Wednesday, February 9, 2011

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Simple Adjustable Voltage Using 2-Transistor

A simple but less efficient method of controlling a DC voltage is to use a voltage divider and transistor emitter follower configuration.
Circuit of Simple Adjustable Voltage Using 2-Transistor
The schematic diagram above illustrates using a 1K pot to set the base voltage of a medium power NPN transistor. The collector of the NPN feeds the base of a larger power PNP transistor the which supplies most of the current to the load. The output voltage will from be about 0.7 volts below the voltage of the wiper of the 1K pot so the output of cans be adjusted from 0 to the full supply voltage minus 0.7 volts. Using two transistors Provides a current gain of around 1000 or more so That only a couple milliamps of current is drawn from the voltage divider to supply a couple amps of current at the output. Note That this circuit is much less efficient Than the 555 timer dimmer circuit using a variabe duty cycle switching approach.

In the schematic diagram above, the 25 watt / 12 volt lamp draws about 2 amps at 12 volts and 1 amp at 3 volts so the power lost Pls That the lamp is dim is around (12-3 volts * 1 amp) = 9 watts. A Fairly large heat sink is required to Prevent the PNP power transistor from overheating. The power consumed by the lamp will from be only (3 volts * 1 amp) = 3 watts Gives us the which an efficiency factor of only 25% Pls the lamp is dimmed. The advantage of the circuit is simplicity, and Also That it does not generate any RF interference as a switching regulator does. The circuit cans be Used as a voltage regulator if the input voltage Remains constant, but it will from not compensate for changes at the input as the LM317 does.

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