Low cost power supply

A versatile power supply

Laboratory power supply

Circuit description

Figure 1.3 shows the circuit diagram of the power supply. The supply is based around the L200C regulator, which is capable of handling input voltages up to a maximum of 40 V and output voltages up to 30 V with programmable current limit. The circuit effectively uses two individual single power supplies i.e. a transformer with two separate secondary windings and two regulators.

A 2 A transformer is used in the design to allow plenty of headroom when the power supply is being used at a current level of 1 A. It is important that the transformer secondary voltage is not allowed to drop below the minimum input voltage for full voltage output from the regulator, as regulation would be lost.

Mains voltage is applied to transformer T1 via primary fuse FS1 and mains on/off switch S1. A mains voltage of 240 V r.m.s. on the primary of the transformer, corresponds to a secondary voltage of approximately 20 V r.m.s. The low voltage a.c. is fed to two separate bridge rectifiers (BR1 and BR2) via fuses FS1 and FS2. The output from the rectifiers is smoothed by electrolytic capacitors C1 and C2. Two completely separate unregulated d.c. supplies are produced which are then individually fed to the input of regulators IC1 and IC2. The output voltage of each regulator is determined separately by a network of resistors, switched by S3. A similar set of resistors (switched by S2) are used for current limiting purposes. S3 is also used to select single or split supply operation. Transistors TR1 and TR2 are used to drive two LEDs which indicate the status of the power supply outputs (single or split). Diodes, D1 and D2 are used to prevent voltage spikes or residual voltages from external equipment connected to the output of the supply from damaging the regulators. Fast recovery diodes are used in this application because of their fast switching characteristics. Resistors R7 and R15 ensure that the diodes are maintained in a conducting state even at very low output current levels. Capacitors C3 and C4 decouple the regulators, attenuate noise and prevent instability.

Construction

Insert and solder the components onto the PCB referring to Figure 1.2 and the Parts List. It is a good idea to start with the resistors, as these are relatively low profile components, and may be awkward to fit at a later stage. Next, using the resistor lead off-cuts, fit the eight links on the PCB, these are marked link on the legend. The fuse clips (used to hold FS1 and FS2) should then be fitted; these must be kept flush with the PCB when soldering as illustrated in Figure 1.1. Next insert and solder the capacitors. It is important that the electrolytic capacitors are fitted with the correct polarity; the negative lead of the capacitor, marked by a negative (−) symbol on the component body, is inserted into the hole furthest from the positive (+) symbol on the legend. Transistors TR1 and TR2 are fitted such that the case of the component corresponds with the outline on the PCB legend. Regulators RG1 and RG2 are fitted in a similar manner with the heatsink tags perpendicular to the PCB; the tags are bolted down to the bottom of the case, using insulating bushes and washers, when the PCB is finally installed. Bridge rectifiers, BR1 and BR2 are inserted and soldered such that the symbols on the corners of the device correspond with those on the PCB legend. The bridge rectifiers each use a small heatsink and these are held in place by a nut and bolt through the PCB. Position the heatsink such that it is clear of any surrounding components. Potentiometers RV1 and RV2 are mounted on the component side of the PCB as shown in Figures 1.4(a) and 1.4(b). Switches S2 and S3 are mounted in a similar manner to RV1 and RV2 and are connected to the PCB using insulated hook-up wire as shown in Figure 1.5. Fit the two LEDs on the track-side of the PCB, ensuring correct orientation, this is indicated by the dotted legend on the component-side of the PCB. Finally fit the two track-side links using insulated hook-up wire. The location of each, is indicated on the component-side of the PCB by a dotted line; a circle at each end of the dotted lines indicates the position where the wire ends should be soldered to the track. Please note, to prevent instability these links must be fitted exactly where indicated by the legend and not at any other point.

For further information on soldering and construction techniques, reference should be made to the constructors’ guide included in the kit.

Enclosure and wiring

Before the supply can be powered up, the transformer, PCB and associated components must be housed in a suitable metal case. The recommended case is Steel Case 1608 (stock code XJ28F) and the drilling details, for those wishing to use this case, are shown in Figure 1.6. PCB mounting information is shown in Figure 1.7.

For ease of assembly, it is recommended that the wiring to the PCB is made before fitting into the case, and that the PCB is fitted before the transformer. With the recommended case, it may be found advantageous to remove the rear panel when installing the PCB and transformer.

It is important that the case used has no large holes as live mains is present inside, on the primary side of the transformer, and it is important that the risk of electric shock due to touching any of these parts is eliminated. Figure 1.8 shows connections to the mains fuse holder (FS1), on/off switch, suppression capacitor and transformer (T1) primary.

Transformer secondary (low voltage) wiring is shown in Figure 1.9. All mains leads should be shrouded using heatshrink sleeving. The wiring should be double checked to make sure that there are no errors. Connection of the earth lead is an essential safety precaution; make sure that the earth lead (colour coded green/yellow) is securely connected to the tag provided, and that the tag is bolted securely to the chassis, so as to make a good electrical connection.

The front panel layout is basically determined by the position of the front panel components, but the actual front panel legend is down to the user. A suggested front panel layout is shown in Figure 1.10; the illustration also shows the different switch positions and approximate voltage settings for the variable voltage controls (RV1 and RV2).

Terminal posts TB1 (+), TB2 (−), TB3 (+) and TB4 (−) are mounted on the front panel of the case and pass through 4 large holes in the PCB. The tags are connected to PCB pins P5 (TB1), P6 (TB2), P7 (TB3) and P8 (TB4) as shown in Figure 1.11.

It is important that RG1 and RG2 are suitably heatsinked. The metal case suggested will provide sufficient heatsinking under normal conditions. However, if a different case is used, it may be necessary to provide additional heatsinks....