CGS ±15V LM3x7 power supply

CGS66 the CGS ±15V power supply.

Note: this project may require wiring a transformer to the A.C mains. This is best left to people who are qualified to do so. Where possible, use a plug-pack, wall-wart or line-lump, or other product where the mains wiring has already been done.

The Power Supply is a companion or alternative to the CGS14 CGS ±15V 7x15 power supply. It is suitable for use with VCOs and more "analog" modules. It is of course quite capable of driving the noisier digital modules too, and in all likelihood will do a better job than the CGS14.

A little on how it works
The incoming AC power is rectified, then filtered. The filtered DC is regulated by the positive and negative voltage regulators. The regulator minimum load requirements are met by placing a LED and its associated resistor between each output and the 0 volt (ground) line. Capacitors are placed close to the input, adjustment and output pins of the regulators to improve stability and transient response. Two additional diodes per regulator protect the regulators should the outputs be shorted together, or should a voltage be fed into the output of the power supply. It is very much a standard configuration, detailed in many data books.

When fed from a 30 volt center tapped transformer (15v-0v-15v) you should be able to pull around 200 mA at +/-15 volts from the PSU without problems, and notably more at +/-12V output. If using for +/-15 volts there will be minimal heat generated by the regulators because they will be running close to their minimum voltage drop, though you do risk ripple getting through to the output as the load increases.

When fed from a 36 volt center tapped transformer (18v-0v-18v) you should be able to pull around 600 mA, at +/-15 volts, from the PSU without problems. There will be a fair amount of heat generated by the regulators and diodes, so make sure there is adequate airflow around the board. 600 mA is close to the maximum DC current you can draw from this PSU when powered from a 1 amp transformer. You may also wish to use 3300 mfd 25 volt capacitors instead of the specified 2200 mfd.

Drawing more than 600 mA, while possible with a larger transformer, is not recommended, as there is inadequate heatsinking on a number of components, and the electrolytic capacitors are much too close to the heatsinks.

Another factor limiting the maximum current that can be drawn from the unit is the size of the heatsink. With 600 mA being drawn, each regulator can easily be dissipating 6 watts, and will require a heatsink with a rating of 12°C/W or better to operate in an ambient of 25°C. As heat in an enclosed space may well be greater than this, I'd recommend going for the best heatsink of the correct profile you can get. Hobby shops in Australia have them available at 8.5°C/W (50 mm) and 11°C/W (40 mm), depending on the shop, though I have used 63 mm heatsinks I salvaged from old switch-mode power supplies. The profile used is simply listed as PCB mount TOP-3 in the local catalogs.

As a note of interest, the prototype was tested running from a HP printer line-lump with a 10v-0-10v coil, the supply outputting +/-12 volts for driving Eurorack modules.

Construction
Before you start assembly, check the board for etching faults. Look for any shorts between tracks, or open circuits due to over etching. Take this opportunity to sand the edges of the board if needed, removing any splinters or rough edges. (With the boards supplied by me, the edges are already milled, and etching faults are very rare.)

When you are happy with the printed circuit board, construction can proceed as normal, starting with the resistors first, followed by the IC socket if used, then moving onto the taller components.

Take particular care with the orientation of the polarized components such as electrolytics, diodes, transistors and ICs.

The easiest way to install the regulators is to mount them on the heatsinks before installing them and the heatsinks on the PCB. The heatsink mounting holes may need enlarging to accommodate the pins of your selected heatsink. The heatsink I have used is of a very common extruded profile and is available in differing lengths. See above for more details.

Parts list
This is a guide only. Parts needed will vary with individual constructor's needs.