Wednesday, April 24, 2024

Transformerless Power Supply

Fig. 1: AC through a series capacitor and resistor
Fig. 1: AC through a series capacitor and resistor

One of the major challenges during a circuit design is to generate low-voltage DC from AC to power the circuit. There are many methods to convert AC voltage into DC. The most common method is the use of a step-down transformer to reduce 230V AC to a lower value AC. This is then rectified and made ripple-free by using a transformerless power supply.

Even though the transformer-based power supply is efficient in providing sufficient current, it consumes much space and makes the gadget bulky. Cost of the transformer is also high. To power low current demanding logic circuits and microprocessor circuits, transformerless power supply is an ideal solution. These power supplies have distinct advantages and disadvantages. These are cost-effective and consume less space, so the gadget becomes handy. But the low current efficiency makes them non-ideal to satisfy high current requirements of most circuits. Moreover, there is no isolation from the mains.

Fig. 2: Capacitive power supply
Fig. 2: Capacitive power supply

The two basic types of transformerless power supplies are capacitive and resistive. Capacitive type is more efficient since its heat dissipation and power loss are very low. Resistive type power supply, on the other hand, dissipates more heat, so the power loss is quite high. If a circuit requires very low current of a few milliamperes, transformerless power supply is an ideal solution. Before designing a transformerless power supply, some facts about AC dropping through a capacitor or resistor are to be considered.

Design considerations

If a non-polarised capacitor and a resistor are put in series with the AC power line, constant current can be maintained through the resistor, provided that the reactance of the capacitor is greater than the resistance of the series resistor used.

The current flowing through resistor R depends on the value of capacitor C. That is, a higher-value capacitor delivers more current to the circuit. Current flow through dropping capacitor C depends on its reactance, and the value of the current passing through the capacitor is represented as:

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where ‘X’ is the reactance of the capacitor and VIN is the line voltage (230V).

Before selecting the capacitor, it is necessary to understand how a dropping capacitor behaves when it passes current. The capacitor designed to operate in AC voltage belongs to the ‘X’ category with operating voltage ranging from 250 volts to 600 volts. If the mains frequency is 50 Hz, the reactance (X) of the capacitor is:



  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 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..

    • 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.

  3. 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 )

  4. 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

  5. Dear Sir,
    Thank you so much for your very useful and informative essay regarding transformerless Power supplies espesially design considerations.
    would you please advice why series resistors and two zener diodes get heated so much and if it is safe to operate the device with such conditions?


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