I don't know how this circuit operating....

DickCappels

Joined Aug 21, 2008
10,153
I wrote about a functionally equivalent circuit in 2002.
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The tapped inductor is equivalent to the transformer in the circuit you show.

Overview
A minimum number of parts yields a compact switching converter that can provide sufficient voltage to drive white LEDs. The resulting lamp is much more efficient, in terms of lumen hours per pound of battery, than incandescent bulbs, and because the color of the light is determined by fluorescence of phosphors within the LED assembly, the color of the lamp does not change perceptibly as the battery runs down. As a result, very long battery life is achievable. This circuit is suitable for powering flashlights, emergency lighting, and other applications in which it is desirable to power white LEDs from one or two primary cell batteries using a low cost circuit.

The Circuit
The circuit could not be simpler than this. The transistor, 1K resistor and the tapped inductor form a blocking oscillator. When the power button is pressed, the transistor is biased on through the 1 k resistor. Voltage that appears from the tap on the inductor to the collector causes the voltage on the 1K resistor to be even higher than the battery voltage, thereby providing positive feedback. Also because there is voltage across the inductance between the tap and the collector of the transistor, collector current increases with time (this is in addition to a starting value that relates to the current supplied to the base, but this part of the collector current is rather small. Because of the positive feedback the transistor stays saturated until something happens to change its base current.


At some point the IR drop across the inductor from the tap to the collector approaches the battery voltage (actually battery voltage - VCEsat). As this happens voltage is no longer induced in the winding from the tap to the 1k resistor and the base voltage starts to drop, and this forces the base voltage to go negative, thereby accelerating the switching off of the transistor. Now, the transistor is off, but the inductor continues to source current and the collector voltage rises.

Quickly, the collector voltage gets high enough for the LED to conduct current, and it does for a little while, until the inductor runs out of current, then the collector voltage starts to ring toward ground base voltage swings positive again, turning the transistor on again for another cycle.

The Inductor
If you aren't designing this as part of a commercial product, you have a lot of latitude in the design of the inductor. The size of the core, its permeability, and its saturation characteristic (Physical dimensions, u and Bs) determine how many amper-turns it can sustain before it saturates. If the core saturates before the IR drop from tap to collector approaches the battery voltage, the circuit will switch quickly anyway because saturating the core makes the coil look like a resistor and coupling between the collector half and the base half (the side with the 1k resistor) drops to very little, so the effect is the same as the IR drop approaching battery voltage. The wire size determines how many amps the circuit will dray (well, ok, milliamps) before the IR drop gets large enough for the circuit to switch. The inductance constant of the core (physical dimensions and u mostly) determine how man microseconds it takes the collector current to rise to the point the circuit switches off, and it also determines how long current will be delivered to the LED when the transistor is off. Nearly every inductor parameter affects the performance of this circuit.

I have made this with ferrite beads a few millimeters in diameter and toroid cores up to a few centimeters across (take a look at the Rust Nail Inductor further down on this page). Here is the general relationship between core size and characteristics:

Large core: Easy to wind, lower frequency operation, higher power.
Small core: Harder to wind, higher frequency operation, lower power.

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