# How to charge a capacitor in an AC circuit



## Custombuilding4 (Nov 4, 2020)

This is a bit of a shot in the dark. So, long story short, I'm in the process of building my O scale Lionel layout and was given some Lionel 1122 non-derailing turnouts that had been modified for accessory power and included a controller that was supposed to prevent the turnout coil from continually energizing if a train/car was parked on it. The latter function didn't work due to a design flaw I discovered and so I embarked to create my own controller. 

I've attached my wiring diagram. My current problem is that I need the capacitor to act as it would in a DC circuit (where when it releases its charge it blocks any further electrical flow). I was under the impression from my research that adding a diode as I have in the diagram would accomplish this. However, as I was testing it today it did not block flow and the turnout's coil is active as long as it's connected.

I've also considered using a bimetallic circuit breaker (similar to how the 132 Auto Station works) which would replace the capacitor circuit. I haven't been able to find one that would trip after a second with only 1.4 amps across it though.

Any help is appreciated.


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## gunrunnerjohn (Nov 10, 2010)

Try one of these designs: *Capacitor Discharge Power Supplies for Twin Coil Switch Machines*


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## Custombuilding4 (Nov 4, 2020)

Thanks, I gave it a try and made some progress. The diode charges now. But the turnouts won't activate. When I move the neutral wire that grounds the turnout (red) such that it bypasses the diode the turnout will activate, but doesn't shut off and acts like the whole circuit is AC again. Also, when I try to bridge the outside rail to the isolated rail to trigger the no-derail, nothing happens in either of the wiring scenarios above.


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## Custombuilding4 (Nov 4, 2020)

Custombuilding4 said:


> Thanks, I gave it a try and made some progress. The diode charges now. But the turnouts won't activate. When I move the neutral wire that grounds the turnout (red) such that it bypasses the diode the turnout will activate, but doesn't shut off and acts like the whole circuit is AC again. Also, when I try to bridge the outside rail to the isolated rail to trigger the no-derail, nothing happens in either of the wiring scenarios above.
> View attachment 583561


I mean the capacitor charges not the diode.


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## Lemonhawk (Sep 24, 2013)

The voltage at + part of the cap is what is needed to activate the coil on the turnout. The way you're configured your switching the - part of the cap which is at zero volts thru the turnout coil to the zero voltage part of the track. You are at the same potential at either end and the middle of the turnout coils, hence nothing happens.


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## Custombuilding4 (Nov 4, 2020)

So, if I’m understanding right, I’ve basically connected all the -‘s to the coils and I need to change my switching for the caps such that I’m switching its + side between source power and the center of the coil?


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## T-Man (May 16, 2008)

Is this suppose to auto derail with accessory power?


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## Custombuilding4 (Nov 4, 2020)

T-Man said:


> Is this suppose to auto derail with accessory power?


Yes


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## T-Man (May 16, 2008)

Evidently 12 years ago it was a hot subject on another forum. If you had a good diagram you would be all set. I looked and found a few post with scattered bits of info with this as a starting page. Design is not my strong point. I know you tried on your diagram but I get confused on the use of the relay or relays. John did have a link to some good circuits.

If I wanted to park a train on the switch I would disconnect power feed to the coils and control it with a on off switch. Just switch back to on for it to work.


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## Custombuilding4 (Nov 4, 2020)

T-Man said:


> Evidently 12 years ago it was a hot subject on another forum. If you had a good diagram you would be all set. I looked and found a few post with scattered bits of info with this as a starting page. Design is not my strong point. I know you tried on your diagram but I get confused on the use of the relay or relays. John did have a link to some good circuits.
> 
> If I wanted to park a train on the switch I would disconnect power feed to the coils and control it with a on off switch. Just switch back to on for it to work.


Thanks, I’ll give it a look. I’d like it to be fully auto if I can. At some point I have to work on the rest of the layout though.


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## T-Man (May 16, 2008)

This is the switch.









You disconnected 1 and attached wires to 2 and 3


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## Custombuilding4 (Nov 4, 2020)

I did some more experimenting and discovered that the switch is not designed with enough insulation to handle having a DC power source from the control side and AC power running through the tracks to trigger the DC system. Everything works great without the AC, but as soon as I add it (even with diodes) the relays just buzz and the switch won’t operate. I’m pretty sure there’s a voltage ‘leak’ and I’d have to rebuild the switch to get it to work. For me it’s not worth it and I don’t want to mar the original design more than I have already. I’ll be content with a slightly modified control switch and the included no drail system. Thanks for all the info and feedback.


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## gregc (Apr 25, 2015)

i'm confused by your circuit. i can possibly understand the use of the relays for remote operation, but typically the capacitor it connected directly across the coils. the reistor and optional diode when using an AC source, should not be between the capacotor and coil


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## Custombuilding4 (Nov 4, 2020)

I’m trying to use the capacitor as a temporary power supply that blocks power once it is discharged. As far as I am aware this only works when it’s in a DC circuit.


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## gregc (Apr 25, 2015)

Custombuilding4 said:


> I’m trying to use the capacitor as a temporary power supply that blocks power once it is discharged. As far as I am aware this only works when it’s in a DC circuit.


seems contradictory -- a temporary power supply that blocks power.
can you explain this is more sensible terms?

in the capacitive circuit i posted, the capacitor becomes a source of current that is capable of momentarily supplying more current than the power supply is capable of.


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## gregc (Apr 25, 2015)

the circuit i posted the capacitor provides high current to the coil, but it is the resistor that "limits", not blocks power otherwise.

the capacitor in the circuit i posted charges thru a resistor to limit the current drawn from the power supply, otherwise the cap may look like a short.

the capacitive discharge circuit requires time to charge, there's nothing to prevent it from being used before it is charged and of course it won't provide as high a current if it is.

so when a switch is pressed to connect it to a coil, the cap discharges (thru the coil) and afterwards the coil continues to see the current from the power supply thru the resistor. of course this is much lower that the discharge current and of course, the capacitor won't start recharging until the switch is released.


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## Custombuilding4 (Nov 4, 2020)

Power being sent through the coil after capacitor discharge is exactly what I’m trying to avoid. Because if I were to stop a train on the no drail trigger track, constant power could melt the coil.


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## gregc (Apr 25, 2015)

the resistor limits that current so the machine won't melt if the button were held down continuously


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