Voltage polarity switch for charger. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Automobile. Batteries, chargers

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The device described below is designed to charge twelve-volt automotive batteries. Its main feature is that it allows you to connect a battery without thinking about its polarity. In a poorly lit garage, this is important, since the marking of the polarity of the charger clamps and the color of the connecting wires are often difficult to distinguish.

Fig. 1

The basis of its design (see the schematic diagram in Fig. 1) was taken "Charger based on the power regulator PR1500i" by S. Muralev, described in "Radio"'. 2009, no. 3. p. 40. I supplemented this device with an automatic switch for the polarity of the charging voltage (in the figure, its circuit is highlighted with dash-dotted lines).

Since the principle of operation of the charger based on the PR 1500i. its advantages and disadvantages are described in the article mentioned above, we will consider only the operation of the automatic switch. When the battery is not connected to terminals X1, X2, the triac VS2 is closed and the voltage at its output is zero. When the positive terminal of the battery being charged is connected to terminal X1 of the charger, and the negative terminal to terminal X2, relay K1 is activated and through the closed contacts K 1.1 and K 1.2 the control electrode of the triac VS2 and resistor R5 current will flow, which will prepare the triac for opening. If you now turn on the charger in the network, the triac will open and the battery will start charging.

If the battery is connected in reverse polarity, relay K2 will operate and by closing contacts K2.1 and K2 2 inverts the polarity of the charging voltage. The fuses RJ1 and FU2 protect the charger from accidental short circuits of the load circuit.

In both cases, even a short-term disconnection of any of the wires connecting the battery to the device will set the node to its original state, the triac VS2 will close on the last drop to zero half-wave voltage from the diode bridge VD1. As soon as the connection of the battery to the device is restored, the node will reconnect the load in the correct polarity.

The machine uses a RP21 relay with a winding for a constant voltage of 12 V. It reliably operates already at a voltage of 8 V. Thus, the charger can work with a battery discharged to a voltage of at least 8 V, which satisfies most practical cases.

It is better to turn on the charger in the network after connecting the battery and with the variable resistor R1 set to the position of the minimum charging current. The mains voltage supply is indicated by the HL1 LED. Then, resistor R1 sets the required charging current of the battery using ammeter PA1.

Transformer T1 must have a power of at least 80 watts. The voltage of the secondary winding can be in the range from 14 to 20 V with a current through it of at least 5 A. Ammeter RA1 - any DC with a scale of 10 A. Resistor R1 - SP-I. It must be isolated from the body and provided with a plastic handle, since it will be under mains voltage.

A properly assembled device does not need to be adjusted.

Fig. 2

If it was not possible to purchase two RP21 relays, the machine can be assembled on one. The circuit of the machine for such a case is shown in fig. 2. It is easy to see that when the battery is connected with a plus to terminal X1 (to terminal 3 of the machine), relay K1 remains de-energized, and with reverse polarity it works and inverts the polarity of the charging voltage. It should only be borne in mind that the relay for this version of the machine must be with four groups of contacts.

Author: S. Klemenov, Smolensk; Publication: radioradar.net

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