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simple Power Supply design help needed
Hi, I'm building a power supply to run a 12vdc car stereo in my room.
The power amp is a 300w ADS Power Plate 300. At 12.5v I need a 24A supply? This seems wrong somehow. You may need _more_ than that. If the amp is actually capable of creating 300 watts of clean audio output power, it's likely to actually be drawing 500-600 watts of total power (amps are not 100% efficient), and you may need 50 amps or more. If the "300w" is marketing puffery, or if you're willing to run the amp at quite a lot less than its full output capacity, the actual needs might be a lot less than that. I thought I'd use the xformer from a surplus 12v battery charger with an LM317 but I have the feeling something needs review here. Darned right. An LM317 by itself is limited to 1.5 amperes, I think. It's not going to cut it. I'd appreciate any observations others may have. Thanks for your time, pm. You're likely to need a hefty DC supply to run an amp of this calibre indoors. Using a battery-charger transformer and rectifier bridge is probably a reasonable starting place. You'll need a good-sized filter capacitor following the rectifier bridge. You'll need a hefty set of pass transistors - the commonest linear-supply design of this sort uses multiple 2N3055 transistors (TO-3 cases) in parallel, with ballast resistors, and a set of big heavy heatsinks. Take a look at the LM317 data sheet - I believe you'll find details on how to use external transistors such as 2N3055s as external pass-elements, to boost the LM317's current capacity. For supplies in the 25-50 ampere range I suspect you'll need an external driver transistor or two inbetween the LM317 and the pass transistors, to provide the necessary amount of base drive current. You'll need to design in current-limiting circuitry (straight limiter or foldback) to keep your supply from frying its transistors, or starting a fire if the outputs are accidentally shorted. If all of this still seems a big overwhelming, you could just buy an Astron switching power supply (see www.hamradio.com for one source), or just decide that using an auto amp in a home setting is more trouble than it's worth, and buy an amp designed for 110 volt operation. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
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