Power supply for fluorescent lamp. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Lighting

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The basis of the lamp, the circuit of which is shown in the figure, is a blocking generator based on transistor VT3. Resistor R7 limits the base current of the transistor. Diode VD1 protects the device from being connected to a power source (battery) in the wrong polarity. The lighting sources are two fluorescent lamps EL1 and EL2 connected in series with a power of 6 W each from the Chinese ROBO lantern. The luminaire has also been tested with single 6 and 20 W LDS. Based on the ratio of brightness and current consumption, the choice was made in favor of two six-watt ones. A low battery indicator is not necessary (all elements included in it do not need to be installed on the board), but it is very useful, especially when using a relatively small capacity battery (for example, a motorcycle battery).

The indicator consists of LED HL1, transistors VT1, VT2, resistors R1-R5, capacitor C1 and is a Schmitt trigger. To achieve a sufficiently small width of the trigger hysteresis loop, the values ​​of resistors R1 and R3 had to be increased, and the positive feedback resistor R5 had to be decreased. Resistor R4 limits the current through LED HL1. Capacitor C1 is noise suppressing.

While the battery is sufficiently charged, transistor VT1 is open, since the voltage at its base is greater than the opening threshold. Transistor VT2 is closed - its base-emitter section is bypassed by open transistor VT1. LED HL1 is off. As the battery discharges, the voltage at the base of transistor VT1 decreases, transistor VT1 will begin to close. Due to positive feedback, the process proceeds like an avalanche. As a result, transistor VT1 closes completely, VT2 opens, and LED HL1 lights up. In standby mode, the indicator consumes no more than 1 mA, and after activation - approximately 5 mA.

The device uses fixed MLT resistors with the power indicated on the diagram. Trimmer resistor R2 - multi-turn SP5-3. Capacitor C2 - K73-9; any small-sized one will be suitable as C1. Transistors VT1, VT2 - series KT315, KT3102 with any letter indices. The VD1 diode must be designed for a current not less than that consumed by the lamp from the battery, and it, in turn, depends on the power of the installed LDS. With one six-watt lamp, a diode from the KD226 series can be used here. LED HL1 - any color, but better than red, most suitable for signaling a situation requiring intervention. Of several transistors of the KT815, KT817, KT819 series tested as VT3, the KT819G indicated in the diagram ensured reliable switching on of the LDS. In addition, it has a fairly large margin of current and voltage limits. The latter is especially necessary when the load is accidentally disconnected from a running generator. For example, the KT815B transistor with a maximum collector-emitter voltage of 25 V worked properly until one of the wires connecting the LDS to winding III of transformer T1 broke. The transistor was immediately broken.

The magnetic core of the transformer T1 - B22 is made of ferrite 2000NM1. Windings I (9 turns of PEV-2 0,45 wire) and I (10 turns of PEV-2 0,3 wire) begin to be wound simultaneously with two wires turn to turn. After the ninth, the end of winding I is fixed in the slot of the frame, then the last turn of winding II is wound. The frame with finished windings I and II is thoroughly impregnated with paraffin and wrapped in thin paper in two layers, ironing each with the tip of a heated soldering iron. As a result, the paper absorbs excess paraffin and fits tightly to the wires of the windings, fixing them and providing the necessary insulation. Next, high-voltage winding III is wound. For one LDS it should contain 180, for two connected in series - 240...250 turns of PEV-2 0,16 wire. The coils are laid in bulk, trying to distribute them as evenly as possible. It is necessary to ensure that those at the beginning and end of the winding do not touch each other. For example, it is very undesirable to place both terminals of winding III in the same frame slot. The coil is once again impregnated with paraffin and inserted into the magnetic circuit, which is assembled with a gap of 0,2 mm between the “cups”, using a paper or thin plastic gasket. Transformer T1 is secured to the board with a screw made of non-magnetic material, passed through the central hole of the magnetic circuit. This method, unlike assembly with glue, ensures reliable fixation of the transformer on the board, and, if necessary, quick dismantling.

The lamp is assembled on a wooden (plywood) base with dimensions of 280x75x6 mm. In the upper part of the base there are two LDs parallel to each other, in the lower part there is a printed circuit board covered with a casing made of aluminum sheet. The casing has holes for the HL1 LED and connecting wires, including two multi-core ones with alligator clips for connecting to the battery. Transistor VT3 is attached to the casing, using the latter as a heat sink. The LDS is installed on two wooden blocks with a cross section of 15x10 mm glued to the base. One of them is located at the upper edge of the base, the other below, at a distance equal to the length of the LDS without leads (215 mm). Under the terminals of the lamps, contacts made of tin are installed on bars. The contact on the upper bar simultaneously serves as a jumper between two LDS, and the terminals of winding III of transformer T1 are connected to the two on the lower bar. The LDS is secured with four screws screwed between its terminals. It is necessary to pre-drill holes in the contacts for the screws, and be sure to place washers under the heads of the latter. This mounting method ensures a reliable connection between the LDS and the transformer and allows you to replace lamps without resorting to a soldering iron. For better light output, the base under the lamps is covered with reflective film or foil.

Before turning on the lamp for the first time, be sure to check the quality of the connection between the LDS and winding III of transformer T1. Poor contact can lead to breakdown not only of the VT3 transistor, but also of the transformer. If, after applying the supply voltage, there is not even a faint glow of the LDS, you should swap the leads of one of windings I or II of transformer T1. Then resistor R6 is selected, achieving the required brightness of the light and taking into account that the current consumed from the battery increases along with it. Typically, sufficient brightness can be achieved at a current of 600...650 mA. If the brightness needs to be adjusted smoothly, resistor R6 can be replaced with two connected in series - constant 680 Ohms and variable 3,3 kOhms. When adjusted, the current consumption will vary from approximately 0,2 to 1,4 A.

To set up the battery discharge indicator, the latter is temporarily replaced with an adjustable constant voltage source with a maximum value of at least 12V. If the source is low-power, you should first turn off the blocking generator by unsoldering one of the terminals of winding I of transformer T1 from the contact pad. By rotating the slider of the trimming resistor R2, we ensure that the HL1 LED lights up when the source voltage decreases from 12 to 10,8... 11 V. The indicator threshold is chosen to be slightly higher than the minimum voltage to which the battery can be discharged (10,5 V), so that after lighting the LED, the lamp does not need to be turned off immediately. A similar lamp with one LDS with a power of 6...10 W can be installed in the car interior instead of a standard lampshade with incandescent lamps. In this case, there is no need for a battery discharge indicator, but the generator must be powered through a noise suppression filter.

Publication: radiokot.ru

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