STABILIZED AND SECURE POWER SUPPLY

yet an other one? Yes.
It's a stabilized power supply (13V variable-5A) with protection for short cut and high output voltage;
It was born because I needed a "secure" power supply for all my TNC-modems (RF feedback, short-cut,high voltage etc.);

Schematics cames from an italian magazine - Nuova Elettronica N. 172-173) but I've made some mods:

1. Added a 220V input filter;

2. I've used a 220V bipolar switch;

3. Added some RF By-pass capacitor in the output;

4. Added a Ferrite Choke in the output to avoid RF feedback;

SHORT CUT PROTECTION

It works in this way:
across R3 there's a voltage drop proportional to the current that flow on the load; the more is the current that flow in the load the more is the voltage drop across R3;
when this drop is >= 0,6V transistor TR3 will open putting pin R of IC1 at ground level;
in this way the output voltage will be 0V;
Varing R3 will change the max ampere there can flow before the protection intervene;
We know that V=RxI,
with R3=0,1 Ohm and 6A on the load the voltage drop across R3 will be 0,1x6=0,6V;

Generally:
A= Max Current before protection intervene;
R= R3 value;
V= for NPN transistor= 0,6-0,7V)

R=0,6/A

so, if we don't want more than 3A in the load:

R=0,6/3=0,2 Ohm

HIGH VOLTAGE PROTECTION (IN THE OUTPUT)

I had some schematics ready when a friend gave me this board; it uses a rele' to break the output when the voltage goes up a preset value:
it's not absolutly the best, because rele' are not very fast, but it's work.

AUTOMATIC FAN SWITCHING

This is an optional, not really necessary, but I had time to spent so....;
It's a simple IC wich compare two voltages: the first is a preset voltage (pin2), the other comes from an NTC monitoring the temperature of the Transistor (pin 3);
when V1 > V2 the output is high and a fan will switch on;
I suggest to mount the NTC not very closed the Transistor and the fan to avoid continuos switching of the fun;
It was sunday and had no NTC home so I've used 3 simple diode (1N4148) instead of NTC  with the anode near R6 (R5-R4 in my schematics);
( NB: add a small resistor in series, 2-5 K);

73 de iz7ath Talino