Power Supplies

Simple power supply concept

THE best-known alternatives to a ‘quick and dirty’ power supply are the three-pin fixed voltage regulator and the Zener-plus-transistor combination. While these basic circuits will suit a good many applications, they do have their limitations, which can be frustrating at times. For example, most types of fixed voltage regulators are limited to an output current of about 1 A only. Where more power is required, a ‘current bypass’ transistor is often added. However, while this boosts the maximum output current, the regulation of the supply is degraded. Fixed voltage regulators with higher output currents (say, 5 A) are not a good alternative because they are notoriously expensive.

The second alternative, the Zener-plus-transistor circuit, has limited use also because of its relatively poor ripple rejection and insufficient stability at output load variations.

The PSU presented here suffers none of the disadvantages mentioned above. and is simple to build a multi-purpose concept. It is the perfect low-cost supply for a host of applications. At first glance, the circuit looks very much like the zener-transistor combination. However, an  essential difference is that feedback is used, which results in a 100-Hz ripple suppression of up to 55 dB-far more than can be achieved with the simple Zener-transistor stabilizer.

The voltage reference used here is D1, a TI,431C from Texas Instruments. The internal structure of the TL431C is shown In the diagram. Here, D1 supplies a base current to T1 that results in 2.5 V across resistor R3. This allows you to calculate the supply output voltage, U0, from
U0 = 2.5 [1 +(P1 +R2)/R3] volts.

The specified component values result in an output voltage of 12 V. For other output voltages, simply adapt the output voltage divider, making sure that the current through P1, R2 and R3 is at least 1 mA. This is required to ensure that the current flowing into the reference input of the TL431 is negligible (approx. 2 pA). The power transistor is a Darlington with a guaranteed current gain of 1 000 or greater at an emitter current of 5 A. This means that only 5 mA of base current is required. Although this is not much, it has to be taken into account when R1 needs to be given a different value. Also, D1 requires a minimum cathode-anode current of 0.5 mA, which results in a total, minimum, current of 5.5 mA through R1. This design information, together with the lowest possible input voltage, Uin (measured across C6). and the base-emitter drop of T1 (approx. 2 V), results in a theoretical value of the current limiting resistor:                                                                                                    R1=(Uin-Ube-U0)/Ir1.

Because the current gain of the Darlington may be up to two or three times the guaranteed value mentioned above, it is often possible to give R1 a higher value than calculated. Since a higher resistor value results in lower dissipation of R1 and D1 some experimenting is certainly worthwhile.

The PCB designed for the supply accommodates the complete rectifier section, that is, a bridge rectifier, a buffer capacitor and a fuse. The buffer capacitor, C1, and the onboard heatsink for T1 are large enough 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. the 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

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


D1 = TL431C

D2 = LED. green. 3 mm

T1 = MJ3001

B1 = B80C5000/3300

KI:K2 = 2-way PCB terminal block, pitch 5mm.

F1 = 2.5 A fast fuse (6.3A)* 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 5 A version only.



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