Stuck on CDU

Discussion in 'FAQs' started by junga, May 11, 2006.

  1. junga

    junga New Member

    Hi, all. I've decided to try CDUs on my Atlas snap switches. The problem is I'm finding all these schematics, but I'm having trouble figuring out how to translate them into reality. I found one item at and I almost had it, but I can't find the transistor that's listed there, or the diodes, at any shop in NYC. I'm trying to build one so I can learn how they work.

    Can anyone send me some pics of CDUs they've built themselves? Perhaps a picture of a circuit board? What I'd like to find ideally is a list of parts, and a picture of what to etch on the back of a circuit board. The CDU MarkII in the link above was so close. I also had another link to the "SwitchWitch" but the schematic and the circuit board don't quite match. That link is at What's missing, for me, in this one is the LED that shows the capacitor is charged.
    Is there software out there that can help me do this?

    I realize it would be easier to just buy, but I really do find the engineering aspect interesting. I hope I'm not way off base here by asking this.

    Thanks to you all.

  2. ezdays

    ezdays Out AZ way


    First of all, welcome to the Gauge. I'm sure there are a few here that have built CDU's, there have been other discussions here regarding them. As far as the first circuit goes, the 1N4001 diodes are a very common diode, as I remember it is rated at one amp/50 volts. You should be able to find one or an equivilent (anything in the 1N4000 series will do). As far as the darlington transistor, another common replacement should be a TIP120. The second circuit uses both and they even give you the Radio Shack part number for each.

    I've not built any CDU's, but maybe someone will stop by that has and can give you more information or perhaps a better circuit. I'm just wondering why this one uses four 2.2K resistors in parallel when they could get by with a single 560 ohm resistor.:confused:
  3. pgandw

    pgandw Active Member


    Welcome. Don identified suitable parts substitutions for the 1st circuit. As to the reason for the 4 parallel resistors, the author states,

    "Assembly of the PC board is straightforward. Four 2k2 1/4watt resistors are wired in parallel to form R1. A 470R 1 watt resistor could be used but ¼ watt resistors will look much neater."

    2.2K 1/4 watt resistors are probably much easier to find also.

    The 2nd circuit is intended to be replicated and installed at each switch machine. Not my favorite option - I like the 1st circuit. You can also increase the size and/or number of capacitors (add additional capacitors in parallel) in the unit if you need to throw more switch machines simultaneously. The drawback of additional capacitance is that the recharge time will increase. If using an AC input power source, the recharge time can be reduced by substituting a full wave rectifier rated at 1 amp, 50 volts or better for the input diode. This will allow recharging of both parts of the AC sine wave.

    yours in powering fun
  4. junga

    junga New Member

    Guys, thanks for your prompt responses. I'm almost there, I think. If I can substitute any 1N400x, then I know I can find those. I'm going to use the included pcb diagram as my template, so I'll go with the 4 1/4 watt resistors. Don, will a single TIP120 replace the BD679, or do I need to have another transistor?

    I prefer the 1st circuit too, especially since it includes an LED.

    Guys, color me naive, but by your reply, Fred, am I to understand that I can use just ONE capacitor discharge unit to power ALL of my switches (albeit one at a time)? I guess I'd wire the AC output to the circuit board's inputs, then wire the output to each toggle switch (I'm using toggle switches on a control panel), then whichever toggle I activate is the one that would close the circuit, and send the charge to the appropriate switch machine, yes? How would I wire these? DaisyChain + with shared common ground?

    If you guys tell me that I would only need one, you'll really have made my day...but don't tell me that just to make me happy! ;)

    Thanks again, guys. This has been really helpful?

  5. ezdays

    ezdays Out AZ way

    They are both darlington's and are both in TO220 packages so even the footprints should be the same. They should substitute one for one, and are much easier to find.

    Again, I have not ever used one, but it is my understanding that you can "daisy chain" one CDU to your toggle switches and use one unit to do all your switching. As Fred points out, you either have to wait for the capacitor to recharge, or increase the capacitance to be able to do multiple switching. Increasing the capcitance will increase the pulse width as well, so that's something to consider.

    Yeah, I didn't read the "theory of operation" all the way through, but if it's power vs "looking good", I'd opt for the larger wattage resistor.:D
  6. pgandw

    pgandw Active Member

    The smaller CDU's use 2200 ufd or smaller capacitors, and are generally designed to be cheap enough to be put at every switch machine. They often will not even bother with transistors, just a simple RC circuit.

    To answer your question, yes. Have a great day. Most modelers prefer a single larger CDU to use as a switch machine power supply, just as you have described. The circuit design article in the first link shows the CDU wired just this way. Commercial CDUs typically have at least 4000 ufds of capacitance to enable them to drive several switch machines simultaneously. This comes into play in wiring a crossover between 2 parallel tracks, or similar situations, where you want to use just one set of push buttons to control multiple turnouts. If you think you might ever want to throw more than one switch machine at a time, use bigger and/or add more capacitance (the linked article describes this as an option towards the end). The slight extra duration of the pulse cited by Don is not a factor in our work.

    I would still recommend using a full wave rectifier instead of a single diode on the input - the conservationist in me hates wasting cycles.

    Thanks Don for the substitution knowledge.

    Enjoy your CDU unit!
  7. junga

    junga New Member

    Thank you both! I'm well on my way. I'll post and let you know how it works out.

    Much appreciated.


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