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12V DC Regulated Power Supply

12v DC Regulated Power


This circuit is pretty simple, but essential if you intend to run any electronic circuits from the 12V accessory output of your loco controller. As stated earlier, the 12V d.c. outlet of some controllers is way above 12 volts, and is unsmoothed, which is fine for signals, turntable motors, and some point motors, but totally unsuitable for any piece of electronic equipment. Most of the projects in this section require smooth d.c., and this circuit will allow the modeller to connect them to an existing train controller, thus saving money on batteries.

The maximum output current that can be drawn is 1 amp, which is adequate for most applications.


How it Works. The “raw” d.c. is first smoothed by electrolytic capacitor C1, before being stabilized by integrated circuit voltage regulator IC1. This component provides a steady output of 12V d.c., which does not vary significantly with variations of either input voltage to the device, or output current demands made by the supplied equipment.

Capacitors C2 and C3 are known as decoupling capacitors, and must be mounted as close to the leadouts of IC1 as possible. Without them, the circuit could become unstable, giving undesirable effects such as oscillation on the output.

Light emitting diode D1 provides a visual indication that the unit is operating, and can be board or panel mounting as desired. Resistor R1 is necessary to reduce the current flowing through D1 to a few milliamps. Its value is not critical, anything from 560 ohms to 2.2k will do, but bear in mind that LED brightness diminishes with higher resistance.

if required, a different regulator may be used in place of IC1, to provide a different voltage output. Use type 7805 for a 5V output, or 7810 for a 10V output.

In the case of a 5V regulator, a 220 to 680 ohm series resistor is recommended for the LED current limiter, to ensure sufficient brightness.

Construction and Testing. Apart from the notes regarding C2 and C3, there are no special precautions concerning layout for this project. Take care to mount C1 the correct way round, as it is an electrolytic and is polarity sensitive. IC1 MUST be attached to a heatsink to prevent overheating. A twisted vane type is recommended, dimensions 40 x 20 x 30mm approx. To prevent an excessive amount of heat dissipation, it is advisable to keep the maximum input voltage below 30V.

Before applying the power, double check ALL your wiring. Check for short circuits across adjacent tracks if using stripboard or a p.c.b., components inserted the wrong way round, etc. When satisfied that all is in order, switch on, and check that D1 lights. If not, switch off immediately, and re-check the wiring.

If the circuit is working properly, the output voltage should be 12V (plus or minus 4%). Shorting together the + and – outputs of the unit should result in the output falling to zero, recovering as soon as the short circuit is removed. That’s all there is to it, the unit is then ready for use!

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