Design of Transformerless Charger Controlled by Thyristor

The charger introduced in this article directly uses 220V AC mains power and adjusts its output voltage from 0V through the control of the trigger circuit. It is suitable for charging 12V-220V batteries (groups).

How it works

The working principle of the circuit is shown in Figure 1. It consists of three parts: the power supply circuit , the trigger circuit and the main control circuit. After the 220V mains is stepped down by the power switch S-S' and the power transformer T1, it is rectified by the full-wave rectifier circuit composed of diodes VD1-VD4 to become a pulsating DC power supply. One path is limited by resistor R1 and stabilized by the voltage-stabilizing diode DW to deliver a trapezoidal wave synchronous regulated power supply of about 18V, which is used as the power supply for the RC delay link of the astable oscillator composed of the time base integrated circuit NE555 and its peripheral components ; the other path is sent out through the three-terminal voltage-stabilizing integrated circuit IC 1 AN7812 to deliver a 12V stable trapezoidal wave synchronous regulated power supply IC 2. The trigger circuit is composed of IC2 NE555 and components such as R2, R3, RP, C1, C2, etc. The oscillation period is less than 10ms and fixed. The astable oscillator and R4 and pulse transformer T2 that can only change the duty cycle of the output rectangular wave form a trigger pulse. The reason why the oscillator uses two synchronous regulated power supplies of 18V and 12V is to increase the duty cycle of the output rectangular wave, that is, to increase the phase shift range of the trigger pulse. The phase shift range of this trigger circuit is greater than 120°. By adjusting the potentiometer RP, trigger pulses with different trigger angles can be output, thereby achieving the purpose of controlling the conduction angle of the thyristor VS.

Experiments have shown that the pulse width output by this trigger circuit is several times larger than that of any trigger circuit composed of a single junction transistor , and it can reliably trigger the thyristor in the reverse electromotive force load and large inductance load circuit to conduct reliably.

The main control circuit is composed of fuse FU, ammeter and thyristor VS. After connecting the battery or battery (group) to be charged, the thyristor VS obtains a trigger pulse and controls the conduction angle of VS with pulses of different pulse widths. By adjusting RP, the charging of batteries (groups) with different charging currents or voltages can be satisfied.

Component selection and production

The components list is shown in the table below.

The power transformer T1 uses a transformer with a primary voltage of 220V, a secondary voltage of 24V, and a power of 5W. T2 uses a MX2000GL22X13 magnetic tank. The primary L1 is wound with Φ=0.17mm high-strength enameled wire for 100 turns, and the secondary L2 is wound with 200 turns of enameled wire of the same wire diameter. All resistors use metal film resistors. RP uses a WXD3-13 multi-turn potentiometer, and VS uses a 10A unidirectional thyristor with a withstand voltage greater than 100V. It is advisable to add a larger radiator to facilitate long-term operation. The charging current of the battery to be charged should be less than 8A.