I’m not knowledgable about CD supplies, either theoretically or physically speaking. I do have a vague sense that solenoid motors may work more effeciently if they can “give back” the charge applied to them that made them move and that I guess is what the CD supply accomplishes. I’ll be using Peco (n scale code 55, electrofrog; layout wired for DC operation) turnouts with Peco switch machines attached directly to the bottom of the turnout on my layout. Given the assistance that I believe the spring wire built into the Peco turnouts provides to ensure a complete throw of the points, is a CD supply necessary? I plan to power the switch motors using the accessory AC terminals of a spare MRC 1300 throttle. Have people using Peco turnouts and motors had disappointing performance? Has anyone added a CD supply and did it provide a noticeable improvement? Thanks in advance.
A CD supply has several benefits over standard AC power.
First, it sends a very short, high current pulse which gives many times more power to the turnout or turnouts. I’m throwing up to (7) Peco t/o’s simultaeneously with a single pushbutton.
Second, because it is a very short pulse, you cannot damage the t/o solenoids by holding the button down. After the pulse the current drops off to a very low state and the CD will not recharge until the pushbutton is released for approx 1/10 second.
The CD board simply installs between your AC terminals and the pushbutton. There are different size CD machines available. Most will handle up to 3-4 t/o’s at one time. I added capacitance to allow more to be thrown.
You will be much happier with a CD supply. Twin coil machines can draw several amps, much more than a 1300 can reliably supply.
A CD supply is easy to construct; you need a low voltage DC supply, a current limiting resistor (to limit current from the supply), and a large capacitor.
The theory of a CD supply is simple. First, the AC is converted to DC with a bridge rectifier circuit. (If you are starting with DC, this step is unnecessary.) Then, the DC flows into a capacitor, where it is “stored” as a charge. It takes something like a second or two to fully charge the capacitor. Then, when you pu***he button, all of that charge goes rushing out very quickly, so you give a big jolt to the switch machine. After that, the “bucket” is empty, so very little current runs through the machines, and after you release the button all current to the machines stops, and the capacitor starts to charge up again.
For the Peco’s, the spring represents a large force that must be overcome to throw the machine. That’s why Peco turnouts require more of a jolt than do, for example, Atlas turnouts.
To clarify the operation of a capacitive discharge power supply:
As soon as it’s turned on, the capacitor is charged by the DC input.
When you press the button to throw the switch machine, the capacitor will discharge through the switch machine coil, giving a quick burst of extra power to throw it quickly.
Since the capacitor has now discharged, if you hold the button down, only the output of the DC side of the power supply is applied to the machine, ther is less chance of burning out the machine, but it will still happen if you don’t release the button.
Think of the capacitor in the circuit as a really short term battery (actually what it is in this case), it charges and discharges very quickly.
I had a problem a few years (and a couple of layouts) ago where some Peco turnouts that were located about 25 feet from my control panel would not throw reliably. I’d get a wimpy buzzing sound, and sometimes they would throw and sometimes they would not.
I built a CD supply and put it in line with the power feed to the buttons on the panel, and instead of a faint buzz I got a sharp snap every time I pushed a turnout button, and the turnouts always threw. I made no wiring changes at all.
Buy or make, with twin-coil switch machines a CD supply is a great thing to have.
Now I use Tortoises, so my CD supply waits in a box for me to find a new use for it.
I’ve tried locating a CD supply module online for purchase but no luck (the ones I found are all overseas). Mainestain, I went All Electronics but could not find one there. Anyone know where one can be purchased?
it takes about 30 minutes to build one…they are really simple to build yourself…a few diodes and a capacitor…Kalmbach has a book titled: Electronic Projects for Model Railroaders by peter j thorne that shows you how to build one for about $3.00 a unit…chuck
My LHS has 2 different models on the shelf. I think one is an MRC, and I don’t know about the other. They are 30-35 dollars, though, which is a lot to pay for a couple of capacitors and resistors. I too built my own, with under $10 in stuff from Radio Shack. I haven’t done a lot of soldering over the years, so this little project was a good “starter kit” for me.