iGo makes a universal power adapter to power things like laptops, displays, and mobile devices. They offer a large variety of interchangeable tips to plug your specific device in. I found an Apple Studio Display LCD monitor at a local surplus and it didn't have a power supply, not to mention that I didn't have the appropriate tip for my iGo Juice 70.
The Apple Studio Display required 24V and up to 1.87A, which I figured the iGo would handle well since it can be configured to output up to 70W and anywhere from 15 to 24V depending on the tip. The only thing left was how to trick the iGo into thinking it had one of the 24V tips plugged in.
Step 1: The IGo Connector
iGo decided to use a 4-pin connector for their tips. After some probing of the connector and of my tip with my multimeter, it was apparent that the first two pins are ground and power, connected directly to the barrel jack contacts. The last two pins are for adjusting the voltage and current limits of the power supply. The tip connects each limit pin to ground through a resistor whose resistance determines how high the limit is. My tip (I only had one to measure) had 13.9kΩ on pin 3 and 162kΩ on pin 4. By hooking up different values of resistors, I was able to watch the output change.
It appears that Pin 3 is the voltage limit, and Pin 4 is the current limit.
Pin 3 can have a resistance of anywhere from 2.5kΩ to infinity (open). 2.5kΩ sets the voltage to 24.5V and open is 15V. Any resistor in between can be chosen to get the desired voltage in that range. My 13.9kΩ tip tells the adapter to put out 16.6V for a Thinkpad laptop.
Pin 4 is a little harder to measure, since current limits require that you actually draw that much current. The tip had 162kΩ in it, which most likely corresponded to an amp or two. I actually found an article on Neripedia about someone else configuring an iGo adapter and he has the resistances listed that he measured from 9 tips that he has. The only discrepancy is that he lists the current limit resistances as voltage limit resistances and visa versa.
Step 2: Making Your Own Config
So my desired output is 24V and at least 1.87A. This is the very top of the range for the adapter, so I need 2.5kΩ. I went with 2.7kΩ and confirmed that the adapter was now outputting 24.25V.
The current limit was much less of an issue, so I decided to go with a 50kΩ resistor. That aught to give me a high enough current limit to supply 2A without worrying.
Step 3: Constructing Your Custom Connector
Since the iGo is a very nice power adapter, I didn't want to destructively convert it. Resistor leads seem to stick very nicely into the pin sockets of the power cable, so I stuck the resistors straight in and hot-glued them to the connector body.
Make sure to leave a long enough lead on the ground sticking out so you can connect your power wires to it. You'll also need an additional resistor lead to stick into the V+ output (Pin 2) since no resistors connect to that.
Once you're done with fixing the resistors in place, you can solder on wires or a power jack of your choosing and you're ready to go! Always coat in ample amounts of hot-glue or liberal use of heat shrink; they're only semi-permanent and cover any exposed conductors!
Step 4: Testing It Out
All put together, and everything seems to work as expected. My Apple Studio Display plugs right in, get's 24.25V and runs great. When the monitor draws a lot of current (over an amp), the voltage output will sag to 24.10V, so being a little bit above 24V is good.
I suppose that if you wanted, you could put a potentiometer on the limit lines and make you're own 15-24V current-limited variable power supply. Should be good for up to 3-4.5 amperes!