For a power supply to be properly effective, it needs to be able to supply enough current to whatever is connected to it. It can be capable of supplying much more but this supply will be connected to only one piece of equipment so we don't need to be too generous.
A power supply has two stages; the reduction stage where the mains voltage is reduced to a much lower level and a smoothing stage where the mains AC is converted to a smooth DC needed for the drive.
Much has been said over the years about the potential dangers of mains voltage. As an electrician and as someone with medical qualifications I am perhaps better placed than most to understand these dangers.
It has been at least eight years since I got a shock from the mains (or a "bolt" as electricians call it!). I avoid it because I know the risks and I am careful. I always check my connections before I connect to the mains. I test connections with a suitable checking device before I touch them. My connections are always tidy with no dirt or bits of wire hanging around. I never say "That will do for now while I check it." I always make sure that every connection is safe before I leave it. And I never trust anyone else to disconnect the mains for me. If somone does, I check that it is really disconnected before proceeding.
Suitable checking devices include a good voltmeter set at 240V AC or as near as. This will be what most people have. Electricians have volt probes but I don't expect most people to have these. Neon screwsrivers are not suitable - they are dangerous, unreliable and useless. I know some electricians do use them but that doesn't make them right. When you use a neon screwdriver, the metal tip touches the mains which travels up through a vert delicate neon indicator, through the metal tip, up your arm, through your heart, lungs, stomach, intestines, kidneys, privates, legs and into ground. You may not feel it but that doesn't meen it's safe. Added to this is the very real potential for failure of the neon's resistance and "zap"...
I wrote a description of what actually happens when you get electrocuted but, after reading it through, decided it was too gruesome. Suffice it to say that electrocution is not a quick death; it is slow and very painful. You may have had a few bolts and so far got away with it. You have just been lucky. There is no method of protecting yourself from a mains shock. Quick reactions are of no use since your nerve endings will be frozen. The only effective method is not to let it happen at all.
These neons aren't even very good for screws since the tips tend to break. There is a way to make them safe however. Place the neon screwdriver on a hard surface, take a large hammer, raise it about two feet high and bring it down onto the handle with some force. Finally sweep up the remains with a dustpan and put it in the bin.
Maplin sell a suitable transformer (Code WB25C) for £9.50. It has 2 12V outputs and is rated at 25VA so each 12V line can produce 1A. A cheaper one could probably be had from a local electronics store if you know of one.
Next you need a suitable case. The case should be large enough to take the transformer and have about 3 inches x 3 inches left for the circuit board.
Ideally the case should be made of metal, pressed aluminium cases are widely available and quite cheap. It must of course have a lid.
The first rule is to put your transformer into a suitable case, make sure your connections are good and make sure that the cable is adequately protected, as it leaves the case, both from damage from the edges of the case. Make sure it cannot be pulled and perhaps come loose. A strain relief grommet will do this quite well. The cable wil need to be 3 core and capable of carrying 3A so chose a grommet for this size of cable. The hole for one of these grommets has to be a rather odd shape, not round. However I have found that if you drill a round hole and push the grommet from the inside of the case out it will be good and strong.
Building the board will be the second stage. Circuits should be built into vero strip board. Careful soldering is needed.
The cable going out to the low voltage power plug could be the same as the mains cable. A strain relief grommet can be used again to protect it.
Check all connections and joints and, when you have finished, check the voltage output in each line before you connect tou your drive. These drives don't like the wrong voltage being connected and once your drive is broken, you really need to throw it away. Repairers are difficult to find and expensive.
Gus Donnachaidh, EUG #26