Introduction to the Low Voltage Regulator Circuits
In the realm of low voltage regulator, there exists a wide array of circuit options. However, as one ventures into higher voltage territories, particularly for applications like valve circuits, the landscape takes on a different character. It was this contrasting scenario that led us to embark on the design of a straightforward regulator capable of handling these elevated voltage levels. This circuit, which we present here, finds a natural synergy with the quad power supply tailored for the hybrid amplifier, as featured elsewhere in this publication. Remarkably, the regulator itself comprises only three transistors, with a fourth one thoughtfully integrated to perform the crucial task of current limiting. Operating as a positive series regulator, the design employs a pnp transistor (T2) to minimize voltage drop.
Operation of the Voltage Regulator Circuit
The operation of this voltage regulator circuit is refreshingly uncomplicated. When the output voltage experiences a decline, T4 promptly lowers the emitter of T3, setting in motion a reaction that drives T2 to deliver more power. This action, in turn, causes the output voltage to regain its former level. R4 plays a pivotal role in regulating the base current of T2, contributing to the circuit’s stability. For added assurance of stability, capacitors C1 and C2 are introduced into the circuit.
Significantly, these capacitors are connected in series to ensure that during startup or in the event of a short circuit, excessive voltage across each capacitor is prevented. It’s advisable to select capacitors with a minimum rating of 100 V for C1-C3. Additionally, the circuit includes diode D1, which serves to shield T2 from negative voltages that may arise in scenarios such as input short-circuits or when substantial capacitors are attached to the output.
Setting the Reference Voltage with Zener Diodes
In this design, a pair of zener diodes, each rated at 39 V, are ingeniously linked in series to establish a stable reference voltage of 78 V, applied to the base of T3. To double the output voltage, R6 is set equal to R7, resulting in an output of approximately 155 V. T4 plays a crucial role as a buffer for the potential divider R6/R7. This configuration permits the use of higher resistor values, ensuring the voltage remains unaffected by the base current of T2, which closely mirrors the emitter current of T3. While lacking temperature compensation, this design proves more than adequate for its intended purpose.
Simplified Current Limiting Mechanism
The current limiting mechanism, centered around T1, is elegantly simple yet effective. When the output current surpasses 30 mA, the voltage drop across R1 triggers T1 into conduction. T1 subsequently limits the base-emitter voltage of T2. To safeguard T1 from rapid voltage peaks across R1, R2 is strategically included.
Startup and Operational Considerations
R3 plays a vital role in the startup phase of the regulator. Without R3, there would be no output voltage, leading to a lack of base current in T2. R3 facilitates a minimal conduction in T2, allowing the regulator to reach its intended state. During normal operation, with a voltage drop of 15 V across T2 and a current of approximately 30 mA, additional cooling for T2 is unnecessary. However, caution is warranted as the junction temperature reaches 70 °C, posing a risk of burns.
Handling Varied Input Voltages and Output Adjustments
The regulator’s ability to supply current hinges on the Safe Operation Area (SOAR) of T2. During short circuits and an input voltage of 140 V, the current hovers around 30 mA, necessitating a heatsink with a minimum rating of 10 K/W under such conditions. Modulating the output voltage involves adjusting the value of R6. Alternatively, a higher reference voltage can be accommodated by substituting T4 with a MJE350. For low current requirements, omitting T4 and R4 is feasible. In such cases, the potential divider (R6/R7) can be directly connected to the emitter of T3. Remarkably, the circuit boasts a ripple suppression of approximately 50 dB, with a quiescent current of 2.5 mA and an impressively low dropout voltage of 1.5 V for small currents.