Jim Grant
EDN
A white LED delivers a wide color spectrum and better visibility than do monochromatic LEDs. However, a white LED presents a higher forward-voltage drop than do its colorful counterparts and thus poses problems for operation from a single 1.5 V cell. The self-oscillating step-up converter in Figure 1 features a minimal component count and an easily assembled transformer, T1.
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| Figure 1. | Two transistors and an easily assembled transformer drive a white LED from a single 1.5 V battery. |
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During the time it takes to charge T1's primary inductance, resistor R1 and T1's added secondary winding provide sufficient base current to turn on Q2. Q2's collector current increases until its base current can no longer hold the transistor in saturation. When Q2 comes out of saturation, T1's magnetic flux and secondary-voltage polarity reverse. During T1's primary-discharge interval, the combination of T1's secondary voltage in series with Q1's base-emitter voltage applies reverse bias to Q2's base and turns off the transistor. When Q2 turns off, the voltage across T1's primary inductance adds to the battery voltage and applies a forward bias to the LED, D1. The current through R1 determines the power applied to the LED and applies forward bias to Q1's base-emitter junction to provide temperature-compensated bias voltage for Q2.
The breadboarded circuit’s transformer, T1, comprises eight turns of AWG #30 insulated wire wound around the body of an unshielded 100-µH axial-lead inductor, producing approximately 400 mV p-p across the secondary winding. (Editor’s note: Observe the winding’s polarity dots. If the circuit fails to oscillate, reverse the connections to either the primary or the secondary winding.) The circuit operates over an input voltage range from just above Q1’s base-emitter voltage drop of approximately 0.6 V to the LED’s forward-voltage drop of approximately 3 V. The circuit’s switching frequency exceeds 340 kHz at 1.5 V input.
