Power Supply Protection and Design

With lightning storms all over, I improved the power supplies for my palmtops and other computers.

Power Board

Too many computers and plugpacks for my wall outlets. Any sensible person would go to the store and buy a multi outlet power board. I had some outlets kicking around so I decided to build my own in a small plastic case. I could fit a dual outlet power point, and two IEEE (computer style) female outlets. Input was via an IEEE male connector, so a spare computer power cord could connect the box to the wall socket.

I had a few ex-equipment LC mains filters on hand in my junk box, and some GE 275L40 and 60 MOVs (metal oxide varistors). With the Australian line voltage 240 VAC, I need MOVs to be at least 275 volts (US readers should use maybe 140 volt units). If I had only one MOV I'd put it on Active to Earth (Ground). Since I had a handful, I put them between all three lines.

I had some fuseholders, so I added a fuse. Then I figured the box would be nicer if it had a neon light to indicate power was on. You just put a 100k to 1 M ohm or so resistor in series to limit the current. However just a neon is boring, so I rigged it to flash if the fuse had failed (which really is overkill, as you can usually figure out a fuse has blown, but gadgets is gadgets).

                   fuse
    Active    --+--++++--+---            Power flows thru neon via
                |        |               R1 and D1 and via R2 and D2
    D1          V        V    D2         so it glows.  If the fuse
    1N4001     ---      ===   1N4001     fails, it flows via R1 and D1.
                |        |               C1, R1 and the neon form a
                |        |               relaxation oscillator.  C1
    R1          r        r    R2         charges to the striking voltage
    2M          r        r    150k       for the neon, so the neon
                r        r               lights.  C1 discharges and the
                |        |               neon flashes.
                +--------+
    C1 0.22uF   |        |
    400v       ___      ___
               ___     |___| neon
                |        |
    Neutral   --+--------+--

Since I'm located down the far end of a long line from the power station, I thought it would also be nice to know if the power had failed, so I added a circuit to indicate that the power had been out. It is a bit primitive, since it only gives a binary indication, and not how long the power was out.