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Build a Homebrew 13.8v (variable voltage) Power Supply

A stable power supply, with overcurrent and overvoltage protection, adjustable from 11v to 15v

Building this design is reasonably easy if you have basic soldering skills and some vero board, however BE CAREFUL when messing with your power supply, as there is a small part of it (the primary side of the transformer) that contains lethal voltages (which can kill you). I'm not responsible for any damage to persons or equipment from reading this page. Always unplug the supply from the mains before working on it

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How to

power supply schematic

Simply follow the diagram (click the small pic to see it full size) and build the circuit on some vero board or similar. Remember that the 2N3055 transistors need reasonable heatsinks as they will run warm with a decent load on the supply. The TIP31 doesn't really run hot as all the load is taken by the 2N3055's, but I would still recommend fitting a small heatsink, just to be extra safe.

This is designed to be a variable 13.8 volt 12 amp supply, however, to make it work at different voltages simply alter the 2.7k and the 3.3k at either side of the 1k pot. To make it handle more or less amperage you can add or remove 2N3055 & Load Resistor pair as required. The maximum amperage is only limited by the transformer that you chose to use. Example: If you add 20 x 2N3055's and Load Resistors, but your transformer is only 5 amps, then the maximum power it can supply will be 5 amps. (also remember to change the 12A quick blow fuse to your maximum power output level)

This circuit is found to be very stable, as the regulator takes its voltage mesurements directly from the outputs (via pin 3 on the LM723). This power supply design will instantly notice any voltage drop and exactly compensate the voltage. It does this so fast that even with a very sensitive analogue volt meter, the needle doesn't even twitch.

The Over Voltage Protection "crowbar" circuit in this design is set at 16.4 volts by the two 8.2v zenner diodes. Originally I had set the crowbar to trip at 15 volts, but this was found to be too close to 13.8v and it was falsely tripping when loads were connected to the power supply due to load transients and stuff... If you make your supply at a voltage different than 13.8v please also remember to change the voltage of the crowbar. Example: For a 5 volt supply you would probably set the crowbar to trip at 6.3v and for an 18 volt supply you would probably set the crowbar at about 22v. Notice that as the voltage increases, so does the gap between the supply voltage and the crowbar voltage. A reasonable idea is to add about 20-25% to the output voltage and set the crowbar as near to that as you can. You can put multiple zenner diodes in series (observing polarity) to obtain the voltage you want, simply add up the voltages... You could for example use an 8.2v zenner with a 6.3v one to give you 14.5v, or 3 x 5.1 volt zenners to give you 15.3v -Get the idea?

If you plan to build a standalone crowbar circuit to go in an existing power supply, then you must put a fuse before the crowbar circuit or you risk severe damage to your regulation circuit if the crowbar trips..

The "startup delay" circuit shown will give about a three second delay... This circuit delays the power getting to the load for a few seconds after you switch on the power supply. This means that if for some reason there happens to be a voltage surge or spike at switch on, the surge won't get to the load (eg. your precious radio)... After a couple of seconds (when the surge has hopefully passed and the regulator has has time to settle down) the load is connected. If you wanna change the delay time you could vary the 4700uf capacitor eg. a 2200uf would make the delay less and a 6800uf would make the delay longer.

Additional things you could add: Voltage meter, Amp meter, status LED's... I have added a cheap analogue voltage meter to my supply, this gives a reasonably accurate reading and lets me know if I have accidentally turned the voltage up or down without realising. I've also added a common cathode (negative) bi-colour LED that is red when power supply is turned on and goes green when the startup delay has switched. The LED connects to the relay: 87 for green, 87a for red and the common cathode leg connects to ground through 1k resistor. You could use two seperate LED's if you want...

If you get stuck, contact me and I'll try to help you if I can. Good Luck! -Norm.

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