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Variable Power Regulator Mystery DAMMITDAMMITDAMMIT!!!!

I suppose I shouldn't be complaining.  After all, the circuit works:

...and more or less this (the cap straddles the gutter on my board  but Fritzing is limited...)

There's no connection to pin 3, which is "output" supposedly.  Yet it works as a voltage controller-from 18v to 3v-12v (range.)

I based this off of the old Mims circuit on p.76 of the Engineer's Notebook II but made a few adjustments, obviously. First, I barely got 1v of variation (4v-5v) with the 50kΩ pot. When I took out the 1.2kΩ resistor going from the wiper to the output rail, I started to get a little more.  With a 100KΩ pot plus a 10kΩ resistor in serial making up "R1," I get a useful range.

My gut tells me it shouldn't work, but my multimeter tells me it does.  Is it just some kind of bass-ackward voltage division at this point?  The output isn't coming from between a pair of resistors.  Is this chip designed to work this way?  I feel like this:


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As you are testing without a load I would hardley say it's working.

I could point out the obvious and say read the datasheet rather than someone elses possibly faulty circuit diagram but in your case I think this might be more useful.


In the good old days, Australian Icon Dick Smith made his money teaching Australians electronics. This is the data section from one of those catalogs. It has a lot of useful information on not only this voltage regulator but a lot of other common electronic components.

Sadly these day Dick Smith is owned by a pack of retards that completely destroyed all Dick Smith stores and turned them into a bunch of second rate consumer electronic stores where the staff couldn't tell you the time if you held a watch in front of there face.

As you can tell I have some strong feelings on the matter as Dick Smith taught me electronics, not the wankers who call themselves the Australian education department.

Thanks for that OddBot! It's almost as good as this one electronics reference poster I got somewhere...;-) Anyway, their version of the 317/350 circuit looks like some of the more complicated implementations I found. (I chose the one I did because I was too lazy to do the math in the data sheet version but I still wanted a low component count.) Dick Smith must be the Aussie version of Tandy/RadioShack when I was a kid.

In this case low component count = low reliability = dumbass!

Following a bad schematic drawn by someone who doesn't know what their doing because it looks easier is like following a guy who's bumper sticker reads "Don't follow me, I'm lost too!"