A versatile PSU with regulated, switched outputs. Design by David Strange

Every time a piece of electronic equipment is needed, so is a power supply. The alternatives for powering equipment are batteries or mains and most equipment is designed incorporating one or both systems. However, in a studio or music laboratory, where lots of gear may be modular or custom built, a mains driven power supply is essential — batteries only being suitable for short-term or low current applications.

Design

The power supply presented here derives five fixed voltages, selected by a switch, and can deliver a minimum of 1.5 Amps. It is protected from short circuit and overload damage by thermal and current 'trips'. One particular feature of benefit in audio applications is the supply's freedom from hum and noise superimposed upon its DC output. This can be seen from the specification shown.

Construction

The power supply consists of two sub-assemblies: firstly the transformer, rectifier, capacitors C1 and C2, and various resistors, and secondly, the regulator section on a heatsink. Since mains voltages are used in the power supply, a suitable enclosure should be chosen and care exercised in layout to avoid the chassis or outputs becoming 'live'. Just follow the layout and there should be no problems. A tag-board construction is recommended as the components are few and therefore the assembly relatively simple. The transformer should be bolted securely in position close to the rear of the chassis and the tag-board mounted close by its output terminals. The board will require raising from a metal chassis by spacers. A couple of nuts run down over bolts pushed up from the underside of the chassis will do quite well.

Before fixing the tag-board into position it can be pre-wired with its components (as shown in Fig 2). The short links between the various components on the board can be made using bare tinned copper wire. Wires to external components need to be of insulated wire such as PVC covered 16/0.2 and preferably follow some colour coding system to prevent confusion later.

Once the pre-wired tag-board assembly is in position the switch can be fixed through the front panel and the necessary wires made to it. Remember when wiring the switch that the wiper will appear to rotate anti-clockwise at the rear when its control knob is rotated clockwise from the front. Ensure that the connections are made so that the voltage from the power supply will increase as the control at the front is turned clockwise.

The transformer can be connected next with the wires from the switch and from the tagboard. The mains input cable to the transformer primary should pass through the chassis enclosed in a strain relief grommet, otherwise over a period of time the cable will chafe and its bare conductors present a hazard. Ensure that the chassis is properly earthed by the earth wire of the mains cable being well soldered to a solder tag bolted to the chassis. As an added precaution an antivibration washer can be used between the chassis and solder tag to bite into the two metal surfaces and create a good contact.

Terminal posts are probably best as output connections from the power supply, and these can be wired to their wires once they are bolted through the chassis. Wires from the tag-board which go to the regulator heatsink assembly need to pass through the chassis since the heatsink, which gets hot, is best mounted on the outside of the case. A good finned heatsink should be chosen, preferably one with double-sided fins so that components mounted on its underside will be protected as will be described later.

The heatsink needs to be drilled to take the T03 (case type designation — Ed) power transistor, and the screw mounting holes opened up so that they clear the top hat mounting washers. It will also be necessary to open up the regulator mounting hole in its tab to the same dimension. Take care to thoroughly deburr all the holes so that no protrusions will break through the mica washers. A sectional exploded view of the heatsink assembly is shown in Fig 3.

Note that one of the top hat washers passes through the regulator tab and mica washer. Neither of the top hat washers needs to be exactly the depth of the heatsink holes and should stop short of passing right through. Once the semi-conductors are bolted tightly down, the assembly must be checked for shorting to the heatsink. If there is a problem, new washers will need inserting between the devices after any burrs are removed. Q1's legs should be covered with pieces of sleeving where they pass through the heatsink. For full thermal protection it is important that the regulator components are mounted as described.

A neat assembly can be achieved following the wiring detail of Fig 4 to complete the regulator, except for setting the components in position with Araldite which can be done after the unit is tested for correct operation.

Testing

With a DC volt meter connected on the output of the power supply, apply the mains to the input and ensure the voltages specified are measured on each range. Verify also, with the volt meter set for reading AC volts, that any residual AC on the output is about that specified (10mV). Finally check the regulation by applying a load to the output and seeing the voltage is stable on any particular range. (Note: some of the voltages measured may not be exactly as stated as the resistor values of R3 to R7 are close to, but not exactly, the value required. Adjustment can be made if the voltages are critical by making up the values with series and parallel combinations of resistors.)