Discussion in 'FAQs' started by cidchase, May 15, 2002.

  1. cidchase

    cidchase Active Member

    I'm new to this, and I keep seeing references to a capacitor used on the AC switch machine circuits. Is this for the purpose of reducing arcing on your control pushbuttons??

  2. shamus

    shamus Registered Member

    Hi Cid,

    Welcome to the gauge.

    A capacitor discharge unit is used instead of using straight 16v ac to power the switch motors. If you just use the 16v ac on its own, sometimes either a momentary switch (Lever) or probe (Stud & Probe) it can weld itself for a second or two. Using the capacitor discharge unit stops this by giving a short burst of electrical current and will not re-charge itself if the lever or probe are still in position. Like you stated, in can cause an arc sometimes.

  3. kettlestack

    kettlestack Member

    Welcome to "The Gauge" Cid,

    Yes, the (transistorised) capacitor discharge circuit dramatically reduces if indeed it doesn't actually eliminate arcing.

    Arcing occurs when a switch carrying current opens, not when it closes.
    The principle being that the capacitor charges up with a fair amount of current. When the pushbutton is pressed the capacior sees about a 3 to 5 ohm load (which is the switch machine coil) and discharges very rapidly. The discharge time is shorter in duration than the time you would normally hold the button down for. When the capacitor is discharged, the transistor prevents the flow of any appreciable current through the pushbutton to the turnout motor. Therefore there is only a few thousandths of an amp flowing when the pushbutton is released. Not only is this insufficient to cause arcing but there is no way the turnout motor can burn out if the pushbutton is held down indefinately or jams in the operated position.

    NON TRANSISTORISED capacitor discharge units do NOT guarantee that turnout motors will NOT burn out.

    If I have been verbose here it is for the benefit of those who are still wondering about why experienced modellers rave about the transistorised versions.

    Believe me, they are worth their weight in gold :) .

  4. 60103

    60103 Pooh Bah

    So, Errol, I should get rid of the ones with the vacuum tubes? :D

    The other reason for the CD units is that you can get a bigger jolt of electricity to throw more machines at a time or kick over a reluctant one. (last reason probably not good).

    They also work with what we call a "diode matrix" which is a set of diodes or rectifiers linking a series of turnouts so that you push one button and line them all up.
  5. N Gauger

    N Gauger 1:20.3 Train Addict


    Vacuum tubes... I remember them!!! They came out just before 8 track tapes & CB's -- right??? LOL :D ;)
  6. Vic

    Vic Active Member

    Don't go out and spend a wad for a CDU. Most all of the model railroad electronics books have a decent circuit for a CDU that you can build for $5 or $6. Might take an hour to do it!:D

    NOW ABOUT THOSE VACUUM TUBES.....:eek: Just sold a vacuum tube radio on Ebay for $150:eek: Paid $1.00 for it at a junk sale.
    Will make a long visit to the train store this weekend:D :p :)
  7. cidchase

    cidchase Active Member


    O fellow railheads???
    Thanx for your responses. After checking some websites as suggested by Vic, I think I've learned::

    1. The "Capacitor" CDU is acually a DC power supply with a large output capacitor for increased power storage

    2. The main purpose of this unit is to provide sufficient power for multiple switches in parallel, at the same time providing the diode logic advantages of DC (matrices, blocking, suppression, etc.)

    3. The AC rated switch machines will function satisfactorily on DC since the coil "on" time is quite short.

    Question? Would the various "slow motion" machines preclude use of the CDU?? I believe they use a motor which might not run on DC??

    This is the most fascinating hobby I've ever been into.
    Thanx again, Cid
  8. Gary Pfeil

    Gary Pfeil Active Member

    Slow motion machines use a steady flow of current to keep the points against the stock rail. So a pulse of power won't work. I use both types of machines on my layout, solenoid types in hidden areas (because I had them) and stall motors elsewhere. I use the same power supply for both (DC) Note the slow motion motors require DC, while the solenoid type work with either AC or DC. I built a capacitor discharge system many years ago and it worked well, but I don't use one now and can throw 4 turnouts at once thru a diode matrix with no problem. Because of this experience, I'm not sure why for the past 20 years or more, the hobby press has been extolling these capacitor systems. I do not recall offhand the specs of the transformer I use, it is large. If anyone is interested, I will check it and post. I use stud and probe mostly, tried various push buttons but welding is a problem. I suppose this is the reason for the cap system. Note that it is DC which gives this problem much more so than AC. An electrical engineer once told me to put a diode across the terminals of the switch (in parallel) it would help prevent the arcing caused when the cicuit was broken. I never did it tho. I have installed turnout switches made by (I think) Acme which have worked so far, but really haven't seen much use yet. They basically consist of a brass bar and phosphor bronze wiper material which is held beneath a push button cap, with a red and a green button intended to be mounted on a panel with track diagram. Again, if anyone is interested in more info on them, let me know I will check.

  9. Vic

    Vic Active Member

    If you decide to go with a "homebrew" CDU most any transformer of 12-18 Volts AC 2Amp rating will work fine. The transformer out of an old power pack will work just fine. If you are using Atlas switch machines a 1 Amp rating will be a plenty. Also if you can't find the exact value capacitor called for in the plans just use the next higher value in microfards. The only difference that will make is that it will take it just a second or two longer to recharge. The voltage rating of the capacitor is not critical either. Just make sure that it is at or larger than the voltage rating called for in the plans. Slow motion switch machines use a motor with a high stall rating to move the turnout points and won't work with a CDU. Can't say that I understand the "arcing" problem. Never had it occur...have never "burned out" a machine or a pushbutton in 20 some odd years. I wouldn't worry about it. Using a CDU voltage and current is applied to the switch machine for only a microsecond and the capacitor can't recharge until the pushbutton returns to the normally open position.
  10. 60103

    60103 Pooh Bah

    We've had a number of Radio Shack pushbuttons fail in the CD circuit -- the worst case is that they stick on and the capacitor never recharges again. Read recently that the problem may be over-enthusiastic soldering that melts some of the innards.

    CD units certainly ONLY work with the coil type switch machines; not with anything with an electric motor in them. Don't expect they work with the old single coil relay machines.
    That said, does anyone have an idea how to hook up a Tortoise type machine to the probe & contact controls that Shamus uses?

    The Wedneday night layout has a string of turnouts with random gatherings of push buttons for some and toggle switches for others.
  11. Gary Pfeil

    Gary Pfeil Active Member

    David, The short answer would be no. The tortoise and other stall type machines require constant current, so a stud and probe won't work, same reasoning why the capacitor discharge won't work with them. Having said that, someone will come along with a way to make the stud and probe work. I would think it would involve a relay, in effect a solenoid being thrown to change the polarity to the tortoise. I assume this could be done with solid state components also, but I don't have any idea on how to do so. What you would really be doing is throwing something to change the polarity of current to the tortoise. I use toggles and implemented a standard so operators will know by looking at the toggle handle which way a turnout is thrown. Toggle levers are mounted to throw vertically, up position is straight, down is diverging route. In case of crossovers, both machines will throw with one toggle, in this case up is normal, down is set for crossover. Toggles can be mounted in control panels or on the fascia of the railroad in front of the turnout it controls.

  12. Vic

    Vic Active Member


  13. jland31

    jland31 Member

    Capacitor Discharge Unit

    Loys Toys has a couple of CDU's advertised.
    Has anyone out there used 'em?
  14. 60103

    60103 Pooh Bah

    Push Buttons

    No Vic, not my house, the other guy's. CDU will lower the fire risk there as well.

    We did have a swirch machine fire at the Great British Train Show 2 years ago. I missed the excitement, but I could smell it.

    I power my CDUs with the little power packs that come in train sets. They seem adequate for that.
  15. kf4jqd

    kf4jqd Active Member


    Hello All:

    There are 2 basic types of capasitors. They are disk and electrolitic. Here are the difereances.

    Disk: They suppress surges from electrical current. They make electricity flow "evenly".

    Eletrolitic: They store an elctrical charge. They are usually polarized. Make sure you connect them correctly. They WILL explode like a large firecraker! :eek:

    As for vaccum tubes, I go to Ham Radio swapmeets and see old recievers and transmitters with them. In large 500watt plus amps, we still use tubes. Why, because the are still more effient than transitors when it comes to amphication.


    Side note:
    There are non-polarized electrolic caps. These are used in audio crossovers and such,
  16. Vic

    Vic Active Member

    Exploding Capacitor...Andy

    Hey Andy, Ever had to clean up after exploding one of those large caps?:confused: Busted a 5k MFD once:eek: I cleaned up those paper discs for a month!!!:D :D :D 73's
  17. kf4jqd

    kf4jqd Active Member


    Yes I had have that happen a couple of times.


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