Utility Power A Resistor selectable voltage - from the DCC supply

Chapter 2a Page 9

Utility Power Supply

This power supply can provide a localized DC voltage source from the DCC track supply. Designed to be used for low current accessories such as crossing lights that do not need a lot of current and where running a special cable from a remote DC supply may be too complicated.

Construction:

Cut the strip board 18 holes wide by 6 rows deep. If you want to allow mounting holes or input/output terminals expand the size to suit. It helps to put masking tape over the extra unused holes. Mark the position of hole number 1 on each side to help alignment then mark the position of the copper cuts (X on diagram) note the board has been flipped vertically to show the copper strip and 1 is now the bottom left corner. Double check the position and twist a #31 (1/8 inch) drill in the marked holes to remove the copper. Clean up the wisps with a knife and check with a meter that there is no electrical circuit across the hole. Some of the component leads are larger than the PCB holes and a #56 (1.12mm) drill will be needed to enlarge the hole.

Solder in the wire cross links. Use solid single strand insulated wire. "Cat 5" communications cable can be used for these link wires. Solder them in and cut off excess. Insert the resistors and diodes. Double check the positions and polarity before soldering them in. Follow them by the larger components, again checking the polarity of the capacitors. The legs of the bridge rectifier can be bent to fit it close to the board or just push it down as far as it will go and solder.

Check the copper side that no solder is bridging between the copper strips. Running a craft knife blade down the insulation between copper strips will usually find solder bridges.

Select the correct resistors (R1 & R2) from the chart to give the voltage that you need - it can be 3, 5, 6, 9 or 12 volts. To get the best voltage range the resistors must be 1% tolerance or selected with an Ohm Meter. With these the voltage variation will be less than 2% on most voltages. Install terminals or wires as required and test.

Chapter 2a-2 Page 10

Soldering notes :-

When you solder, touch the hot iron to the lead and the copper strip, push the resin cored solder onto the joint and when the solder runs along the copper and round the lead, slide the iron up the lead to leave a clean wick of solder running a little way along the copper and up the lead. Do not make solder blobs, they usually mean the solder has not attached to the metal. i.e. something was dirty.

How it works

The LM317 series regulator I.C. is used as it can be easily setup to a specific voltage. The 1N4004 diodes will damp any high voltage spikes from an inductive load that could damage the regulator.

Most DCC boosters run a track voltage in the 15 to 18 volt range with current capacity of at least 4 amps so the supply of a low current accessory circuit will not impose any great load on it. The regulator does need at least 3 volts more than its output, so a 15 volts DCC input is the minimum for a 12 volt output. The DC generated is pure DC without as much voltage ripple of the sort you would get from a mains transformer driven supply. The D C output is not isolated from the track power, so this is best used for stand alone accessory circuits.

The LM317 Integrated Circuit Voltage Regulator

This multi voltage regulator is used in many applications. It has a regulation range of 1.2 volts to 37 volts from a 40 volt max. DC input. The regulator has a basic 1.2 volt regulation between the output and the adjust pin. The output is then raised above the 0 volt supply line by the resistor connected between the adjust pin and the 0 volt line. The IC is protected internally from thermal overload and short circuits. The output current can go to 1.5 amps. If over half an amp fit a heat sink to it.

The voltage output depends on the ratio of the There is no protection from voltage spikes selected resistor and the 240/270 ohm one (R2). The on the output line so to protect against more accurate this is the closer to the voltage you will these damaging the IC from inductive or get. capacitive loads, two diodes must be Resistors of 1% tolerance are recommended and included in the circuit. these will give a 0.6 volt spread at 12 volts. These are more expensive but becoming more readily available. Most resistors now are sold in at least packs of 5 so a selection can be made. The reference resistor change allows a more accurate voltage to be obtained.

Table of 1% resistor values Voltage = 12v, 9v, 6v, 5v, 3v R1(Sel) = 2,200 1,500 1,000 750 360 R2 (Ref) = 240R 240R 270R 240R 270R

Components Capacitor 0.1ufd disk capacitor [0.1u] Resistor 1% at R2 shown as Electrolytic capacitors 240 ohm resistor [240R] 10ufd at 16 volt working [10u] Or 270 Ohm as per table [270R] 100ufd at 25 volt working R1 is a 1% Resistor, as required [100u] LM317 Integrated Circuit Regulator W04 Bridge Rectifier 1.5 amp - round Diodes 1 of 1N4004, 1 of 1N4148 Perforated Strip board 18 holes wide by 6 rows

Most layouts will need an extra power supply to run utility circuits. Signals, lighting, signs etc are all going to need some power source. Here is one way to obtain a multi voltage, high current and readily available supply

There is available a ready built utility supply in the Personal Computer that most people now have. Most computers die or are replaced every 5 or so years so why not recycle a bit of it for the layout. You might get one for the asking at the local computer shop if they throw them away or there might be a collection point for old computer gear at the local refuse collection center. Or they can be bought new for under $100 at a computer hardware supplier. The part we want is that box that the power cord plugs into. It may have an on/off switch on it or the switch might be on the front of the PC case. There will be a lot of red/orange/yellow/black wires coming out of it and going to parts of the PC. Unplug them, then remove the 4 screws holding the power supply box to the rear of the PC case and wriggle it out. Once out you will see a lot of dust, a fan also covered in dust and lots of wires with plugs on. Take the top off and get to work on the dust. Turn the box upside down and brush as much as you can off of it and into the bin. Remove the 4 special screws from the fan mount, note which way it goes, slip it out and give it a good brush down, including the air slots it sits against. Remove the wires from the exit holes. Separate them and their plugs and note the ones that went to the socket on the main board. Cut all the wires to the other plugs close to the circuit board in the box and discard them. If the power switch has been mounted on the front panel of the PC you must decide what to do with it. I use it as is, mounted somewhere on the layout. Or wire across the leads and eliminate the switch, which would mean you need to switch off at the wall when leaving. Any PC less than 10 years old will have a switch on the back of the power supply. It will use a control wire that is grounded by the front panel switch to start the computer. The power switch on the box only puts the supply into standby mode. There should be a label in the top cover that gives you the specifications and with luck the color codes of the various wires.

Construction

Mount a terminal strip on the top of the box and route the wires to it. The black wires are all common and are the ground [0volt] return wires. Color codes in general are:- For older supplies with mains power switches, red for 5 volt, orange for +12volts and blue for -12 volts.

WARNING: Fuse each voltage line

Newer supplies [less then 10 years old] also have a 3.3 volt wire and this is orange. Then the +12volt wire will be yellow. There will also be a green or gray wire going to the 4th pin in the plug. This is the power on control [p-on] you must connect this to the black wire beside it [pin 5] for the supply to come out of standby mode when the power switch is put on. Join the 2 wires and insulate them off. Snip off any other unused wires. Having connected every thing up, replace the top cover and the supply is ready.