Power Supplies and Safety


Working With High Voltage AC Power

- Don't* -

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

  Always include a Ground Fault Circuit Interrupter in your household wiring or as part of an extension cord to supply AC power to home built circuits and your whole layout for that matter.

Ground Fault Circuit Interrupter

  Also, it is far less expensive, safer and easier to buy a new or used, regulated and efficient power supply that is UL or CSA approved than making one yourself using an old iron cored transformer.

  In the end, you will be safer, and your insurance agent will be happier, if you use an approved power supply.




Getting The Supply Polarity Wrong.

  This photo shows what can happen if the polarity of the power supply to a circuit is incorrect. The charred IC was once an LM556 timer.

  The 556 is particularly suceptable to dammage by a reversed supply voltage. Not all ICs are this likely to burn up, some may survive short revered polarity exposures while others will be dammaged electrically.




Using Old Throttle Power Packs As Power Supplies For Electronics

- Don't Even Consider Them -

  The throttles that come with inexpensive train sets will almost certainly have no voltage regulation and will likely not have filter capacitors. Setting the TRACK power output of a throttle to "12 volts" could supply full wave DC to the circuits with a peak voltage of 17 volts which may dammage some components.

  Even the more expensive DC power packs should not be used to power electronic circuitry.

  Using the ACCESSORY output of a throttle to feed a separate rectifier and regulator circuit is not a good choice either as these will still be supplied by a poor quality transformer.




Using Plug-in Transformers (Wall Warts) As Power Supplies For Electronics

- OK, But -

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

  Some plug-in transformers have no filter capacitors and may not even have rectifiers. These supplies 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 for the load and adding a regulator is a good way to make use of surplus transformers.




Using 9 Volt Batteries As Power For Electronics

- OK, But -

  9 Volt batteries have the current capacity of a AAA size battery. If the circuit draws more than a few milliamps the battery will not last long. If the current draw is too high for the battery, the voltage will drop-off.




Laptop Computer Power Supplies.

  An excellent source of high quality, regulated DC power supplies is AC power adapters for laptop PCs. The best choices are 12 and 15 volt models with a current capacity of 3 amps or greater.

  Power supplies with 3 prong plugs should be not be used for throttles and DCC boosters as the common ground connection will cause short circuits when the power is reversed between blocks. The Toshiba brand supplies are more likely to have the 2 prong power cord that is desirable for use on model railroad throttles and for testing power supplies.

A similar circuit using one transformer.

  15 volt power supplies would be ideal for home built DC throttles and accessory decoder boosters for DCC systems.

  Even though these power supplies will likely have built in overcurrent protection, a fuse should be used at their outputs as the internal protection may pass more current than downstream circuits can handle.

  Used laptop power supplies can be can be purchased inexpensively at many computer and electronics recycling facilities. (Have them tested before purchasing.)


Other Power Supplies.

  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.

  Battery substitute power supplies typically have an output that is fixed at 13.8 volts DC and are available in a variety of current ratings.



Regulated Power Supply Circuits

  The circuits below are for various, regulated power supplies.

  The iron cored transformers and bridge rectfiers are shown in the circuits are typical of text book power supply examples.

  The transformer, bridge rectifier and filter capacitor in these circuits could be replaced by 15 volt laptop power supplies. This substitution would reduce costs and avoid high voltage AC exposure.


Fixed Voltage Regulators

5 Or 12 Volt Supply

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


A Test Bench Power Supply

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

Test Bench Power Supply Schematic

Data sheet for LM317 Adjustable Voltage Regulators - http://www.national.com/ds/LM/LM317.pdf

  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


1.5 Volt Power Supply - LM317

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

1.5 Volt Power Supply schematic


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Please Read Before Using These Circuit Ideas

  The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.

  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.

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22 March, 2016