The relationship between capacitive reactance (X), capacitance (C) and output current (I) is expressed as:


As a safety measure, R2 (470-kilo-ohm) is placed in parallel to C1 as a bleeder resistor. It removes the stored current from C1 when the power supply is disconnected from the mains. This prevents shock hazards, since the capacitor stores more than 400 volts when fully charged. This current may be present in the capacitor for many days even after the circuit is switched off.

Power considerations

Since the circuit is directly connected to high-voltage AC, power rating of the components is important. Capacitor C1 must be X-rated type with a voltage rating of 400 volts or more. The power rating of resistors R1 and R2 must be high enough to handle AC.
Current through R1 is full-wave current, so it is equivalent to the line voltage divided by the impedance of C1. Therefore R1 and R2 must be rated at 1 watt.

Diodes D1 and D2 are rectifiers. Maximum RMS current passes through D1 and D2. So the peak inverse voltage of the diodes must be sufficient to handle this high current. Diode 1N4007 is a good choice as it can handle 1000 volts and 1A current.

Capacitor C2 smooths rectified DC and acts as a buffer to release current to the load. The value of C2 should be double the voltage rating of the zener diode. A 25V electrolytic capacitor will work smoothly.

Zener diode ZD is used to regulate the output to 9.1 volts. It is subjected to most of the current in no-load condition and will get most of the full-wave current once C2 is charged. So the power rating of ZD must be sufficient to handle this high current. Maximum power dissipation that can be allowed in the zener is the zener voltage multiplied by the current flowing through it. If the 9.1V zener passes 35mA current, its power dissipation will be around 318 mV. So a zener rated at 400 milliwatts or more can be safely used.

Resistive power supply

In contrast to the capacitive type, a resistive power supply uses the property of resistance to limit the current. As with the capacitive type, the output voltage will remain stable as long as the output current is less or equal to the input current. Resistive power supply is cost-effective and requires less space for its accommodation. But severe power loss through heat is an important drawback. Like capacitive type, it also has no isolation from mains. The output supply is less energy-efficient compared to a capacitive power supply.

Fig. 3 shows the design of a simple resistive power supply. Before making a resistive power supply, power rating of the components needs critical consideration. All the components should have sufficient current capabilities. Since the resistor is used to drop voltage and current from the mains, power rating of the resistor is important to avoid burnout. Table II shows the current-handling capacity of various types of resistors.


  1. This article is an example of what we get when a professor of zoology tries to write articles on Electronics!
    Lot of nonsense has been written in this article to confuse hobbyists and students.
    For example, 230V/14.4 = 15.9 mA? It is simple Ohms Law, and the answer is not mA but AMPERES, yes, the answer should be 15.9 AMPERES! Mr Mohan Kumar, just try to connect a 14.4 ohm resistor across the 230V mains and you shall get your answer….It seems that the author has never actually done any practical projects in electricity or electronics himself.

  2. Hello sir, I need help about smps circuit it is ac 175 to 230v input and output 24v 2.5amp. Dc so pls give m circuit design. In my mail id. Thank u.

  3. Hello sir,
    I had tried this circuit. I had got perfect output from this circuit. But I had found some problems in this circuit. When the 230v ac is connected to this circuit both the zener diodes got heated in this case. I need solution for this issue. Plz tell me sir..

  4. Mobile charger with all protection like over voltage/under voltage protection over charge cut with indicator with 2AMP cap.5v

    • You will never know whether neutral or connection is proper or not is the are interchange and you touch the ground of the circuit and same time touching the earth.. It will definitely give you a 240 rms shock.

  5. The author of this article has clearly mentioned that It is dangerous to touch this power supply in page number 5 safety Overall this is a Non isolated power and harm you any time. please refer completely. If it is mandatory to touch better take a insurance and move. (Kidding )

  6. i made this circuit (Fig.9) and used to operate Arduino mini it’s work properly, but R1 resistor is heated too much even I used 3W rather then 1w. please tell me, sir, what is the problem or how can I sort out it?

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  7. Hi dear Schrödinger
    Thank you so much for your very useful and informative essay regarding transformerless Power supplies.
    would you please answer my questions 😕
    1. Is it possible to increase the power of Zener Diods by simply paralleling them? for instance to get one watt by paralleling two 0.5 watt of them?
    2. what about increasing Zener Diods’ voltage? for example: to join two pcs. 9.1 and 3.1 volts in series to get a 12 volts Zener diod?
    Thank you again

  8. Hello sir, I need help about smps circuit it is ac 175 to 230v input and output 12v 3.5amp. Dc so pls give m circuit design. In my mail id


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