## Power considerations

Power rating of the dropping resistor depends on the current flowing through it. Before selecting a resistor, it is important to assess how much current is required for the load. The output voltage also depends on the value of the resistor and the zener diode. Table III helps to select the resistor and zener diode to get the desired output voltage.

A high-wattage type resistor is necessary to reduce the heat generation and power loss. A 10-watt resistor can be used safely without much heat generation. Instead of using a single resistor, it is better to use two resistors in series to get the total value of the resistance required. For example, instead of using a single 10-kilo-ohm resistor, it is better to use two 5-kilo-ohm resistors in series. This will prevent high-voltage transients appearing in the circuit and also lower the potential across the resistor, so the chance of arcing can be reduced.

Power rating of the zener diode is also important as it handles large current in no-load condition. It is better to use a 1W zener diode for regulation. Electrolytic capacitor C1 should be rated twice the value of the zener diode. For fine smoothing, it is better to use a 1000µF, 25V capacitor.

To increase the power supply efficiency, the circuits shown in Figs 4 and 5 can be used.

In both the circuits, current is supplied from the source during both the halves of the AC cycle. So the output voltage remains more stable than with half-wave rectification. Since the output voltage is not referred to phase or neutral, these power supplies cannot be used to control thyristors.

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.

• You are right. Teoretical is one thing and practical work is the other. The difference between these two is big.

• And mentioning theoretical work earlier, in this article the author doesn’t seem to work a lot on theoretical knowledge either 😀

• Chinmoy Mitra what you have told is wrong. Mohan kumar is right since the reactance of capacitor for 0.22uF capacitor is 14.4 K ohm and thus the 230/14.4*10000=15.9mA.

• It’s because the capacitor’s value is incorrect. It is supposed to be 225K value (22uF), which will then give you the rest of these values. 0.22uF or 220nF will give you 14.4K Ohm.

• He may have missed to write 14.4KOhm, just a typing error. But don’t say this way. He has described very well.

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.