Driven more than one high-brightness white LED, design engineers need to select LED is connected in series or parallel connected LED. Of course, each connection will be brought to the design trade-offs.
Simply connected in parallel across each LED low voltage applied, but need to use ballast resistors or current sources to ensure consistent brightness for each LED. If the flow through each LED bias current of different sizes, then their brightness is different, resulting in uniform brightness of the light. However, the use of ballast resistors or current sources to ensure consistent LED brightness will shorten battery life.
Series connection with nature can well ensure the consistency of the current, but requires a higher voltage applied to the LED string. To achieve appropriate lighting levels, ordinary white LED requires 20mA 3.6V bias voltage and the maximum bias current. Figure 1 shows can adjust the brightness of 7 low-cost white LED string inductive boost circuit.
This circuit can be divided into two parts: Q1 and Q2 by the composition of the boost circuit, and by Q3 and JFET1 composed control circuit. Suppose Q1 off, when the battery voltage is slightly higher than Q2 of VVB time, Q2 base current will flow through the positive (iB = (battery voltage VBE) / RJET1). At this point, Q2 turns on, inductor L1 to ground.

With current L1 on the di / dt of the velocity increases, the energy saved in the L1 magnetic field. As the current increases, it also flows through Q2 resistance RSAT (SD1 and the LED string is off). Q2's collector voltage is high enough to make Q1. Q1's base voltage through R1 and C1 formed by the feed-forward network is connected to the collector of Q2. R1 can also be used to limit the Q1's base current.
After Q1 turns on, driving the base of Q2 ground, so the end Q2, L1 decreased as the magnetic field energy is released into the LED string.
L1 moves in the fast return to zero applied to the LED string forward bias voltage above 26V, the LED white light. Since the human eye can not feel the high-frequency flashing LED, so the circuit can provide constant brightness lighting. When the discharge end of L1, Q1 to return to the cut-off state.
Normal operation, the self-oscillation action is repeated until the battery voltage drops below the VBE and JFET1 Q2 drop (about 1V) of and, when Q2 is no longer turns on. L1, Q2 of RSAT and Q1, Q2's switching characteristics also affect the oscillation period and duty cycle.
Battery pack (4 alkaline batteries) or more voltage is increased to 26V in order to the 7 series LED string composed of white LED provides positive bias.
A small DC current flows through R4 (less than 20uA) for Q3 is biased to regulate JFET1 the channel resistance, thus regulating the battery drain current to extend battery life. JFET1 the gate voltage higher than the battery voltage is about 0.9V. Here p-JFET is used as depletion-mode device, when VGS is equal to zero, p-JFET conduction.
ET's source connected to the battery terminals. Design engineers can improve the gate voltage (positive voltage higher than the battery) to turn off the channel. The higher gate voltage than the battery voltage, the channel resistance will be.
Therefore, when the battery voltage drop from 6V to 3V, the oscillation frequency decreased (JFET1 the VGS will be a slight change.) At this point, LED brightness slightly. Ideally, the LED current control loop will remain unchanged. However, subject to the human eye sensitivity to light quasi-logarithmic relationship, so the battery voltage down to 2V or so ago, the small magnitude of the linear decline in brightness is not easy to be noticed.
Another solution is to keep the battery output power (the product of current and voltage) constant. Loss due to battery internal resistance, although this LED brightness can be maintained unchanged, but will shorten battery life, in addition to the complexity of the circuit will also be greatly enhanced. In short, this simple circuit LED brightness throughout the battery life of little change.
Can slightly adjust the brightness of LED strings, such as design engineers to slightly change the resistance R2 and the LED for the manufacture of transistor bias adjusted, so that light output (unit: lumen) can be set to a fixed value.
When the battery power is running out, you can dim the LED light string short-circuit, but only to connect a LED, then there is 1V as long as the remaining battery voltage, you can let the LED light issue. This single LED connections can be used to provide the final disposal of batteries emergency lighting.
From safety considerations, when using alkaline batteries for all battery must match. When the battery pack in energy at least the energy of the battery is completely empty, while others have enough battery power runs out of battery power on the formation of reverse bias, it will lead to energy depletion and leakage of milky acid batteries overheating, resulting in security issues.
Match for the realization of the battery, should ensure that the new battery with the same package at the same time replacing all the four batteries. 4 AA alkaline batteries rated capacity of 4 × 1000mAh, which means that LED lighting can be about 61 consecutive hours. The test results show that the prototype is the time for two days of continuous illumination (48 hours) more.