Notes On "Simple" Pedalboard Power Supplies

[smallbearelec]

Re the questions about which power supply to build, I may turn this reply into an FAQ, because the issues that are raised--and not raised--in that thread keep coming up.

The AC wall wart as input to a small box with a filter/regulator has some serious advantages. The transformers in those bricks are usually either fused or designed to be self-current limiting, so you don't necessarily have to build in that feature on the DC side. The wall wart also eliminates potential for shock hazard. Biggest disadvantages, IMO, are that it is a two-piece solution, and plugging in a wall wart isn't always convenient.

Some of my customers who have done the "Spyder" thing, have been very happy with the GEOFEX design based on the small Mouser 41PG006 transformers. Only be aware that they are 110-volt only. I believe that one of my European accounts added a small 220 to 110 transformer inside the same enclosure and was off-and-running.

For a small, inexpensive, higher power 12 VCT transformer, you would be hard put to do better than the flat-pack on my Stock List, SKU 2401. The primary of that unit can be wired for 110 or 220 volts, and a number of my European customers have had good results with it. If you do use it with 220, buy a proper three-prong inlet and cord set similar to the ones I offer for 110 Volts. They will be much more common  (and cheaper) overseas than here.

Do not build without some form of current limiting and short-circuit protection! The popular three-terminal regulators will go into a "shutdown" state under short circuit, which reduces the output voltage to 1 Volt or so. This protects the chip, but it does Not keep the transformer from pumping current into the input and overheating!

A number of the PS designs out there show a small fuse in the primary circuit for current limiting/short-circuit protection. In prototyping my Small Wart kits,

http://www.smallbearelec.com/Categories.bok?category=Kits+And+Designs

I found that the lowest-rated fuse I could find would not always pop under a short-circuit condition. That's why I designed with resttable fuses; they are cheap and simple to install, and the limiting action is quick and positive. The construction articles:

http://www.smallbearelec.com/Projects/ProjandProd.html

give suitable part numbers for types to use in the nominal 60 ma. and 200 ma. builds.

If you already know what you are doing, the construction ideas in the Small Wart articles may save you some time and trouble if you are building a supply into something else. If either one would be your first PS project, please, do not deviate from the construction shown. In all cases, when you are connecting to line voltage, treat the job as though you are building life-support equipment, because You Are.

Regards
SD