I've missed some classes and I'm struggling to do this. I need to "design a linear power supply with 12v output, with simple series regulator, providing a 2A current from 220V AC (60Hz)". I don't know how to calculate the values for the components.

Is there any document that can help me with this?


Here's my design so far: sch

is this a valid solution?

  • 1
    \$\begingroup\$ It would be easier to help you if you added the stuff you do know to your question: Which components are you worried about? Can you use an IC regulator like a 78S12? \$\endgroup\$
    – us2012
    Aug 20, 2013 at 23:21
  • \$\begingroup\$ I can't use an IC regulator. I should use a transitor and a zener diode. Since I'm not from the electrical engineering field, I'm feeling a little lost in everything. I mean, I do know some basic calculations, but nothing I know seems to fit. I've found some texts on the web, but when I try to simulate it (I'm using Proteus), the results are not OK. \$\endgroup\$
    – Fernando
    Aug 21, 2013 at 0:48
  • 1
    \$\begingroup\$ Post a link to a publicly available image or insert a schematic (the little circuit symbol button ) of your idea so far. \$\endgroup\$
    – Spoon
    Aug 21, 2013 at 9:30

2 Answers 2


Your basic design is workable, but requires that R1 be small enough that even when the supply voltage is at minimum D1 will be kept in saturation. Variations in supply voltage (e.g. 120Hz hum) will cause the current through D1 to vary, which may in turn cause the output voltage to vary. Performance may be improved by using a three-transistor circuit with negative feedback. Since it sounds like this is a homework assignment, I'll just describe the essentials. Feed power from the source to the load by a PNP transistor Q1 whose base is fed by an NPN transistor Q2 to ground (you may want to experiment with having a resistor between the Q2's base and ground). Use NPN transistor Q3 to implement a circuit which bypass current away from Q2's base when the output is at or above a voltage controlled by a Zener.

Using such a circuit, the current through the Zener will be independent of the input voltage, remaining constant at the level where Q3 starts to turn on. Further, the circuit can work with input voltages that are within a few hundred millivolts of the desired output voltage. It should be possible to implement the basic circuit with the aforementioned three transistors, two resistors (not counting the load), and a Zener. Adding more resistors may improve performance, but the described circuit should work pretty well as a starting point. Note that in this latter circuit one may replace the Zener with a resistor in exchange for less precision on the output voltage.



simulate this circuit – Schematic created using CircuitLab

Start with a transformer, rectifier and capacitor to get a raw DC voltage. The transformer must be rated for more voltage than you expect to get out of the regulated circuit. We'll assume 15V. The zener diode must be rated for a voltage equivalent to the output voltage plus the Vbe of the transistor. We'll assume 12.6V. The wattage of the zener diode will help you calculate the resistance needed for the limiting resistor. If we assume a 1kOhm resistor, the current through the resistor will be (15V-12.6V)/1000, or 2.4mA. Wattage for the diode is is 2.4mAX12.6V, or 30mW. If you use a 500mW zener, you can reduce the resistance of the series resistor. The 2.4mA passing through the resistor is multiplied by the Hfe of the transistor. A 2N4401 is too small for your purposes, it won't pass enough current. Try a TIP122. It has an Hfe of 1,000 or more so it will amplify the 2.4mA current nicely. To calculate how much power will be dissipated through the transistor multiply maximum output current by the voltage across R1.


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