120-volt power supply in a 220-volt network. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Probably, many have faced such a situation when a purchased imported household appliance (for example, a telephone set or a calculator) turned out to be equipped with a remote power supply unit for a mains voltage of 120 V. This case, of course, cannot be called pleasant, but a radio amateur is quite capable of work normally on 220 V mains voltage.

How to modify a 120-volt remote low-power power supply to be connected to a 220 V network? This can be done in several ways. Let's consider them briefly.

As a rule, the entire "stuffing" of the block consists of a network transformer, a rectifier and a smoothing capacitor. Therefore, the first method is to disassemble the transformer, remove all windings from the frame, recalculate them already at 220 V, rewind the coil and assemble the transformer.

The calculation of the transformer is not difficult [1], but winding will require a lot of trouble and, of course, skill.

An insurmountable obstacle when using this method can be the fact that the magnetic circuit of an imported transformer is often made non-separable - the plates are "tightly" connected by a narrow weld. In this case, we can only recommend throwing away the transformer and choosing a replacement suitable for it with a magnetic circuit of the same (or slightly larger) cross section.

For those who find transformer rewinding unacceptable, we offer another obvious way - to connect a ballast resistor in series with the mains winding of the transformer, having previously calculated its resistance (in ohms) using the formula:

Rbal \u12000d XNUMX / RG, where RG is the overall power of the transformer in watts, usually indicated on the unit case.

The method is very simple, but if you calculate the power that will be released on this resistor (and it will be approximately equal to the power of the transformer!), It will become clear that the applicability of the method is limited.

Instead of a ballast resistor, you can use a ballast capacitor [2]. Then there will be no problems with the thermal power released in it - it is close to zero, but the capacitor will need an impressive size. Suffice it to say that its rated voltage must be at least 520 V!

To connect to the network of low-power electrical appliances with constant power consumption, another method is sometimes used, based on the phenomenon of current resonance. It can occur in two parallel branches of an electrical circuit powered by alternating voltage, if the nature of the resistance of one branch is inductive and the other is capacitive (see diagram). Here Req and Leq are the equivalent active resistance and inductance of the power supply transformer, respectively, reduced to its mains winding, and the elements R1 and C1 are additionally introduced to implement the current resonance.

It is easy to see that R1 is the same ballast resistor, but the capacitor C1 here compensates for the inductive component of the primary winding current, so the power released on the ballast resistor is 30 ... 50% less. The amplitude voltage across the capacitor C1, even at the moment of switching on, does not exceed 200 V.

Thus, it is only necessary to determine the values ​​of additional elements, and for this you need to know Req and Leq. The power supply usually indicates the input nominal voltage of the power supply UBX, the total overall power Рg, the output voltage UByX, the load current lH and sometimes the consumed current lBX. An ohmmeter should measure the resistance of the mains RI and secondary RII of the transformer windings.

The calculation begins with the determination of the consumed current (if it is not specified):

lBX=Rg,/UBX. Next, calculate the active power consumed by the power supply from the network: Pa \u2d I2BX RI + IXNUMXH RII + IH Uout

(it is assumed that the load of the unit is purely active and losses due to eddy current and remagnetization of the magnetic circuit are not taken into account), and reactive power: Рх= √ Rg² - Ra².

Based on the values ​​of active and reactive power, the equivalent active resistance and inductance of the transformer are calculated, reduced to its mains winding:

Req=Pa/|2BX; Lequiv \u2d Px / ω I2BX, where ω - XNUMXπ f;

f - mains voltage frequency - 50 Hz.

The capacitance of the capacitor C1 is determined from the condition that the reactive conductance of the circuit formed by the parallel connection of the capacitor and the transformer is equal to zero:

C1=Leq/A, where A=ω2 L2eq + R2eq.

The resistance of the ballast resistor R1 and its power PR1 are calculated by the formulas:

R1=A/Req(UC/UBX-1); PR1=UBX-Req(UC-UBX)/A, where UC=220 V.

The proposed method was used to refine the remote power supply for the calculator, which had the following parameters: UBX=120 V; Pg=3 V-A; Uvyx=5,6 V; lH=0,2 A; winding resistance measured with an ohmmeter, RI=764 Ohm; RII=3 Ohm. According to the initial values, the parameters of the elements were calculated: Req=2748 Ohm; Leqv=12,54H; C1=0,54uF; R1 \u6987d 1 Ohm; PR1,48=0,5W. We select a MBGCH capacitor with a capacity of 250 μF for a voltage of 2 V and an MLT-6,8 resistor with a resistance of 120 kOhm. Calculations have shown that when turned on, the voltage on the capacitor does not exceed the value corresponding to the steady state (4 V), and when turned off, it exceeds only XNUMX%.

In conclusion, some recommendations. It is desirable to choose the capacitance of the capacitor C1 as close as possible to the calculated one. This is achieved by connecting the required number of capacitors in parallel (the capacitance values ​​are added together). The rated voltage of all capacitors must be at least 200 V. Paper capacitors (MBGCH, MBGP, etc.) should be used, designed to operate in an alternating current circuit; when choosing the type and rated voltage, you must use the reference book on electrical capacitors.

The power of the resistor R1 is chosen more than the calculated one. Sometimes a resistor adjustment is required, which is best done when connected to a rated load power supply. With a reduced output voltage, the resistance should be less, with an increased one, more.

The capacitor and resistor can be placed inside the power supply if there is free space (do not forget to drill ventilation holes in the walls of the block), or in a separate housing made in the form of an adapter.

Literature

1. Nikolaev Yu. Homemade power supply? There is nothing easier. - Radio, 1992, No. 4, p. 53, 54.
2. Biryukov S. Calculation of a network power supply with a quenching capacitor. - Radio, 1997, No. 5, p. 48 - 50.

Author: V. Chudnov, Ramenskoye, Moscow Region

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