Power supplies are the basis for any piece of electronic equipment, whether a computer, base station radio, mobile radio or even a portable radio. Since this site is dedicated to repeater stations this discussion will focus on base and repeater equipment at a fixed location, usually on some remote mountain top. Because of this, reliability is the top priority, with ease of maintenance right behind. Since cost is usually an issue with (non-profit) Amateur projects most of the equipment is either home-made from scratch or home-modified with commercially made components.
Power supplies come in two basic types,which are the analog (with the heavy transformer) and switching (lighter weight) type. While either type works satisfactory the Author prefers the former (old school). The former does have an issue that will be discussed. 12 volt (nominally 13.8) supplies start with 110 VAC line in, with a fuse and/or EMI filter protection. A power switch is usually in this circuit for convenience only. Next, this feeds a step-down AC transformer. Most manufactures spec the secondary side anywhere from 12 to 30 volts, AC. From here it's rectified and filtered. At this point is the secondary/unregulated section. More on this later. From here the changing DC voltages (from loads) and AC ripple is filtered out, by various methods. Some use simple transistors, while others use a DC amplifier from an I.C. "op amp". This, in turn controls one, or several (in parallel) pass transistors, with a well-regulated output of 12 volts from zero to full rate load, rated in amps.
That big, bad heat problem
Energy cannot be created or destroyed and comes in many forms. It can be converted from one form to another. With the field of electronics, hence, electrons flowing, will generate a certain amount of heat, if restricted, such as flowing through a common resistor. That's why resistors have power ratings, expressed in "watts". Transistors can also be "resistors", hence, called semi-conductors and also rated the same way.
Power supplies are rated both for intermittent and continuous duty. The latter means a 24/7 load on it. For this rating design people have to know their "stuff". Focusing on the unregulated section, this probably is the most problematic area of low voltage supply design. This section, due to the voltage doubling effect of a full-wave rectifier can have DC anywhere from 20-35 volts (no load) or higher. The regulator section controls the output by causing resistance in the final pass transistors. (For further details on controlling bi-polar with bias, seek technical publications, such as the A.R.R.L. Radio Amateur's Handbook). During no load, this works fine. However, when substantial loads are drawn this resistance (transistors; semi-conducting) will cause a large amount of heat from the current flowing. The very basic formula is "E" x "I" = "P", or simply voltage times current = power, expressed in watts. For example, if a regulator has to take 35 volts DC down to 12, that's a 23 voltage drop. At 1 amp that would be 23 watts, which is easy to cool. In the case of large radio transmitters that draw 20 amps (or more) that's not so easy. In fact it's a big problem, or at very least a big challenge. A 20 amp load (23x20) would cause 460 watts of power to be lost in the form of heat in the final pass transistors. A single transistor won't pass that much power and stay together. Several pass transistor can share the load, nevertheless, this heat has to be dissipated with heat sinks. Most sinks do not have enough surface area to safely dissipate this amount of hear, therefore, the typical "fix" is to install air fans to increase (greatly) the cooling effect. Fans wear out, are noisy (at sites) and suck in dust and other contaminates. Unfortunately, most manufacturers depend on fans rather heavily for all cooling needs. This would include PCs, servers and radio transmitters; anything that draws a lot of power.
Astron is one company that trys to keep the unregulated section low, but still enough for proper regulation. Typically you need about 5 volts to play with for proper regulation. For a multiple equipment station the Author took a 35 amp model Astron 35 amp desk top, changed into a rack mountable with 14 individual (fused) circuits on a barrier terminal strip on the rear. This permits easy connections for present and future pieces of equipment. Since this supply has a conventional secondary voltage, some fans where added for cooling under high current conditions. This is controlled with a thermostat mounted on the inside of the (rear) heat sink. At this time pictures or a schematic is not available. The diagrams for the two smaller units are available and are similar to the bigger brother, the 35 amp, with the main difference in the number and type of final pass transistors.
Custom suppliesFor the SRG projects two supplies were designed and built, with emphasis on keeping the IF (heat) problem to a tolerable level. This is accomplished by keeping the unregulated at a very minimum, thus minimizing the voltage drop, thus, heat. Fans are still used, but only come on during long, high current loads, controlled with a thermostat (or several for redundancy) to come on at a specific rise in temperature of the heat sinks for the pass transistors. These were built from "scratch". The second one addresses the heat problem in high powered radio transmitters, specifically, the Motorola Micor P.A. section. Stock supply is 15 volts, while reducing it to around 10 volts lowers the heat, while keeping a stable transmitter. For details click on the one of your choice.
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