Electronics > Beginners
DC/DC converter with uC interface
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paulca:
I think Simon is trying to nudge you towards using a proper variable speed PWM fan.  These are not expensive, they are pretty standard PC fans.  12V PWM control.  A basic 4 wire fan is about £3 on a PC supplier website, less on ebay.

You get your DC +- an RPM sense wire and the PWM control wire.  Some even work with 5V on the control line or you can use a mosfet to level step the PWM.

The downsides of course are that the PWM control mechanism is not well specified and manufacturers are free to implement it as they choose with regards to things like minimum speed and what they do below it.  For example out of 8 PWM fans in my PC, 5 will not stop at 0 PWM, of those 1 will snap to 100% rpm if PWM falls below about 15% the others will just continue at minimum RPM.  The other 3 will stop at minimum PWM (Arctic 140mm F14s).

With a PWM fan you can (google the spec) and just send the right duty cycle from the uC pin, possibly via a 12V mosfet level changer (your 5V PWM pulses the mosfet gate to pull 12V across the fan PWM pin.

You can also connect the sense line to the uC (via a divider to get 5V) and count the pulses to work out the current RPM.

https://folk.uio.no/kyrrens/diverse/viftekontroller/developer-specs-REV1_2_Public.pdf
Simon:
Yes, once you have paid for the built in electronics to drive the motor (DC to 3 phase converter) actually bringing a wire out for input control is a trivial exercise for the manufacturer. The fans I use a lot at work come in varieties with and without the speed wire and rpm feedback probably for people that don't want to have to deal with unwanted wires as you can't always just leave them hanging around if it is say a wet environment but the fans themselves cost exactly the same with 2 or 4 wires.
aix:

--- Quote from: paulca on May 06, 2019, 06:50:16 am ---I think Simon is trying to nudge you towards using a proper variable speed PWM fan.  These are not expensive, they are pretty standard PC fans.

--- End quote ---

That would certainly be a good solution.  The reason I'm not using one is that the form factor (7530 blower) is a hard constraint.  When I looked for fans in that form factor, I found 2-wire to be common and cheap, 3-wire to be rare and generally expensive and 4-wire to be like gold dust.

For future-proofing, I thought it desirable to keep my choice of fans as wide as possible.  And building my own speed controller felt like a good learning exercise, which is the main reason I am doing this in the first place.  Though I now think that having a fixed efficient step-up to 12V followed by current control might have been a more fruitful approach.  It just did not occur to me when I started this, though it's not too late to play with that as well. :)

Of course if any of you know of a good supply of inexpensive 4-wire 7530 blowers (maybe they're also known under a different designation?), I'd be all ears.  I attach a drawing for reference.
Simon:
Presumablp no one does a brushed version?
aix:

--- Quote from: Simon on May 06, 2019, 09:15:09 am ---Presumablp no one does a brushed version?

--- End quote ---

I've not seen one.
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