45ohm Mylar Speaker,Mylar Speaker for telephone,Mylar Speaker Used in telephone Gaoyou Huasheng Electronics Co., Ltd. , https://www.yzelechs.com
To drive multiple high-brightness white LEDs, engineers must decide whether to connect them in series or parallel. Each configuration comes with its own set of trade-offs. In a parallel setup, each LED only requires a low voltage, but to maintain uniform brightness, ballast resistors or current sources are necessary. If the current through each LED varies, their brightness will also differ, leading to an uneven light output. However, using these components to balance the brightness can reduce battery life.
On the other hand, a series connection ensures consistent current flow through all LEDs, but it demands a higher voltage across the entire string. A typical white LED needs around 3.6V and up to 20mA of current for proper illumination. Figure 1 illustrates a cost-effective inductive boost circuit designed to power seven white LEDs in series.
This circuit consists of two main parts: a boost section made up of Q1 and Q2, and a control section involving Q3 and JFET1. When Q1 is off, and the battery voltage slightly exceeds the VBE of Q2, a positive base current (iB = (battery voltage - VBE) / RJET1) flows into Q2, turning it on. This causes the inductor L1 to be grounded.
As the current through L1 increases at a rate of di/dt, energy is stored in the magnetic field of L1. The increasing current also flows through the saturation resistor RSAT of Q2, while SD1 and the LED string remain off. Once the collector voltage of Q2 rises enough, Q1 turns on. The base of Q1 is connected to the collector of Q2 via a feedforward network consisting of R1 and C1, with R1 also serving to limit the base current of Q1.
When Q1 turns on, it grounds the base of Q2, causing Q2 to turn off. The energy stored in L1 is then released into the LED string as the magnetic field collapses. The rapid discharge of L1 creates a forward bias voltage exceeding 26V across the LED string, resulting in bright white light emission. Since the human eye cannot perceive the high-frequency flicker, the circuit provides steady illumination.
Once L1 discharges completely, Q1 returns to the off state. This self-oscillating process continues during normal operation until the battery voltage drops below the sum of VBE for Q2 and the voltage drop across JFET1 (approximately 1V), at which point Q2 stops conducting. The resistance of RSAT, the characteristics of L1 and Q2, and the switching behavior of Q1 and Q2 all influence the oscillation period and duty cycle.
The battery pack, consisting of four alkaline cells, is boosted above 26V to provide sufficient forward bias to the series-connected LED string of seven white LEDs. Additionally, Q3 is biased by a small DC current (less than 20µA) through R4, which helps regulate the channel resistance of JFET1 and reduce battery leakage, thereby extending battery life. The gate voltage of the p-JFET is about 0.9V higher than the battery voltage. As a depletion-mode device, the p-JFET is normally on when VGS is zero.
July 29, 2025