Car power supply for a laptop, 12/19 volts 3 amps. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The Toshiba-Satellite laptop's power supply voltage is 19V with a maximum current of 3A. Unfortunately, this is not at all suitable for working in a car, since it is 12V.

The figure shows a voltage converter circuit that boosts car 12 V to laptop 19 V, and maintains this voltage stable. The circuit is based on a DC-DC converter based on the LT1070 chip.

The voltage from the vehicle's on-board network through a 5-ampere fuse and noise suppression circuits is supplied to the power supply point of the A1 microcircuit (pin 5).

The chip generator starts working and sends pulses to the key in A1, connected between pin 4 and the common minus (pin 3).

The key, periodically opening, passes a pulsed current through the primary winding of the transformer T1. In the secondary winding T1, an alternating voltage is induced, which is rectified by the VD2 diode and smoothed out by the capacitors C9, C10.

Further, through the circuit L2-C11-C12, a constant voltage of 19 V is supplied to the output of the source.

To control the level of the output voltage and stabilize it, the internal stabilization circuit A1 is used. The essence of her work is that she thus changes the duty cycle of the pulses entering the primary winding of the transformer so that there is a constant voltage of 2 V at pin 1 A1,24.

In order to obtain a stable output voltage, it is necessary to apply a constant voltage through a divider to terminal 2 A2 from the output of the secondary rectifier to VD1. The divider resistor ratio should be such that, with the correct output voltage, there is a voltage of 2 V at pin 1 of A1,24.

The exact selection of R4 sets the required nominal stabilized output voltage. In this case, it is 19 V. For winding the transformer, a ferrite ring with an outer diameter of 32 mm from 2000NM ferrite was taken. The ring must be wrapped with a thin layer of fluoroplastic film or varnished cloth. You can not wrap the ring with anything, but cover it with a layer of epoxy pack. After it dries, you can wind the windings. It is quite possible that a ring of a different diameter and brand of ferrite can also be used to wind the transformer - you need to experiment.

The primary winding contains 40 turns of a winding wire, consisting of two PEV 0,43 wires folded together. You can use a single wire with a cross section of 0,9, but it will be more difficult to wind. The secondary winding contains 70 turns of the same double wire. First, the primary winding is wound, and then the secondary winding is wound on its surface, laying the wire in the same direction as the primary was wound. In the diagram, the beginning of the transformer windings are marked with dots.

For chokes, rings with a diameter of 18-20 mm are used. They contain 30 turns of the same double wire as for winding the transformer.

The converter circuit is assembled on a printed circuit board with a one-sided arrangement of printed tracks.

The microcircuit and diodes must be mounted on radiators. A metal case in which the converter is assembled can serve as a common radiator (diodes and a microcircuit are specially located at the edge on the board). The microcircuit can be installed on a heatsink-case using heat-conducting paste, without insulating, since its heatsink plate is connected to the 3rd output, and it is connected to a common minus (to ground). And the diodes on the radiator must be placed through mica gaskets on a heat-conducting paste, using ebonite washers, so as to ensure the isolation of their cathode plates from the case.

With proper installation and serviceable parts, the adjustment comes down to checking the output voltage. If it differs from the required one, you need to change the resistance of the resistor R4. A decrease in resistance leads to an increase in voltage, and an increase to a decrease in it. The operation of replacing the resistor should be done only after turning off the power. Therefore, it can be temporarily replaced by a variable resistance of 1.5 ... 2,5 kOhm. With it, you need to set the required output voltage, check it on the load, for example, by connecting a 36 V incandescent pump (or two car bulbs connected in series as for rear lights).

After the setup is completed, after turning off the power, unsolder the variable resistor and measure its resistance. Then install a constant resistor of this resistance or very close to the one obtained on the board. It is possible that the required rating will not be available and you will need to dial R4 from several resistors using serial or parallel connection.

Turning on a converter without a heat sink for the A1 chip is risky, therefore, even during the adjustment process, especially when testing on a load, you need to install it on some kind of radiator and periodically monitor the temperature.

The same converter can also be used to power other devices from the car's on-board network. The desired output voltage is set by selecting resistors R5-R4 and the number of turns of the secondary winding of the transformer.

Author: Kuzyansky L.

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