In desktop computers, it's common to have headers that control fans with a 0-12V analog output, capable of up to 1A. This works fine for one or two fans, but with more than three fans, you can exceed the 1A rated current.

The obvious solution to me is to make a voltage buffer with power fed from the power supply's 12V rail, and I'm amazed this isn't available already. I know how to do this with an op-amp, but that would not supply enough power and it seems that a buck regulator would be better suited to this, but I can't figure out how to do it. Are there any buck regulator ICs that would be able to implement a 0-12V 5A voltage buffer, fed by a 12V input? Is there a better way to do this? I don't care about high frequency response, and a ~10Hz lowpass filter would be ideal.

  • \$\begingroup\$ I just learned these are also known as voltage followers, and according to one company, "Switched-mode Voltage Mirrors". Avatekh seems to be doing exactly what I want, but not selling any products yet :( \$\endgroup\$ – ahalekelly May 27 '15 at 6:04
  • \$\begingroup\$ A couple of notes: if you exceed the rated current your fan controller would probably not get damaged, but the fans will be slower. As per your project, time and money wise you're probably better off buying a bigger controller. \$\endgroup\$ – Vladimir Cravero May 27 '15 at 7:50
  • \$\begingroup\$ Good to know. I have a manual fan controller with plenty of power and channels, but right now I have to check CPU temperatures and adjust the sliders. I have one fan output on the motherboard that's controlled by CPU temperature right now. I'm trying to figure out how to run all my fans off of it, and I can't seem to find any product that will do that. \$\endgroup\$ – ahalekelly May 28 '15 at 14:15

I assume this doesn't need to be particularly precise output voltage, you just want the secondary supply to approximately match the varied voltage on the primary supply. You may be able to use a two-input comparator as the "controller chip" that controls the gate of a P-ch MOSFET in a typical inductor-based buck topology.

Since the comparator would probably operate at less than 12V supply (possibly 5V) you would need a small regulator, and consequently also want resistor dividers to provide a lower reference to each input.

Basically, this would be a circuit to switch on the FET whenever supply 2 is lower than supply 1, and switch off the FET when supply 2 is higher than supply 1. Probably also want to design in a small amount of hysterisis on the feedback side, this should allow some control of the circuit's switching speed, otherwise may be overly-sensitive with constant rapid switching.

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