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Regulated Power Supply Circuits

Regulated Power Supply Circuits

There are power supplies with greater current capacities on the same Digi-Key catalog page as those shown above. These should be avoided as they have a negative terminal that is connected to earth through the grounded, three prong plug. This could cause isolation problems between various parts of a layout and could cause short circuits or ground loops.
The schematic is for a general purpose, variable voltage power supply. It is ideal for designing circuits or testing locomotives at the workbench.

The output is adjustable between 1.25 and 13.5 Volts with a maximum current of 1.5 Amps. The LM317K regulator is internally protected from current and thermal overloads. A heat sink will be required for most applications however.


The values of R1 and R2 are not critical but R1 should not be larger than 270 ohms. For a fixed voltage output R2 should be selected to give the desired maximum output voltage for a given R1 value or vise versa. See the calculation on the drawing.

The diodes D1 and D2 in the circuit prevent damage to the regulator during certain adverse conditions such as the output voltage being higher than the input voltage to the regulator or the capacitor across R2 from discharging through the adjustment terminal of the regulator. These conditions will be rare but ten cents worth of protection can save the cost of a new regulator.

An Ammeter could be added to the circuit by placing it between the filter capacitor and the input terminal of the regulator. The ammeter circuit located at the following link would work well for this purpose.

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