Capacitor discharger for switches?

Discussion in 'FAQs' started by engineshop, May 6, 2005.

  1. engineshop

    engineshop Member

    I checked out this web site

    that has a diagram for a capacitor discharge unit for switch machine that take 16V and changes it to 35V to get better results for two-coil switch machines like Peco.
    I was wondering why not get a 35V Radio Shak transformer for the switch machines.
    I am not an electrician and maybe I am missing something important.
  2. ezdays

    ezdays Out AZ way

    I'd have to think long and hard before I'd do this. The point seems to be that the capacitor will charge up to the peak voltage of the AC signal and when you push the button, this higher voltage is placed across the coil until the capacitor discharges. Now unless the coil is designed for this higher voltage, even for a short duration pulse, it's going to take a toll on it and probably burn it out a lot quicker than you'd want.

    Now this is just my opinion, but I know the coils I've played with can't take a lot of sustained voltage without winding up with an open coil and a puff of smoke.:eek: You could use a higher voltage transformer, but that will give you the higher voltage (and even higher peak voltage) for as long as you have you finger on the button. This at least will only give you a short spike at peak voltage, then decay rather quickly. Risky at best, again, not from experience, but only in my opinion:wave::wave:
  3. Russ Bellinis

    Russ Bellinis Active Member

    I may be wrong here, but I think the capacitor discharge machines use a high voltage; but it only lasts for the millisecond that it takes to discharge the capacitor. The circuitry won't allow the capacitor to recharge until the button is released. If you try to use a higher voltage transformer to put more voltage to the switch machine instead of using a capacitor discharge circuit, you will probably hold the button on too long and let the smoke out.
  4. ezdays

    ezdays Out AZ way


    Yeah, that's basically what I was saying, that if the switch machine is designed for the high voltage, than this would be the only time one would use it. You could hold the button in and get a full discharge spike, then the voltage would decay to the input voltage until the button is released. I'm not familiar with the specs of the coils on these machines, but they'd better be able to take that spike.

    I guess I gotta ask out of ignorance, why would someone design a component that takes a higher voltage than most other gear? Or are these for larger gauges that use higher voltage equipment?
  5. Russ Bellinis

    Russ Bellinis Active Member

    I'm not sure that the voltage is higher for capacitor discharge machines, just that they will discharge in a millisecond, and then cease to work until the button is released. I think the main advantage of the capacitor discharge machine is that you can't burn out the coil by holding the button down too long. It also eliminates the possibility of a button sticking and burning out the coil. If they do use a higher voltage and are designed for ho, then I would hope the designer knows enough about the machines they are designed to operate that they aren't misleading anyone by selling them for use in ho scale. I think that generally electrical components can take a higher voltage than they are rated at for short periods of time. It is just that the higher the voltage, the shorter the time frame before damage occurs. twin coil switch machines will burn out if you hold the switch on too long with 12 volts. If the voltage is only 6 volts, you might be able to hold the power to the coil much longer.
  6. 60103

    60103 Pooh Bah

    The points for the CDU are that it gives a large kick to the switch machine(s), it won't run on too long and it won't overheat the coil if a button sticks.
    Against: needs time to recharge; won't work with motorized machines (Tortoise & similar).
    We had an incident at a show a few years ago where a switchmachine caught fire (don't know if it was the machine or a bit of layout around it) when something stuck.
  7. ezdays

    ezdays Out AZ way

    Well, I think I gotta look into this a bit more, since I obviously know nothing about switch machines like this.I can see the advantage, but I would think I could accomplish the same thing using a one-shot to trigger a pulse (DC) or a few cycles (AC) and limit the peak voltage to whatever the device is rated for. I'm thinking about doing this when I build my control panel since I have smoked a few Atlas coils :eek: and don't look forward to doing it again.:D
  8. Zman

    Zman Member

    I use Peco turnout motors, and experimented with a CDU. I found that it was totally unnecessary. The higher voltage doesn't do anything at all to improve the performance of the motors. I would recommend getting the Peco passing contact switches that are designed for use with the motors (I think the catalog number is PL 24). These switches are mechanically designed to allow only a momentary contact.

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