MAKING A POWER SUPPLY

How much power supply do you need, how many volts, build or buy? Lots of questions.

Successful operation of most decoders requires low amperage motors to be installed in your locomotives. Many newer locomotives are DCC ready. Older steam and diesle locomotives often have to be remotored. Systems like these include Alliance Helix Humpers for older Mantua and Bowser type steam locomotives. Mashima and Canon and Sagami make can motors that are suitable for older Athearn and PennLine diesels.

Typically a single engine will require a half to three quarters of an amp of current to run well. An example of a typical decoder might be the Digitrax DH121 decoder which can deliver 1.5 amps. What this means is, that if you wish to operate three or four or more locomotives simultaneously, you will need a power supply capable of delivering three or four amps output. Sometimes it is possible to operate even more locomotives by assigning sections of track to additional power stations, but this is a more advanced topic that we don't need to deal with right now, besides, anything you build now will probably be able to be used in a larger system later.

We will be looking for something around four amps, which is a pretty 'healthy' power supply, and should easily run four or so locomotives.

The primary component of a good power supply is the transformer. A center tap 24 VAC with a four amp rating is what you should be looking for. Also, several power supplies are available ready made from Digitrax, and others, including Radio Shack and Jameco Electonics. In this case you will want an 18 VDC regulated unit.

If you haven't built anything like this before, get some help. You need to be careful with what you do until you get it into the box. It needs to be well engineered! You need a strong reliable cord that will not pull loose. You need an On/Off switch, an indicator light so you know when it is turned on. All wires must be soldered securely and protected from all possiblility of shorting out. The voltage should be well smoothed with capacitors so the voltage is even and steady. The case should be grounded. .... and you want it to look good too.

A transformer this large will be heavy and somewhat expensive. Figure on thirty dollars for the transformer alone. You want a good strong box to put it in too. So add another ten dollars or so for that. The only other item of any major cost will be the filter capacitors and you will want at least 5000 mfd of capacitance. A fuse holder, fuse and indicator light are needed also.

Here is the circuit I am using for my 18 VDC power supply and it works well for my home layout running three or four trains.



Notice that the wall outlet "hot" goes to the fuse and then the switch. Then to the transformer primary, then back to the wall outlet neutral. The box or chassis is grounded to the wall outlet ground wire prong. A neon 120 VAC on/off indicator bulb is used to indicate when the system is turned on and that the fuse has not blown. The secondary winding is used full width. The center tap is output as DCC system booster ground. The secondary wires are wired to the bridge rectifier AC inputs as labeled on the device. The plus/minus outputs of the rectifier are labeled on the device. Only the plus output is used. This goes to the +18 VDC booster connection output on the back panel of the box. There are capacitors on the output to smooth ripples. This is a full wave rectified filtered DC supply. It is not regulated until it reaches the boooster.


The perfboard is mounted on two aluminum channel pieces that are bolted to the sides of the box. I am using a Stancor P-8663 24 VCT 4 Amp transformer that I purchased from Mouser Electronics. The transformer is fastened to the floor of the box with machine screws and nuts. The on/off light and switch are mounted on the front panel of the box. You can see the bridge rectifier and the capacitors used to derive and smooth the 18 Vdc output. The rectifier has labeling on it and must be placed into the circuit correctly. It passes electricity only in one direction, so it serves to convert the AC into DC current. The blue wires from the transformer are the two ends of the the transformer secondary winding, while the yellow wire is the center tap. The center tap is booster ground. I am using a 5 pin DIN socket to bring the output to the rear of the box. One red +18 Vdc and one black ground carry each output. I used the DIN socket and cable because I had it on hand, and it provides polarity assurance to avoid connecting the wrong polarity to the voltage regulator and 18VDC in the booster.