AVR controlled power

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I'm just starting off building up a AT power supply replacement (Xbox) to go into a friends car. I'm familiar with h bridges, AVR programming and car electrics but no expert. I'm throwing this out there and hoping for some constructive criticism.

At the moment I'm thinking a linear regulator (7812 + pass transistor)for 12V (~3A) rail with fastPWM controlled switching into LC low pass networks, independent for the 5volt (~10A) and 3.3volt (~5A) rails. The PWM would be fuzzy controlled though a very heavily filtered/averaged ADC. The electronics would run off a 7805 with good heatsink and the entire enclosure would be shielded and well away from the engine bay. I have some mega8s lying around so I'll use one of them.

In general Xboxes are very tolerant of thier power supplies so there is little chance of catastrauphic failure.

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you know about the commercially available PC power supplies designed for use in cars with 12V as the prime power, right? I haven't looked in a while, but prices were falling.

one place to start looking is
http://www.mini-itx.com/Default.asp

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Thanks, I didn't realise they had gotten so cheap... and small.
Over here (Australia) they are still asking $150+ through the usual sources.

There'd still be a bit of hacking to do though, and then there's the question of actually getting one posted here for a reasonable price. Never know...

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You aren't going to like the results of running a 7812 regulator in a car. It's only specified for inputs geater than 14.5 volts which isn't supposed to happen. A typical boost transistor configuration increases that input requirement by about another volt (15.5 total).

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Good point, LM1084 (low drop out version LM317) and a resistor bridge should fix that

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On further consideration I think I'll go for all three voltages done as switchmode. Once I have the fuzzy tuned (which will be a pain) the rest is childs play.

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This isn't a straight buck or boost design, since you want 12V out with 10V...14.4V in. But the books show plenty of circuits to do the job.

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Why not just get a cheap 12v to 110vac inverter and simply plug the XBOX into that?

We use one we got at Wal-Mart to run a laptop in the car...it cost like 35$

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There is no WAY in the world u can get 3A @ 12 out of 7812 unless u use a FAN or a greate heat-sink.

My consideration, simply make a Switching power supply. it needs an extra inductor, but u can find
alot of components on Analog-Deivces, ON-semi, TI,
Fairchild-semi, .....

Good luck
Nasser