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Linksys 3.3v PSU

S

Sinclair

Limp Gawd
Joined
Aug 5, 2004
Messages
242
Not sure if this really belongs in this forum, but since it does relate directly to a power supply question, I'll start here :p . A little history: I've had a Linksys 16 port switch for a few years now, maybe 6 months ago the 3.3v walwart decided it had had enough and killed itself. I've been running it off a spare ATX supply's 3.3v line for awhile now but would like to try to find something a little more compact for it. I built up a little LM317-based linear regulator to drop a 12V to 3.3V, which seems to work just fine. I'm a little worried about how hot it gets tho, finger-scalding hot hot hot even with an oversized heatsink (hot as in burning the spit off your fingers hot). What I was wondering was if anyone had any idea where I could score a replacement 3.3v walwart or perhaps a little better designed regulator (used these plans for the regulator). I've been able to find a few on the net, but wasn't too sure about the stores that were carrying them. Any suggestions or ideas would be appreciated :D .
 
I had a set of speakers that was missing the 12V wall wart. Radio Shack had a replacement that fit. Even if Radio Shack carries a 3.3v that doesn't fit your application, they have a universal adapter that looks like a star shape with all sorts of male power plugs on it. Of course as it's Radio Shack we're talking about, you will be forced to buy one that's plated gold for 3x what it's worth. No really I'm not joking. Radio Shack has an obsession with gold plating everything...
 
I thought about checking the local stores for a replacement, and did, however the closest I could find was a 3V and a 4.5V.. neither of which the switch really liked =). It would be so much easier if they used a more common voltage, 5V, 12v, etc.
 
Well, if you're dropping 8.7 volts at, say, 500mA... that's 4.35 watts, which is a lot to dissipate off a TO-220 package.

See if you can get a 6V wall wart or something instead of 12, you'll bring the dissipation down considerably. Or if you're feeling creative, you could use a Maxim MAX788 or similar chip and build a switching supply. Or if you're feeling lazy, go on national.com and use their online "SMS" design software - for a few bucks they'll even send you a miniscule custom board with a switching regulator that includes everything. :D
 
Yeah I had originally planned to put 5V in and drop that down, however when I tried 5V, the best I could get out of the regulator with a load was about 2.7V or so. Saw something looking around usenet about having to have a 3V difference or so between the outputs, maybe that has something to do with it. In either case, it was still pretty hot putting 5V into it :p . I'm sure I have a 7V wallwart around here somewhere I can try.. have to try that and see what happens. As a note, according to Linksys's tech docs, its only supposed to pull about 300mA (or 500, forget now). For 20$ tho I may just end up picking up that one linked above.
 
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