Power Supplies

Simple Power Supply Concept Schematic Circuit Diagram

Enhanced Power Supply Alternatives:

Simple power supply concept: Common alternatives to a ‘quick and dirty’ power supply include the three-pin fixed voltage regulator and the Zener-plus-transistor combination. While these basic circuits are suitable for many applications, they have limitations that can be frustrating. For instance, fixed voltage regulators are typically limited to an output current of around 1 A. Adding a ‘current bypass’ transistor for higher power can compromise supply regulation. Regulators with higher output currents (e.g., 5 A) are costly. The Zener-plus-transistor circuit, another alternative, faces challenges like poor ripple rejection and stability issues at varying output loads. The presented PSU overcomes these drawbacks, offering a simple, versatile, and cost-effective solution for various applications. Unlike the basic alternatives, this circuit incorporates feedback, achieving an impressive 100-Hz ripple suppression of up to 55 dB—far surpassing the capabilities of a simple Zener-transistor stabilizer.

Comparing Limitations and Advantages:

The traditional alternatives, while functional in many cases, come with inherent limitations. Fixed voltage regulators struggle with higher output currents and increased cost for higher capacities. The Zener-plus-transistor circuit faces challenges with ripple rejection and stability. In contrast, the presented PSU stands out by addressing these limitations. Its innovative design incorporates feedback, significantly enhancing ripple suppression and overall stability. This circuit proves to be a superior, low-cost solution for diverse applications, providing efficiency and reliability where other alternatives fall short.

Utilizing TI 431C as Voltage Reference:

The voltage reference in this design is D1, specifically a TI 431C from Texas Instruments. The internal structure of the TL431C is depicted in the diagram. D1 supplies a base current to T1, resulting in 2.5 V across resistor R3. This configuration allows for the calculation of the supply output voltage, U0, using the formula U0 = 2.5 [1 + (P1 + R2)/R3] volts.

Adapting Output Voltage and Component Values:

The specified component values yield an output voltage of 12 V. To obtain different output voltages, adjust the output voltage divider, ensuring that the current through P1, R2, and R3 remains at least 1 mA. This precaution is necessary to maintain negligible current flow into the reference input of the TL431 (approximately 2 pA). The power transistor, a Darlington with a guaranteed current gain of 1,000 or more at an emitter current of 5 A, necessitates only 5 mA of base current.

While this is relatively low, it’s a factor to consider when assigning a different value to R1. D1 requires a minimum cathode-anode current of 0.5 mA, resulting in a total minimum current of 5.5 mA through R1. Considering the lowest possible input voltage, Uin (measured across C6), and the base-emitter drop of T1 (approximately 2 V), the theoretical value of the current limiting resistor is determined by R1 = (Uin – Ube – U0)/Ir1.

Optimizing Resistor Value and PCB Design:

Due to the potential higher current gain of the Darlington, which may be two or three times the guaranteed value, it’s feasible to experiment with giving R1 a higher value than calculated. A higher resistor value reduces dissipation in R1 and D1. The designed PCB accommodates the entire rectifier section, including a bridge rectifier, a buffer capacitor, and a fuse. The buffer capacitor, C1, and the onboard heatsink for T1 are sufficiently sized for output currents up to 2 A.

As already mentioned, this PSU is a concept. Those of you who do not need the rectifier section may omit it, and connect a d.c. with a voltage of 16 V to K1. Note, however, that this requires wire links to be fitted in the positions indicated by dashed lines near the bridge rectifier.

If you require more output current (say, up to 5 A), simply move the power transistor off the board and fit it on a larger heat sink (see parts list). Also, increase the buffer capacitor to 10,000 pF. Since such a capacitor (or array of capacitors) will not fit on the board, connect it as an external part via heavy-duty wires and two spade terminals (marked ‘+’ and ‘-‘ on the component overlay). A continuous output current of 5 A also requires the bridge rectifier to be cooled. This is best achieved by leaving it on the PCB and clamping it on to a side panel of the metal enclosure used to house the supply.

 

 

Parts list

Resistors:
  • RI = 470 ?, 0.33W (see text)
  • R2 = 6.8 k?
  • R3 = 2.2 k?
  • R4 = 1 k?
  • P1 = 2.5 k? preset H
Capacitors:
  • C1 = 4700 pF, 40V
  • C2 = 10 pF, 35 V tantalum
  • C3;C5 = 100 nF
  • C 4 = 100 pF, 40 V
  • C6 = 10 pF, 40 V
Semiconductors:
  • D1 = TL431C
  • D2 = LED green, 3 mm
  • T1 = MJ3001
  • B1 = B80C5000/3300
Miscellaneous:
  • KI:K2 = 2-way PCB terminal block, pitch 5mm.
  • F1 = 2.5 A fast fuse (6.3 A)* and PCB mount holder
  • Heat sink: SK201 (6 K W-1) or
  • SK71/75mm* (1.25 K W-1).
  • Two `fast-on’ spade terminals for PCB mounting
  • PCB Ref. 924024.
  • * for the 5A version only.

 

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