Power Supply 13.8V 20A by Transistor TIP3055

A heavy duty 13.8V power supply is a fine thing to have in the shack, but unless you acquire one secondhand, is an expensive little beastie to buy. This means building one should be considered, not only for the cost savings, but also because you can brag about it on air to your mates. Of course, careful consideration must be given to the properties of the completed supply, and after talking to a few of my friends who have built their own and fallen into all the traps, here are the printable ones : RF proof, easy to make, commonly available parts used, but above all CHEAP.

Well, last things first. Breaking down the construction costs of a heavy duty regulated supply, they are in order:

  • The transformer (around $A80)
  • The main filter electrolytics - around $A80
  • The case - a metal case is well beyond the workshop capabilities of many amateurs and is quite expensive to buy (if you can).
  • The meter - around $A20-$27 (either digital or analogue)
  • The electronics - transistors, resistors, diodes, etc.
  • All the bits - fuseholders, terminals, switches, solder tags, nuts and bolts, power cords, etc.

Dealing with these in turn, we can reduce the cost greatly by rewinding a microwave transformer (about $A5 total), scrounging old computer grade electrolytics (lots around), and designing the electronics to be so RF proof that a wooden case can be used - yes, that's right - wooden! If you are really stuck for a dollar, then good supply regulation and overload protection also allow all metering to be deleted. Finally the wooden case allows 1/4 inch bolts and washers to be substituted for expensive terminals. If you can't put the whole thing together for less than $A50 then frankly you don't even qualify for the junior scroungers league.

Moving on to the other points, manufacture is easy as no etched PCB is used. The PCB is simply made by using a hacksaw to cut through the copper overlay on the PCB material breaking it up into separate pads. Details are given in the drawings.

Keeping the supply RF proof is another matter entirely. During development, several designs were tested, based around such chips as the 723 regulator, the 3140 op amp. and a 7912 three terminal regulator with bypass transistors. In all cases, the high gain of the control amplifier forced the use of a PCB with a ground plane to which everything was heavily bypassed. This technique limits RF interference and also prevents motorboating and high frequency instability (a common problem in high current circuits such as power supplies and audio amplifiers) as the ground plane acts as both an RF shield and a single point ground.

However, for home construction, the use of a double sided PCB is undesirable and anyway, the performance of each of these circuits is totally over the top. After all, ham rigs powered from 13.8 volt are designed for use in a car where the supply voltage wanders all over the place. Two volts of variation is quite typical. Regulator circuits which hold the output voltage constant within a few millivolt for all conditions of load are simply not required. It is much more important that the output voltage is free of noise and ripple, and the published design does this very well. Noise and ripple are well under 5 millivolt peak to peak, and output regulation (no load to full load) is around 200 millivolt. A simple control circuit is used without overall feedback and the result is a cheap, very stable design. RF proofing is provided by physically earthing the heatsink, and also by using it as a ground plane. The collectors of the TIP3055s are physically earthed to the heatsink (no micas), and so a good section of the circuit is actually at earth potential. Two other advantages are easy assembly and excellent heatsinking.

Source :: http://www.users.on.net/~endsodds/ps20.htm

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