Mech. Engineer so not too much electrical circuit knowledge.  My question is what the easiest (cheapest) way to step up DC voltage 12v to 24V in ranges?

Thank You
William

A few questions:  how much power does the 24 volts need to supply?  How well regulated do you need it to be?  What do you mean by "ranges"?

There are many switch mode power supply ICs from companies like National Semiconductor, Maxim IC, Linear Technologies, Texas Instruments, Micrel, Microchip, that can do this with the addition of a handful of inductors, capacitors, resistors, and perhaps a schottky rectifier and MOSFET that can provide anywhere from a watt or so to several hundred watts.

A quick and dirty method is to make a square wave (a 555 timer and some resistors and capacitors can do that) and then a voltage doubler (two diodes and two capacitors) will get you a low current supply a volt or two shy of 24 volts.

For one-off applications, you just buy it. The good news is that nominal 12 volts DC input, with 24 volts DC output, even at relatively high power levels - these are readily available.

E.g. http://www.powerstream.com/dc24.htm

Many more. Google: 12 volts to 24 volts converter

Even if you're designing a new product that requires a circuit design, then it might still be worthwhile to order a few commercial examples for "competitive analysis."

I think both of you guys gave me two solutions that I needed.  For prototype I will get a ready made unit.  For final product I need low cost-low cost-low cost and footprint.  I think the numbers I'm playing with is IN= 12V, 5 Amps; OUT=24V, 2.5Amps.  It's driving DC motor so I don't think sygnal conditioning or exact voltage is detramental.  As far as ranges go I just wanted to be able to select voltages in that range may be in 5 equally spaced increments.   

Lowest converter cost is to buy a motor that will run on the voltage you have.

Mike Halloran
Pembroke Pines, FL, USA

Or two batteries wired in series.

Understood- but not an option.   

For your final design I don't think the capacitor doubler will get you what you want.  Your desired current is too high.  Better to look at a Boost converter topology.

Maybe this is too late to help you, but if Boost Converter gives less performance than you need, the Boostbuck Converter will provide smoother, quieter ouput; and easier to control with feedback.  If you've already built the Boost, you can just follow it with a Buck Converter in series!

Go to ebay ans search 280754070785  For $15 you likely wont do better building it on your own.  It is 10A and I wouldn't go smaller unless you want to inatall a fan.  Control voltage with a pot or set of resistors to step.