Transformerless charger 20 volts 140 milliamps. Scheme, description

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Transformerless charger 20 volts 140 milliamps

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Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

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I offer a low-power charger (charger) with a quenching capacitor (Fig. 1). It is designed to charge batteries with a maximum output current of 140 mA and a voltage of up to 20 V. The transistor threshold circuit allows you to set the charging voltage of 13,8 ... 14,4 V (for batteries - 12,6 V), at which the charging current is turned off , i.e. overcharging of the battery is prevented. This is facilitated by a gradual decrease in the charging current with increasing voltage on the battery.

Rice. 1. Schematic diagram of the charger (click to enlarge)

In the memory circuit, special attention is paid to safety. The phase wire "Ф" of the 220 V network is connected through a fuse and an inrush current limiter R1 to a quenching capacitor C1, the other output of which and the neutral wire of the network "0" are connected to a capacitor voltage divider.

Through the diode bridge VD1 ... VD4, the voltage from the capacitors C2, C3 is connected to the key circuit on VT1 ... VT3. Resistor R7 - shunt charge current indicator VD5. The charging current in the form of wide pulses with a frequency of 100 Hz enters the battery through the VT1 key and the VD7 diode. In the pauses between charging pulses, the battery is discharged for desulfation through a threshold circuit on VT3 and VD6.

Resistor R12 sets the maximum battery charge voltage. When it is reached, the transistor VT3 opens, and VT2, VT1 close, the charge current stops, and the green LED VD5 goes out, indicating the charge. After some time, due to self-discharge, the voltage on the battery decreases, and the threshold trigger on VT2, VT3 turns on the charging current again, opening VT1. Flashing VD5 with a period of about 5 s indicates the charged state of the battery. In this mode, the battery can power the bell circuit or the emergency fluorescent lamp. With the current "fan" outages, this is an important property of the memory.

The most critical part of the memory is capacitor C1. Here you can use 2 capacitors of the K73-14 type (1 μF x 400 V) or 4 K73-17 (0,47 μF x 630 V) connected in parallel. Electrolytic capacitors C2, C3 - K50-35 (22 uF x 63 V). It is undesirable to use imported "electrolytes", because they have large losses during reloading.

Diodes VD1 ... VD4 can be used with any U_obp> 100 V and I_Max> 200 mA. KD103A and 1N4007 work well. Transistors - with U_ke> 80 V.

When you turn on the charger for the first time, you need to set the R12 regulator slider to the lower position according to the diagram. The green LED VD5 should be lit. During operation, it is worth checking the absence of heating VT1. Overheating can be eliminated by reducing the resistance R9 or replacing VT1, VT2 with transistors with a large β.

Upon reaching U_ak = 13,8 V by rotating R12, you need to turn off the charging current.

Connect the charger to a 220 V network using an indicator screwdriver or a neon bulb TN-0,2 with a 240 kOhm (0,5 W) resistor to determine the phase wire in the outlet.

To charge 6-volt batteries, the VD6 zener diode must be replaced with KS133 or KS147.

When the battery is disconnected from the charger, the voltage at the output of the charger (cathode VD7) is zero. Relative to the neutral wire of the network, both output wires of the charger have a potential of about 30 V. Closing the output wires of the charger does not disable it, because. the maximum current is limited by C1 at 140 mA.

Literature

O. Khovaiko. Power supplies with a capacitor voltage divider. - Radio, 1997, N11, p.56.
A.Sorokin. Charging and desulfating machine. - Radio amateur, 1998, N10, p.30.
A. Trifonov. Ballast capacitor selection. - Radio, 1999, N4, p.44.
S. Biryukov. Calculation of a network power supply with a quenching capacitor. - Radio, 1997, N5, p.48.
S. Biryukov. Digital devices on the IC, 1999.
R. Levitsky. On the use of capacitors in AC circuits. - Radio, 1969, N8, p.49.
Pulse charger. - Radio, 1995, N8, p.61.

Author: Yu.Semenov, Voronezh; Publication: cxem.net

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