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Fig. 9: Efficient capacitive power supply with series regulator
Fig. 9: Efficient capacitive power supply with series regulator

AE2_ShockBy adding a few more components, efficiency of the power supply can be further increased. Transient protection is achieved in the capacitive power supply by adding an metal oxide varistor (MOV) across the phase and neutral (Fig. 6).

Resistor R2 acts as a bleeder to discharge C1 at power-’off’ and to filter EMI in the line. The MOV connected across the phase and neutral will suppress transients in the lines.

A resistive power supply using two series resistors instead of one (Fig. 7) reduces high-voltage transients and the potential across the resistors. Capacitor C1 and resistor R1 connected across phase and neutral lines act as filters to prevent EMI flowing into the AC lines. MOV also reduces transients.

Capacitive power supply with more power efficiency

An efficient power supply that can deliver 9V DC at 100 mA is shown in Fig. 8. 230V AC is stepped down to 24V AC by capacitor C1. This low-voltage AC is rectified by a bridge rectifier to give 20V DC. Capacitors C2, C3 and C4 eliminate ripples from the DC and store current to stabilise the output. Resistor R2 attenuates EMI generated in the circuit and capacitor C1 discharges through it when power is switched off. MOV gives transient protection.

Instead of a zener diode, a series regulator of the 78XX series can be used to give regulated output as in Fig. 9. Zener diodes ZD1 and ZD2 drop 24V AC to 15 volts, which is then rectified and filtered to get ripple-free 15V DC. IC 7812 regulates the output to 12V stabilised DC. The output current will be around 50 milliamperes, which is sufficient to drive most of the light load circuits.

Safety considerations

Capacitive and resistive power supplies are directly powered from 230V AC. So adequate precautionary measures must be taken when building and installing these circuits. An AC-type switch should be placed in series with the phase line to break the power supply. Fixing a fuse will give an additional safety. Power supply section and the electronic circuit must be separated with sufficient spacing while enclosing in the cabinet. Proper shielding and sleeving are required in the AC side of the power supply.

If costly ICs are present in the circuit, do not use a transformerless power supply since these may get damaged in case of a short circuit. Instantaneous power surge in the lines, lightning and short circuits can cause damages in the circuit. Use shockproof cabinets with proper earthing. The circuit can give lethal shock if handled carelessly. So take utmost care during testing and installation. Do not touch any part of the circuit when it is powered.


The author is an associate professor and head of Department of Zoology, Government College for Women, Thiruvananthapuram, Kerala

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21 COMMENTS

  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
    Bye

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