Unless you are very familiar with the precautions and safety regulations that are in force in your area, do not work on high voltage AC portions of power supplies. Have a qualified techician help you or have them build this portion of the circuit for you.

  Always include a 'Ground Fault Interupter' in your household wiring or part of an extension cord to supply AC power to home built circuits.

  The ones that came with inexpensive train sets will likely have poor voltage regulation and may not have filter capacitors. Even more expensive DC power packs should not be used to power electronic circuitry.

  Generally, plug-in transformers are OK but most of them are poorly filtered and not regulated. The output is voltage may drop considerably, as the load current increases and the ripple voltage will increase as well.

  Some plug-in transformers have no rectifiers and may not have filter capacitors and are not usable without an external rectifier and regulator circuit.

  Using a plug-in transformer with a higher current and voltage rating than will be needed by the load and adding a regulator is a good way to make use of surplus transformers.

  9 Volt batteries do not have much current capacity. If the circuit draws more than a few milliamps the battery will not last long. If the current draw is too high the voltage from that battery will drop-off.

  The following is a list of regulated, wall-plug type 12 Volt power supplies of various current capacities that are suitable for use with the circuits on this page.

  There are many others of similar characteristics form other sources as well.

  NOTE: There are switching power supplies with higher 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.

  Another regulated, high current power supply that may be useful are those from of similar to PYRAMID Regulated Power Supplies. These supplies are often used for automobile battery substitution circuits.

  The battery substitution supplies have an output that is fixed at 13.8 volts DC but are available in a variety of current ratings.

  The circuits on this page are for various regulated power supplies.

5 Or 12 Volt Supply

  Data sheet for LM78xx Series Voltage Regulators - http://www.national.com/ds/LM/LM7805C.pdf

  The next circuit is a classic on every LM317 datasheet. It's simple, reliable and a great place to start on building more advanced circuits.

Test Bench Power Supply Schematic

  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 limit 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. Shunt Ammeter Circuit information

  This circuit is for a 1.5 Volt power supply using the LM317T Adjustable Voltage Regulator.

1.5 Volt Power Supply schematic

  If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.

  Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.

06 August, 2009