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Transformerless power supply


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Suppose we give the mains supply directly to the bridge rectifier and then attach a zener diode of suitable power rating and required voltage,then put the filter capacitor, and then give it to regulator IC.
Will it work?
I want to ask before experimenting.

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It is as LDanielRosa has said, but allow me to add some comments since this is an International forum.
Where to put the fuse depends upon the mains. Usually, the mains will have a hot lead or leads and the other is considered common. For example in the US and Japan 120 VAC and 100 VAC, the black is hot and the white wire is a return or common. You will find that the circuit breakers on 100 VAC, 120 VAC are only on the black wire. This is because if you break the circuit on the common while the other is still connected, then you will have more of a danger as the circuit is live with no ground and looking for a way to complete.
If your mains are 220 VAC, then the method described by LDanielRosa is perfectly correct as both wires are hot. Using only one fuse would still leave you with a hot circuit. You want to be sure there is no problem with the ground return path in either type.

MP

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Ldanielrosa,
Input capacitors will not provide isolation from the mains, since they pass the AC.
If you use their capacitive-reactance like a series resistor, except without heating, then they will cause the rectified voltage to fluctuate with load changes (very high with no load and quite low with a load).

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A series capacitor is a good idea, but it will require a large one if the filter capacitor is large. This would lead to a pack of capacitors with the rated voltage. This is because a low value series capacitor will drop most of the DC and a large value filter capacitor will not give you much DC.

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I have supply of 220 volts.
A resistor has to be placed before the zener,of what wattage?
An insultion of sponge or if the circuit is placed in a plastic box should work but then there will be problem of heat.

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MP, I reccommend both wires regardless of the configuration (hot-neutral-ground or hot-neutral/ground-hot) because this is for a single phase bridge rectifier. This means that on the negative half cycle the "hot" input will be clamped to ground while it pushes the "neutral" positive. This requires both inputs to be isolated.

If it were for a voltage doubler, then the neutral could be left alone- but I wouldn't advise it. There is no guarantee good enough that the socket was wired correctly. Only on a device with no chance of human contact and no need of ground would one capacitor (and one fuse) do.

Audioguru, you're right- it is an imperfect solution. One (possibly arrogant) assumption I make is that size and weight are issues because the device being powered is small, light, and possibly portable. If this is the case then hopefully it will not require a lot of power.

Kevin Weddle, I'd set the limit at 10uF. Anything beyond this and the capacitors will be too expensive and hard to find. Not to mention that they may be bigger than the transformer they replace.

Siddarth, the chief concern is what you are powering. A zener regulator is best designed for one device only. The calculations can't begin until the requirements are known. Also, with a limit of 10uF on the capacitors, you'll be limited to about 40mA.

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siddharth, the resistor before the zener is calculated as follows:
You have a different voltage on each side, input voltage on the one and zener voltage on the other. Subtracting these voltages gives you the voltage drop of this resistor. Figure out how much current you need through this circuit using this voltage drop and use ohm's law to figure R. Then figure Watt rating needed for the resistor from there with the power formula.

LDanielRosa, you might have misunderstood my comments, but I was describing a circuit where one leg of the input is only common. In the countries where this is used, it is actually against the electrical code to put your breaker or fuse on this side of the circuit. This is because if this fuse went out but the other one did not, you would have a live circuit waiting for a path to ground through any means.

MP

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There is a safe way- in a grounded steel box that also contains the device being powered with no wires leaving without being isolated.

If there is no denying that you need a small, light power supply for more than 30V and less than 30mA then build one. If you do then be sure that you use insulated knobbed banana jacks (or something similar) to reduce the shock hazard as much as possible.

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Siddharth, have you been hearing ante? It is dangerous. But if you must, then the answer is yes. You can build one of these for any voltage that you need. Just choose the appropriate zener value and calculate the series resistor for the current. Those are the only changes that will matter for functionality of the circuit. I built such a power supply several years ago for power amps. The difference was, I used a 1:1 transformer for isolation. I was working with the same voltages but I was isolated from the mains when working on the equipment powered.

MP

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