Motel Power Supply

Joe, WMQY, and I were talking (err.. typing) on PSK one evening about a regulated power supply for motel QRP ops. We thought that a wall wart might be a good place to start. So I scrounged around in the wall wart stash (we all have one dont we?) and found one rated at 14v at 700ma output. That's nearly 10 watts so this thing should work. It's a GE model 5-2154C and I found a bunch of references using the Google and Yahoo search engines. Most were for sales offers for $2.99 new!!

Testing is prudent with junk box parts so I hooked up a 27 ohm load and plugged it in. 15 minutes later the wart was just barely warm and load was too way hot to touch. The output voltage was constant at 15 volts the whole time. Thats a little over 8 watts so I figured it passed the test. Test results of the final configuration are listed at the end of this article.

I've built several wall wart power supplies in the past by just gluing a regulator on them and wiring it up. Below is one I stuck together a coupla years ago for my Nescaf filter, a fine audio filter kit offered by the New England QRP Club. One thing to remember about warts is that they're usually only at the rated voltage when the output is at the rated current. Other wise they're over or under the marked voltage, depending on the load. This particular wart was rated at 9 volts at 200ma. The NEScaf filter expected 12 volts at a light load, so I used a 7812 and it worked fb, with plenty of headroom for the regulator.

They've all worked well when built like this, so I thought I'd build the Motel Power Supply (MPS) the same way. The two pictures at the right are of the final MPS showing the wart, heat sink, and 7812 before I goobered up the wiring with silicone to stabilize and insulate it. Thats a .33ufd cap on the input and .1ufd on the output.

This regulator application would need a heat sink and I happened to have one in the junque box. Mounting the regulator on it, epoxying it to the wart, and wiring it up was an easy job. The grey blobs are JB Weld epoxy.

Heres's some Test results:

Load: 440 ma
Regulator input Voltage: 15.75
Regulator output: 11.95
Heat Sink Temp After 10 Min: Pretty warm but could keep finger on it.

Load: 665 ma
Regulator input Voltage: 14.73
Regulator output: 11.98
Heat Sink Temp After 10 Min: Very warm couldnt keep finger on it, but not destructive to the components..

The duty cycle of Morse is probably less than 50%, perhaps 40% (just a guess), so the MPS won't run this warm in actual practice. Last but not least I checked out the ripple on the DC by listening with an audio amp, and nothing heard. Not a scientific effort, but probably an ok method.

So there you have it. If you have a well stocked junque box, you can build it for next to nothing. It's cheap, easy, and works. All good ham radio attributes, but you've heard that from me before. If you build it, pse email if you have any questions, and let me know how it works for you.

Revised 27 November 2012 ... WAITP
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