Most projects require some form of voltage regulator. The Universal Power Supply project from August 1988 has been so popular, the kit is still on sale some 23 years later!
Basically, that design allowed you to build one of four different voltage regulator configurations on a single PC board. It could be configured in both split (positive & negative) rail and single rail versions and could be used with a variety of power transformers, with or without centre-taps. It could also be set up for a variety of output voltages, depending on the regulator(s) used.
In view of its popularity, we thought it was time to make some improvements. Accordingly, we have made the following tweaks to improve the original design:
(1) Designed a smaller PC board;
(2) Added terminal blocks for inputs and outputs;
(3) Made it easier to build;
(4) Made better provision for regulator heatsinks;
(5) Added LED indicators/bleeders for both rails;
(6) Added reverse-biased diodes at the output for regulator protection; and
(7) Made provision for a wider range of electrolytic capacitor sizes.
This project is called a “Universal Regulator” because it’s so flexible. Most commonly, it will be used to convert the AC output from a transformer (or an AC plugpack) to a regulated DC output. It can also be used to regulate an unregulated DC input voltage or it can be used to step-down a DC input voltage to a lower (regulated) output voltage.
As with the original design, the unit can be built in both dual-rail and single-rail versions. The output voltages can range from ±5V to ±24V at currents of up to 1A per rail. It all depends on the transformer and the regulators used.
Because this board can generate split (ie, positive and negative) rails, it is ideal for powering op amp circuits. It is also very handy for circuits which only require a positive supply (eg, +12V), in which case some components can be omitted.
Before going further, let’s take a closer look at how transformers are marked. Sometimes you will see a transformer labelled as “9 + 9” or “2 x 9”. This usually means that it has two 9V separate windings which can be connected in series or parallel. If you connect them in series, you have an 18V transformer with a centre tap.
If you connect them in parallel and in phase, you have a 9V transformer with twice the current rating of the centre-tap configuration (if they are in anti-phase, you will get no output).
A “9-0-9” label implies two 9V secondary windings with a fixed centre tap. These can not be connected in parallel because they will be in anti-phase and so there will be no output.
If a transformer has a VA rating (and most do), you can calculate the maximum theoretical output current by dividing the secondary voltage into that figure. So for example, a 60VA transformer can provide 2A if its secondary is 30V (60 ÷ 30) or 5A if its secondary is 12V (60 ÷ 12). Some transformers have multiple secondary taps so you can select the best combination for your circuit.
Fig.1: the circuit for use with a centre-tapped transformer to generate split rails. Diodes D1-D4 form a bridge rectifier, while capacitors C1 & C2 filter the rectified AC. Regulators REG1 & REG2 provide a steady output voltage while LED1 and LED2 indicate operation. Different output voltages are obtained by changing the transformer and regulators.