Output

Fig. 1, Schematic diagram. F1 =0.5 amp line fuse; T - J J 5 VAC primary, 20 V-flC secondary at 0.5A; DJ = j 8V I W zener; D2tD3 = smalt diode combinations; D4 -D7 = 400V 1A rectifier diodes; Rs = series current setting resistor; SI - AC 3 pole, 5 positioj] rotary switch ;M - 0—30V voltmeter; Q1 = GE-14 transistor; Q2 = 40466 transistor.

provide additional switchable increments. Just use lots of diodes and a big switch.

Construction

The construction is non-critical and the parts can be assembled in almost any manner. The only critical component is 01 - This transistor dissipates the entire power that is "shorted out" in the regulation process, ("he power must be dissipated as heat, which requires a good heat sink. In this case the transistors were bolted to the largest piece of meta! available; the chassis. Almost any silicon power transistor can be used for Q2, so long as it has about a 60 volt rating. Q1 must have about a 25 watt power rating, a 60 volt voltage rating, and handle about 4 amps. These ratings are conservative, of course, to a ¡low for transient problems and endurance under high temperatures. The ratings of the transistor used are actually much greater, since the increase in cost was nominal. A one watt zenei was used because it was available, but a few hundred milliwatts should be adequate, i he small diodes can he selected by experiment to determine type and amount of them necessary to obtain the voltages desired.

Operation

Setting the power supply up for operation is just a matter of selecting voltages by tiving different combinations of diodes and setting current by adjusting the series resistance for maximum current without overheating QL Q] will normally run quite hot.

particularly in the lower vollage ranges where the regulator works the hardest. If a higher voltage transformer is used. QI is too hot, or lower voltages are desired, a dropping resistor will be necessary. The resistor can be about 10 100ii. depending upon the parts used. ; he parts used as shown in the schematic gave the following results: Switch

Position 12 3 4

Voltage 18V 20 V 23 V 25V

Current 500 Ma 400 Ma 300 Ma 100 Ma

(no regulation)

The current readings were made for a one percent drop in output voltage as the load was increased. The ripple in the worst case was about one percent. One unusual feature is that the chassis is connected to the positive B+ lead. This is because the transistors were bolted to the chassis without: the insulating mica spacer and the necessary silicon grease. The chassis really should be grounded, but I don't expect any troubles.

Conclusion

This power supply was buih out of junk box components in one evening. The builder should use the schematic as a guide to use whatever parts he has available to experiment to build a power supply to obtain whatever voltages he desires. This project is not difficult and will make a very useful "first project11 for the experimenter to gam experience as well as power for his future experiments and projects.

1; .WB6BIH

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