I have a full-wave center-tapped transformer circuit with input voltage of 220 V.

How do I get an output voltage of 15 V? What equations should I use?

Here is the circuit:

enter image description here

  • 1
    \$\begingroup\$ What is a centre tapped rectifier? \$\endgroup\$
    – Andy aka
    May 16, 2020 at 21:27
  • \$\begingroup\$ Updated the question \$\endgroup\$ May 16, 2020 at 21:31
  • \$\begingroup\$ It's the transformer that's centre-tapped. You need to fix the title and the question. \$\endgroup\$
    – Transistor
    May 16, 2020 at 21:32
  • \$\begingroup\$ Sorry about that, I am still a beginner \$\endgroup\$ May 16, 2020 at 21:34
  • \$\begingroup\$ You must reply. \$\endgroup\$ May 17, 2020 at 0:57

2 Answers 2


enter image description here

Figure 1. Centre-tapped transformer with full-wave rectification.

Because there is no smoothing capacitor you just need a 15-0-15 V transformer. (You will lose about 0.7 V because of the diode drops so your actual output voltage will be 14.3 V DC.)

If you want smoothing you will be adding capacitors which will increase the average DC output voltage. There are many, many articles explaining this on the web.


simulate this circuit – Schematic created using CircuitLab

Figure 2. A more usual way of drawing the same circuit with the power-supply on the left and the load on the right.

From the comments:

I want to understand it and what about the value of resistance between the two diodes.

It should be clearer in Figure 2 that the resistor is the load (whatever is being powered by the power-supply). It could, for example, be a light bulb or a motor.

One thing to watch is that the transformer will have a maximum power rating, usually in VA (volt-amps). In a real circuit you need to make sure that the current drawn by the load (R) is not greater than the transformer can supply without overheating.

See Electronics Tutorials for a good introduction.

  • \$\begingroup\$ I am a newbie into this. I want to understand it and what about the value of resistance between the two diodes. I need to do a circuit simulation using a program like proteus \$\endgroup\$ May 16, 2020 at 21:58
  • \$\begingroup\$ See the update. \$\endgroup\$
    – Transistor
    May 16, 2020 at 22:05
  • \$\begingroup\$ Using Max VA rating compute V/I and raise the R by 30% as the minimum R derated for power factor when you add a Cap. The RC =T must be about 80 ms = 8/2f for ~10% ripple is a rough estimate \$\endgroup\$ May 16, 2020 at 22:06
  • 3
    \$\begingroup\$ @Tony, that's likely to be as much use to the OP as a chocolate teapot. \$\endgroup\$
    – Transistor
    May 16, 2020 at 22:08
  • 1
    \$\begingroup\$ There is no requirement in the question for a smoothed DC output. Give the guy/gal a chance to figure out how a rectifier driving a resistor works first. \$\endgroup\$
    – Transistor
    May 16, 2020 at 22:17

How about you do your KVL homework on sinewaves, source voltage, turns ratio and peak to full-wave conversion charts to proof and I'll show you the answer.

I used 240Vrms 50Hz because I assumed you were not in Canada. Correct me if I was wrong.

My diodes conducted 0.73Vpk into 100 Ohms. But you can assume 0.7V

enter image description here

My simulation using arbitrary load R of 100 Ohms and Vrms is computed on plots with Vmax and Vmin shown.

  • \$\begingroup\$ I don't really understand what you mean, Like I said I am a total beginner to this \$\endgroup\$ May 17, 2020 at 1:59
  • \$\begingroup\$ You must understand how it works before the math \$\endgroup\$ May 17, 2020 at 2:00
  • \$\begingroup\$ Well, I know that it unifies the direction of the current by using the diode as a switch that only lets current flow in one direction \$\endgroup\$ May 17, 2020 at 2:04

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