AAA Carbon battery.Analysis and design of circuit principle using battery to drive seven high-brightness white LEDs

　　Using a series connection can essentially ensure the consistency of the current, but it requires applying a higher voltage to the LED string. To achieve proper lighting brightness, ordinary white LEDs require a 3.6V bias voltage and a maximum 20mA bias current. Figure 1 shows a low-cost inductive boost circuit that can adjust the brightness of 7 white LED strings.

　　This circuit can be divided into two parts: the boost circuit composed of Q1 and Q2, and the control circuit composed of Q3 and JFET1. Assuming that Q1 is turned off, when the battery voltage is slightly higher than the VVB of Q2, a positive current will flow through the base of Q2 (iB=(battery voltage VBE)/RJET1). At this time, Q2 is turned on and the inductor L1 is grounded. As the current on L1 increases at a rate of di/dt, energy is conserved in the L1 magnetic field. As the current gradually increases, it also flows through Q2's resistor RSAT (SD1 and the LED string are in the off state). The collector voltage of Q2 is high enough to turn on Q1. The base voltage of Q1 is connected to the collector of Q2 through a feedforward network consisting of R1 and C1. R1 is also used to limit the base current of Q1.

　　After Q1 is turned on, the base of Q2 is grounded, so Q2 is turned off, and the energy of L1 is released into the LED string as the magnetic field weakens.

　　The rapid return action of L1 applies a forward bias voltage higher than 26V to the LED string, causing the LEDs to emit white light. Since the human eye cannot detect the high-frequency flicker of LEDs, this circuit can provide lighting with constant brightness. When L1 is discharged, Q1 returns to the cut-off state. During normal operation, this self-oscillation action is repeated until the battery voltage drops to less than the sum of Q2's VBE and JFET1 voltage drop (approximately 1V), at which point Q2 no longer conducts. The RSAT of L1 and Q2 and the switching characteristics of Q1 and Q2 will also affect the oscillation period and duty cycle. The voltage of the battery pack (4 alkaline cells) was boosted above 26V to provide forward bias to the LED string consisting of 7 white LEDs connected in series.

　　A small DC current (less than 20uA) flowing through R4 biases Q3 to adjust the channel resistance of JFET1, thereby regulating the battery leakage current to extend battery life. The gate voltage of JFET1 is about 0.9V higher than the battery pack voltage. Here p-JFET is used as a depletion mode device. When VGS is equal to zero, p-JFET is turned on. The source of ET is connected to the battery terminal. Design engineers can turn off the channel by increasing the gate voltage (higher than the positive battery voltage). The higher the gate voltage is than the battery voltage, the greater the channel resistance.

　　Therefore, when the battery pack voltage drops from 6V to 3V, the oscillation frequency drops (VGS of JFET1 will change slightly). At this time, the brightness of the LED drops slightly. Ideally, the control loop will keep the LED current constant. However, the human eye's sensitivity to light obeys a quasi-logarithmic relationship, so until the battery pack voltage drops to about 2V, a small linear decrease in brightness is not easily noticeable.

　　Another option is to keep the battery's power output (the product of current and voltage) constant. Due to the internal resistance loss of the battery, although this can keep the LED brightness unchanged, it will shorten the battery life and the complexity of the circuit will also be greatly increased. In summary, the LED brightness of this simple circuit will vary very little throughout the battery life. The brightness of the LED string can be slightly adjusted. For example, the design engineer adjusts for the manufacturing deviations of the transistor and LED by slightly changing the resistance of R2, so that the light output (unit: lumens) can be set to a fixed value.

　　When the battery pack is about to run out of energy, you can short-circuit the dim LED string and connect only one LED. At this time, as long as the battery pack still has 1V of remaining voltage, the LED can be made to emit strong light. This single-LED connection can use discarded batteries to provide last-minute emergency lighting.

　　For safety reasons, all batteries must be matched when using alkaline batteries. When the energy of the battery with the least energy in the battery pack is completely exhausted, and the other batteries still have enough energy to form a reverse bias on the exhausted battery, it will cause the exhausted battery to overheat and leak milky acid, resulting in Security Question. To achieve battery matching, ensure that all 4 batteries are replaced at the same time with new batteries from the same package. The rated capacity of 4 AA alkaline batteries is 4×1000mAh, which means the LED can illuminate continuously for about 61 hours. Test results of the circuit prototype showed that its continuous illumination time was a little more than two days (48 hours).

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