Switch throwing with capacitors

[lionelsoni]

I got the following gratifying message from Jay, which I am posting here so that others who are interested can read my answers:

"After reading your many posts regarding re-wiring switches to run from a capacitor, I decided to try it and it does work great! I was wondering, though, if there is a way to cut down on the amount of time it takes the capacitor to re-charge. Since I am novice at electronics (but no stranger to engineering) I thought I would ask you before I tried to figure it out myself.

"Thanks for all the great posts!
Jay Maria"

First of all, I wonder whether you are using the same component values that I am, since mine recharge pretty quickly. I use a 5000-microfarad capacitor and a 100-ohm resistor, which together have a time constant of .5 second. The recharge is exponential; and the time constant of .5 second is the time for the voltage to recover to 63 pecent of its full value. It recovers to 95 percent in three time constants, or 1.5 seconds, which I find completely tolerable.

The time constant in microseconds is the product of the resistance in ohms and the capacitance in microfarads. So you can shorten it by reducing either or both of these values. I use 5000 microfarads mostly because I have a big box of these that I got for free years ago. I have successfully operated two switch machines from one of these; so it is likely you could get away with cutting the capacitance, and therefore the charging time, in half.

The resistance determines not only the recharge time but also the steady current that flows in the coil when a train is parked on the switch. This is, for example, 160 milliamperes at 16 volts with a 100-ohm resistor. You could experiment with a smaller resistance to see whether the coil heating is tolerable; but I haven't done that myself. Reducing the resistance should be combined with re-evaluating the resistor's power rating. The power that it dissipates is the square of the voltage divided by the resistance. It is wise to use a resistor with two or three times that rating. I use 10-watt resistors, although the power dissipated is only 2.5 watts.

I have seen some rather elaborate circuits intended to replace the resistor, to recharge the capacitor faster. I don't think they are worth the trouble.

There is another slightly more complicated circuit that keeps the current completely away from the coil when the switch is occupied. In this scheme, you wire the common of the coils directly to the dc supply and put a capacitor in series with each coil, between the coil and the control rail of the switch. The resistor goes between the control rail and the dc supply. You also have to add a diode in parallel with the coil. In this circuit, the resistance can be as small as you want, as long as the resistor and the power supply can handle the power involved, since the resistor's current does not flow through the coil. So you can get a very short time constant. However, you obviously need more components: twice as many resistors and capacitors, and two diodes per switch.

In either of these schemes, you could replace the resistor by an incandescent lamp to get some improvement in recharge time.

[FJ and G]

Some useful info, Bob,

Now how do I get my Tortoise Switch Machine to speed up? :-)

[lionelsoni]

I don't know; but, in any case, trying to speed up a Tortoise is a hare-brained scheme.

[underworld]

Dave,
Try hooking it up to 110AC!!!!! [:p][:p][:p][:p][:p]

underworld

[:D][:D][:D][:D][:D]

[FJ and G]

BB the beagle is fond of retrieving box turtles out of nowhere. I keep her away from my tortoise switches.

Really tho, I plan to lengthen the control rail to compensate for its lack of speed on turnouts.

[Anonymous]

Thanks Lionelsoni,

Great response. I begin to understand what's really going on here. My accessory power supply is the constant 19.2 volts measured across the D and U posts from my KW transformer. I can't measure it at the moment, but I think the voltage (in DC) across the N4001 diode is something like 9v. This in turn feeds my 100 ohm 10w resistor, and on into the 4700uf 25v capacitor. Does this sound correct to you? I am guessing about the recharge time, but I think it takes a good 5 count for the thing to be snappy again after discharge. Any comments or things that I can check?

Thanks again,
Jay

[lionelsoni]

Jay, that's a lot more voltage than I use. If I understand your situation, you have a 1N4001 between a 19.2-volt-rms supply and your 100-ohm resistor. This should charge the capacitor to about 26 volts, slightly under the peak voltage from the transformer. (I wouldn't worry about the slight overvoltage on the capacitor, by the way. They are very tolerant of that.)

Charging from a rectifier is a little different than charging from straight dc supply, but not that much. It is fast at first, then slower as you approach full charge. But, with that much voltage, I would expect good operation in a short time, if not full voltage. However, you could create a pretty good 26-volt dc charging supply simply by putting another capacitor between the diode's cathode and ground. It could be as small as about 470 microfarads, but you might just use the same 4700 as for the switches. One charging supply should suffice for any number of switches.