Level shifter question

[disphenoidal]

A friend recently purchased a fan controller that allows him to adjust his fan voltages from 12 to 7V. However, he would like to run a wider range of voltages, say 12 to 5V. My idea was to build a level shifter out of a transistor and resistor. The drain of the transistor would be connected to the fan and a resistor to 12V. The source would be grounded, and the gate would be connected to the existing fan controller. As the gate voltage increased, the output would be pulled down to ground. Has anyone tried anything like this? I thought I'd see if anyone had any experience with something like this before I started breaking things.

[Felger Carbon]

A friend recently purchased a fan controller that allows him to adjust his fan voltages from 12 to 7V. However, he would like to run a wider range of voltages, say 12 to 5V. My idea was to build a level shifter out of a transistor and resistor. The drain of the transistor would be connected to the fan and a resistor to 12V. The source would be grounded, and the gate would be connected to the existing fan controller. As the gate voltage increased, the output would be pulled down to ground. Has anyone tried anything like this? I thought I'd see if anyone had any experience with something like this before I started breaking things.

Assuming your friend only needs to occasionally adjust the voltage, and can live with (say) 10v to 5v in the winter and 12v to 7v in the summer, it would be a lot cheaper and simpler to get a "2V wire" (Akasa) or "3V wire" (Jab-Tech.com). This is a multi-juntion silicon diode that's in series with the power lead with 3-pin connectors (male, female) on each end. No "Xvolt" wire, 12-7V. With a 2V wire, 10-5V.

If your time is worth anything at all, this is the cheaper way to go.

[dinofx]

How does the fan controller work? Are its internals exposed? I know on mine, I can replace 2 of the resistors and change the range from 7.4-12 to 6.0-12.

[disphenoidal]

I had considered using diode(s) in series, but wanted to know if there were other options. I usually use LEDs, actually, since you can choose how much voltage drop you want based on the color of the LED, and they drop more voltage than regular diodes, so I usually only need one. And while I don't go in for the crazy cases some people have these days, I think an LED or two can be tasteful.

As for the internals, I really don't know what's going on in there. My friend says there are some integrated circuits in it, so it's not just potentiometers. But what they do, I don't know.

[TomZ]

I usually use LEDs, actually, since you can choose how much voltage drop you want based on the color of the LED, and they drop more voltage than regular diodes, so I usually only need one

Most LEDs are only rated for maybe 40mA (0.040A) maximum, so unless you are using either a small fan, or a large LED, it would be easy to exceed the power-handling capacity of an LED in this application.

Of course, if you are finding it works well in practice, then that's great.

[disphenoidal]

Most LEDs are only rated for maybe 40mA (0.040A) maximum, so unless you are using either a small fan, or a large LED, it would be easy to exceed the power-handling capacity of an LED in this application.

Honestly, I hadn't thought about that. I'm running an 80mm fan, there's already the psu fan controller driving the diode and fan, so I don't know what voltage I'm seeing right now (And am scared to run the psu while probing around with a voltmeter) or what current the fan pulls at that voltage. Oh well, I'll let you know if anything dies.

[cpemma]

Zener diodes are the thing to use to drop several volts.

The 1.3W BZX85 Series zener diodes come with voltages of 2.7V, 3.0V, 3.3V, 3.6V, 3.9V, 4.4V, 4.7V, 5.1V, 5.6V, 6.2V, and so on, so there's a good selection of under-voltages possible.

More here.