as the topic suggests, i'm looking for a device which operates on +12-0V rails, and when given and input voltage will provide the same voltage out (i know there will be a little clipping at 12V - not an issue) HOWEVER with a much larger current caperbility (potentially 3A). ive been searching for ages but cant find anything which fits the bill, anyone know of such a device? a 4 pin device is what ive been hunting for like a +V, 0V vin and Vout type layout.

reason - being driven from a 12V D-A with a +5V offset for fan control. will need many of these, and some will power potentially 3 fans in parallel.

B

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Hi,
If you make it linear then it's going to get hot and will need a metal tab and a heatsink. When you apply 6V to a 12V/3A fan, it might draw 1.5A. That same current will conduct through your linear control device which will have the remaining 6V across it. Therefore it must dissipate 9W. A small transistor is rated for only a few tenths of a Watt. You need a 3A power transistor that is mounted on a heatsink, and a small driver transistor. A 3A power darlington transistor will do the same thing.
A switched-mode power supply circuit can be arranged to follow the output of your D-A and with its high efficiency may not need a heatsink.

 

From the description given, this isn't a power supply at all (so it's rather misleading!). From what he says it appears he just wants to control the speed of a fan? - presumably a standard brushed DC motor? - as he's thinking of varying the supply to it!.

You don't need the complexities of a switchmode PSU, a simple PWM speed control circuit would be far simpler, and do EXACTLY what is required.

L0D|Mr_B - what are you feeding it from?, and do you have access to that device?, and is it a micro-controller you are programming? - if YES you should be able to provide a suitable PWM signal from the micro on a single pin. If it's a PIC it may have hardware PWM available as well, which would make things really simple! - some other micro's also have hardware PWM.

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ok.

i'm building a control circuit to control the array of fans in my PC (i have 12 of the buggers - dont ask!). to control this lot i'm using a VB app ive already 80% coded (needs some tweeks which can only be done when hardware is available). device is to be powered from standard molex power pins (0v, 5v and 12v lines available). communication was to be using the parallel port, but using it serially (as i dont want to wire up so many wires), however i'm now thinking about USB, however i have no idea how to code for, or hardware required for USB. if anyone has any linkage, please fell free to post in this thead, even if it going sideways to this original post.

anyway.

i was thinking of putting some inteligence into the device, such as if no contact from the software after 30 seconds the device goes into "full protection" mode - all fans on 100%, but at the end of the day it doesnt need it - a bit overkilled, besides i have no abaility to program the device - writing the code no probem but i dont have time or money to make a programmer aswell

I have read up about PWM, but ive read in places this is not actually that good - can make fans have a "growling noise" and across 12 fans would be a nightmare esp. when playing doom3 LOL

my idea was first to have each fan connected to 2 relays, so i could put 0V, 5V 7V and 12V across each fan respectivley, but i want a little more control, so i want to use D/A'a to convert an 8 bit number to a voltage with a range from +5 to +12 (a 5V offset). my idea was to plug that voltage via some current buffering straight into the fans - hence my previous question.

if i havent answered what i was asked, sorry! atleast i may have cleared up what i'm doing given the above project profile, what you tink i should do. do you think PWM is the best way to go, or can a current buffering device be applicable here?

B

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Mr B.

With 12 fans running at full power I hope you have tied your computer down or it will run away like a hovercraft. :lol: This site has a lot about fan control: http://www.cpemma.co.uk/pwm.html

Ante :roll:

 

PWM can make audible noise unless filtered, which should be done with an inductor. If you know what the max surge load you have is, you might be able to design a useful RC filter.

It's certainly possible though to just build a reg with an op amp and a power transistor, I think that's what you want. You may need to slow down the amp's feedback path to prevent oscillation. It will dissipate heat in a predictable way, it's 7 watts per amp when going from 12V to 5V. Now you CAN shift the heat away from the sensitive transistor by adding a resistor in series. If you know your circuit never needs more than 1 amp, and your 12V power is regulated and guaranteed not to drop, you can add 7 ohms of resistance if it's a MOSFET transistor or about 6.5 ohms if it's a bipolar transistor. The transistor will dissipate little or no heat at full power, its max heat is at the 0.5 amp point. You would need to select a resistor capable of handling the power dissipation.

 

It seems rather silly pumping all this heat into the case, when it's intended to run cooling fans :lol:

However, many computer fans are brushless, and don't accept PWM control well - they may not even be very happy about low power feeds?.

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