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Posted by David Sherman on Wednesday, April 01, 2009 at 20:15:26 :
In Reply to: Just curious... posted by Hank on Wednesday, April 01, 2009 at 02:42:07 :
That's what I was describing. Half a dozen parts would do it. However, to step UP in voltage is a lot harder than to step DOWN. It takes a switching power supply, and even with custom chips, they can still be fussy. They can also generate a lot of radio noise. You'd have to know a fair bit about electronics to build one that would withstand the abuse of a vehicle electrical system and would also not generate radio interference. To do it right is more than just a "throw it together from a schematic in a book" kind of project. I'd guess that in addition to the half-dozen parts for the actual power supply, you'd be in another half dozen parts for the electrical abuse protection and interference suppression, and you'd have to know how to design all that stuff. Then you'd have to construct a circuit out of it, probably by hand-wiring all the parts on a piece of vectorboard, and then once you had it working (it would not work the first time, until you'd futzed with it a while), you'd have to mount it in a sealed box with some sort of terminals or connectors on it.
In terms of engineering, it's fairly simple to somebody who knows what they're doing, but packaging it up in a way that will be reliable and maintainable in an automotive environment is not a trivial piece of work. I recently used a "simple" three-terminal regulator to make 6 volts out of 12 to run the gauges in one of my sno-cats. In the simplified schematic, it would just be the regulator plus 2 resistors, but by the time I had all the protection around it, I'd added 2 capacitors, a zener diode, and 2 regular diodes, and then I had to mount it in a gasketed box with screw terminals sticking out through the sides in a sealed way. That simple little regulator became a fairly elaborate construction by the time it was ready for real-world use, and that was a step-down converter, not a step-up one.
The difference between a piece of electronics that works perfectly when new but breaks at the slighted provocation, and one that is rugged and will survive a long time in the real world can easily be twice the number of parts and 3 times the cost. Much aftermarket automotive electronic gear will not survive load dump faults (generator running while battery is intermittently disconnected due to a bad connection), reverse polarity (stupid use of jumper cables, or hooking the battery up backwards), high temperatures, or dampness. Commercial temperature range electronic parts are only rated for use up to 70 degrees C (158 F), which is easy to exceed in a vehicle. Industrial goes to 85 C and military to 125 C, with the price going up as well. The circuit boards in most consumer electronics are not cleaned well after soldering, so they have lots of flux residue on them, and they're never conformal coated, which means that moisture condensing on them, which is very easy in an unsealed housing in a vehicle that's exposed to environmental extremes, will cause all sorts of intermittent malfunctions.
I guess my point is that even when the concept is simple, making a reliable automotive electronic device takes a lot more than just a schematic and some parts.