Simple transformerless power supply, 220/5 volts. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Simple transformerless power supply, 220/5 volts. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The proposed power supply has a very simple, even primitive circuit. It is deprived of such a heavy and bulky thing as a transformer, and does not contain any scarce elements. I designed it to power a portable receiver. Since the block has small dimensions, I managed to hide it in the receiver's battery compartment.

Simple transformerless power supply, 220/5 volts

Many can attribute the lack of galvanic isolation from the network to the disadvantages of this scheme, but you have to pay for everything good. Another disadvantage could be considered the ability to get your hands on the phase wire, but you should not rush to such conclusions. Imagine a situation where you touch a phase wire. If you are not “grounded,” i.e., do not conduct current through your body, then you can hold on to this very phase as much as you like.

From here, the conclusion suggests itself - it is not so much the fact of touching the phase wire that matters, but the magnitude of the current passing through your body. For this very reason, quenching capacitors are installed in both power supply lines. Now, how not to plug the power plug into the mains socket - at least one capacitor will be between the phase and the rest of the device, and you may “twitch” a little (or a little more than a little).

It all depends on your resistance and the AC resistance of the capacitor. But still refrain from such experiments. The resistance value can be calculated by the formula:

Rc = 1/2πFC,

where Rc is the resistance of the capacitor, in ohms; F - frequency, Hz; C is the capacitance of the capacitor, F. The resistance of two resistors connected in parallel:

R = R1R2/ (R1 + R2).

Knowing these formulas, Ohm's law can be applied to calculate the necessary ballast resistance in the circuit to provide a given load current. Let's determine the capacitance of the capacitor. In the simplest case, we multiply the resulting capacity by two C1 = C2 = 2C. according to the diagram in the figure. Resistors R1, R2 are designed to discharge the capacitors they shunt. We rely on the diode bridge VD1 for the corresponding current in the circuit. Its maximum operating voltage is determined by the voltage that the zener diode provides in the load. Accordingly, we select the capacitor C4 necessary for voltage and capacitance.

Elements R3, VD2, VT1 constitute an analogue of a powerful zener diode. The maximum current and power dissipation of such a zener diode are determined by the maximum current and dissipation power VT1. This transistor may require a heatsink. But in any case, the maximum current of this transistor should not be less than the load current.

Elements R4, VD3 form a circuit for indicating the presence of voltage on the load. At low load currents, the current consumed by this circuit must be taken into account in the calculations. If there is no need for this circuit, simply exclude it. Resistor R5 works as a load resistor, loading the power supply circuit with a small current, which stabilizes its operation.

Setup and accessories. Quenching capacitors C1 and C2 - type KBG or similar, they are the most reliable. You can also apply K73-17. The best option is when their maximum voltage is 400 V, although 250 V is possible, since they are connected in series. The output voltage depends on three factors: 1) on the resistance of the quenching capacitors to alternating current; 2) from the actual load current, if it exceeds the calculated value; 3) from the zener diode, more precisely, from its stabilization voltage. Choose the one you need. Correctly calculated and assembled from serviceable elements, the block does not require adjustment. Finally, I note that having assembled this power supply and measured its ripples at the load, I was pleasantly surprised by the almost complete absence of ripples and noise at its output.

Author: V.B.Efimenko, Kiev

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