Electrical Projects The power supply can be used for bench operation of low-voltage ac and dc appliances and control devices. Transformer T may be a Stancor P-8364, which has three 12V secondaries of which one is centertapped. The load current rating of each of the windings if 0.5A. Jacks Jl, J2, J3, and J4 are three-conductor phone jacks. Switch S2 is an SPDT toggle or slide-type which in the position shown short circuits diode CR so that the output will be ac. In the other position, the short is removed from across the diode and capacitor C (100 uF, 150V) which filters the now-dc output voltage.

For convenience, there are only two output terminals to which the load is connected. Voltage selection is achieved by means of a three-conductor phone jack whose tip and ring terminals are shorted. The desired voltage is passed through the selected jack and plug. By having only one plug available, inadvertent application of more than one voltage at any one time can be avoided. Note that the frames of the jacks are not connected to anything and therefore need not be insulated if mounted on a metal panel.

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October 6, 2008