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