highlandsniptechnology.com: