5196106366

SWlb shorting out the feedback resistor Rll when the 1.5V setting is selected. A level of 0.75V was out of the linear region of the inputs of the LM741, and resulted in poor output regulation. Since SW1 was a double pole switch, utilising the second half was a simple solution to the above problem.

Similar advantage was taken of the current selector switch SW2b, uscd to boost the current capacity of SW2a on the 500mA rangę.

Diodę D5 is included to effectively give the LM741 a slight negative supply raił of 0.7V, once again to keep it in its linear region.

Capacitor C3 and resistor R13 pro-vide a high frequency load to the output of IC1 to ensure stability, and C4 re-moves any remaining ripple from the output.

The AC voltage from the plugpack feeds the bridge rectifier of Dl to D4 and the 22()0uF filtering capacitor Cl, whilst further filtering for the reference and IC1 is provided by R1 and C2. A green LED indicates a DC source volt-age is present.

Construction

The prototype was assembled on a PCB (codę 87ps9) measuring 62xl22mm, which mounts dircctly by the switch shafts into a Iow cost aluminium box. The box has sufficient area to double as a heatsink for the series pass transistor Ql. This style of construction virtually eliminates any mount-ing hardware and wiring within the box, the Circuit board simply having AC in and DC out connections. Once the com-ponents have been mounted on the board, it is then a simple matter to mount it in the box and be under way!

Start the construction by mounting all the smaller components on the PCB, following with the larger components such as the main capacitors, and finish with the rotary switches.

The switches used in this project are the sealed PCB mount rotary type, which have a removable ring under the locknut to set the number of positions. This ring has a tab that may be placed in a choice of slots labelled from 2 to 11, the 12th position being available with the ring rcmoved. Therefore, SI and S2 are set to the 6 and 4 positions respectively. The PCB holes for the switches will need to be quite large, be-ćausc the large number of pins makes. them a little tricky to install.

Construction of the rest of the unit is quite straightforward, the components fitted to the board as shown in the com-ponent overlay. Particular care must be taken with the polarity of the semicon-ductors and electrolytic capacitors. The resistors should be mounted a couple of millimetres off the board to allow enough airspace for cooling.

The TIP31 transistor is bolted to the box between the output terminals, a mica washer being uscd to insulate it from the chassis, and heatsink com-pound applied for a good thermal path. To reach this mounting position, the transistor legs were bent so that it hung bclow the PCB.

When the Circuit board is completed, the height of the LEDs can adjusted to protrude the correct amount through the top of the box. Smali lengths of wire (component leg offeuts) can be used if the LED legs are too short. Only the top scction of the LED mounting elips are needed to provide a neat finish to the front panel.

The rest of the wiring is very simple. A couple of pieces of hookup wire link the PCB to the banana sockcts, and the pług pack lead enters the box via a rub-ber grommet.

The negativc output banana socket (black) may be electrically connected to the box (as in the prototype), or left disconnected if a “floating” case is de-sired. For cxperimental work the latter is often preferable.

The front panel artwork has been reproduced herc, and can be used by those w ho w ish to make their own. A Scotchcal panel was used on the proto-type.

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