The contemporary table lamps are widely used low-voltage halogen lamp 12 V To power supply 220 is often used In step (220/12 V) transformers that doubles as a cargo, which increases the stability of the structure to rollover. Power halogen lamps table lamps usually does not exceed 20…50 W, on the same power calculated and step-down transformers. It is important that the primary and secondary windings are wound, usually in separate sections, which increases the safety.
The power supplies on the basis of such a transformer, made in the form of an independent design, discussed in the articles [1, 2]. But if the size of the base (stand) table lamps, the power supply can support it. This source is useful for power players (3 In), portable radios (6 and 9), charging cell phones (5 In). It can also be used as a laboratory source of stabilized voltage of several fixed values. Refusal smooth adjustment due to the desire to simplify the use of the instrument.
Fig. 1
The scheme is embedded in a table lamp to the power source with a set of specified values of the output voltage is presented in Fig. 1. It contains a bridge rectifier diodes VD1-VD4, the capacitor C1, smoothing the ripple of the rectified voltage, and a voltage regulator based circuits DA1. The desired voltage select switch SA2 connecting in parallel with the resistor R3 shunt the resistors R4-R6. LEDs HL1-HL4 different colors of light - indicators of inclusion of a source and a selected value of the output voltage. Button SB1 - staff switch the lamp, the switch SA1 is used to turn the system off (for example, if the lighting is not necessary).
Fig. 2
Most of the details of the source are mounted on a printed circuit Board (Fig. 2) of one-sided foil fiberglass with a thickness of 1.5…2 mm. Fee calculated on the use of fixed resistors R1-4, S2-23 and import of capacitors (for example, series TC Jamicon company). The switch SA1 - PT or other small-sized, the switch SA2 is any compact disk, for example PH-9-PIN, socket XS1 - small, are used to connect headphones in mono equipment. HL1 led - super-bright white glow, and the rest - the usual (HL2 - yellow glow, HL3 - green, HL4 - red). Chip DA1 is necessary to provide a heat sink with a cooling surface 15.. .20 cm2 in the form of a plate thickness of 2…3 mm of aluminum alloy.
Author's variant of the source is mounted in the lamp model KD117N company Chameleon (Fig. 3). Board glued to the transformer, the LEDs are fixed with glue in the holes drilled in the left side of the base, the switch is in its rear part, the breaker - upper, socket XS1 - in right. The lamp is designed for use with a halogen lamp up to 35 watts. If it is to replace 20-watt, you will remain a reserve of strength that can be used for power source. If the required maximum output current (1.5 A), the lamp will have to disable to avoid overloading the transformer. To connect powered by devices use two-wire cords with the corresponding connectors on the ends.
Chip LM337T you can replace the IC regulator LM317T, but he has a different Pinout (input / output 3, output - pin 2), so we have to change the configuration of the respective traces on the Board. In addition, in this case must be reversed the polarity of diodes, LEDs and capacitors. The polarity of the output voltage will be positive with respect to a common wire.
In order to obtain a stabilized output voltage of 12 V, the diodes 1N4004 need to replace the diodes Schottky (for example, 1N5819) and select resistor, shunt R3. Note that the maximum output current in this voltage should not exceed 0.5 A (when a larger number may be the appearance of large ripple)
In conclusion - another useful function is described power source in a table lamp. The use of super-bright LEDs, white color luminescence (HL1) allows you to use it as a night light. It is enough, without connecting the load, set the switch SA2 in position "9".
Literature
1. I. Nechaev Block-based power step-down transformer halogen lamps. Radio, 2008, No. 12, pp. 28, 29.
2. I. Nechaev Small laboratory PI. Radio, 2009, vol. 5, pp. 21-23.
Author: I. Nechaev, Moscow; Publication: www.radioradar.net