Powering an Intel Mac Mini from 12V using a Kensington 33196 Universal Laptop Adapter

Edwin Olson, eolson@mit.edu, 7/6/2006

We wanted to power an Intel Mac Mini from an unregulated 12V supply. (In our case it's a robot, but it could also be a car.) The Mini takes about 19V, so we purchased a Kensington Universal Laptop Adapter (PN:33196). It's tiny, and it says it can source 120W!

Mac mini power pinout

1 Gnd
2 +18.5V
3 iSense
4 +18.5V
5 Gnd

Understanding the Kensington Universal Adapter

Using a set of interchangable tips, the adapter can be configured to output a variety of output voltages ranging from about 14 to 20V. Our first step was to figure out what the 5 pin "laptop adapter" was doing... 5 pins go in, and only power/ground come out (to the laptop). Truth is, we haven't figured out what the hell Kensington was doing, it seems silly. But here's the schematic:

4 --.
5 --*
1 --*
2 --*--- GND

3 ------ VCC

Ra, Rb, and Rc are resistors. On the tips that came with the adapter, Rc is always 750 ohms. Ra and Rb appear to be a resistor divider whose output controls the voltage on pin 3. Playing with a bunch of tips, it appears that the output voltage is approximately:

Vout = 24*Ra/(Ra+Rb)

Additionally, it appears that Ra+Rb is always about 2.68k. But not quite. We don't get it. What the heck is Rc doing? We assume pin 4 is a reference voltage from which two other voltages (pin 5 and 1) are derived, but we don't know why two reference voltages would be useful. Or maybe pin 5 is a reference voltage, and Rc sets a current limit?

Powering the Mac Mini

Chop apart the Apple-supplied power cable roughly 1/3 of the way from the tip. There are two thick wires (power and ground) and one thin gray wire (iSense). Put mating Molex 3 pin connectors on them so that you can still use the Apple power supply if you need/want to.

Now, we just need to put a 3-pin connector on the Kensington. You can do this by chopping the Kensington cable and wiring up the resistors on a proto-board. Or, you can notice that tip N3 is already configured for 19.1V (close enough) and just force the thing open, de-solder the tip, and wire up the molex connector to where the barrel connector was. We had a hell of a time breaking apart the tip-- it's superglued. Next time, we'll probably just use proto-board and make our own. When you're done, you've taken care of power and ground.

Now you need to take care of iSense. iSense is another little mystery. We're not quite sure what it's doing. However, the Mac refuses to boot if iSense is left unconnected. According to the carpc folks, wiring iSense to ground through a 3.3k resistor is the "right thing to do". It works. We're still not sure what it actually does. (If it was a current sense, the Mac could sense it itself, without running a cable to the power supply.)

Power Consumption

Sitting idle at the finder, the Intel Mac Mini draws about 19W. Running multiple processes, Xbench, and anything else we could think of, we couldn't get the power to go over 38W. (This was a 1.66 GHz Intel Core Duo Mac Mini.)

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This page (C) 2003-2004 by Edwin Olson, eolson@mit.edu. All Rights Reserved. Feel free to link to this page, but do not copy it or its figures.