One-transistor Voltage Converter

Taking apart a solar-powered lamp  revealed a single-transistor voltage  converter circuit that allowed an LED  to be driven from a 1.2 V cell. The l/h  diagram shows the circuit (with slight  modifications). The circuit oscillates at  about 500 kHz and, at a cell voltage of 1.4 V, draws 11 mA with a respectably bright LED. The circuit works down to a supply voltage of 0.8 V.

One-transistor Voltage Converter Circuit Diagram:

Converter Circuit Diagram

The oscilloscope shows 3Vpp at the LED, as expected. The left-hand coil and the capacitor form a series resonant circuit, excited by the collector of the transistor which alternates periodically between conducting and blocking. When the transistor is off the upper coil dumps its stored energy so that the voltage on the collector rises to about double the cell voltage.

A sinewave voltage of 35Vpp (!) was measured across the capacitor in the resonant circuit. Using a two-channel oscilloscope showed the phase relationships: the resonant circuit shifts the phase by about 90 degrees. The base resistor coupled with the base capacitance and the Miller capacitance (http://en.wikipedia.org/wiki/Millereffect) of the transistor add a further phase shift.

The voltage increase obtained using the series resonant circuit can be used to make a bipolar voltage converter, for example to power operational amplifiers (see r/h diagram). Two electrolytic capacitors and two diodes rectify the voltage. The circuit can deliver a volt-age difference of 9 V at 0.2 mA, which is enough for a low-power opamp.

Author By:  Burkhard Kainka (Germany) Source By: Elektor



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