Can a capacitor be too big for a relay?

That would depend on the circuit.  Obviously, it'll take a healthy current surge to initially charge that cap.  I'd probably have a very low resistance in series with the cap to limit the maximum inrush current.  Again, some circuits won't be phased by a large capacitive load, I'd have to know more about what it's connected to.

should be OK. That much inrush current could drive a modern transformer a bit nuts if it has a sensitive breaker (K-Line power chief for example) but a standard breaker no problem. There is a slight possibility that you could tickle the contacts but train relays are not used under severe conditions usually. If there is a problem put in a small value resistor as John advised. The relays I use I put up to 10,000 uf on them. The relays I use have a 480 ohm 12VDC coils and  DPDT 15 amp contacts. The time delay is about 1 second per 1000uf of capacitance in practice so I actually use them as crude timers. For example using 4 relays and the insulated rail method I make such a circuit where the whistle is automatically triggered as the train approaches a crossing. The whistle sounds long,long,short ,and then long as the train passes through the crossing.  The amount of time lag depends on the relay used. This crude timing method is actually more realistic as it does not always ring exactly the same.

Normally just to keep a relay from chattering 470 uf usually works well.  If you want long durations you could use a timer module or 2 relays in a latch-unlatch circuit.

Dale H

Dale and John,

I guess since I'm only feeding 14 or 15 volts to the circuit I should be all right.  Is it worth putting any low level resistance in ahead of the capacitor anyway?

Bob

Is the relay with capacitor being energized by wheels on a section of insulated track?  If so, there might be an arc when the wheel first makes contact to charge the capacitor.  I would recommend a few ohms of resistance.  This might induce a very short delay on the charge-up time, but probably not a significant amount compared to the charge decay time.

I'm assuming you're using DC to charge the cap right?  AC won't do the trick.

John/Dale,

Yes I'm using DC to charge the capacitor. Is a 470 Ohm resistor adequate and does it go between the rectifier and the cap or between the cap and the relay?

Bob

470 is probably way too high a value.  I'd be thinking more like something around 47 ohms max.  If it is fast enough, that would be great, if not lower the value.

John,

The shack sells a 33 ohm resistor - I'll try that.  I assume it is on the power (hot) side between the rectifier and the capacitor. Am I correct?

Bob

10 to 33 ohms is fine.  Just need a little bit of resistance so the wheels do not spark. try it small you can always add by putting 2 in seres.

Dale H

The resistor needs to be anywhere in the circuit.

Thanks for all of your help John.

Bob

Anywhere in series on the input side to the capacitor, but not in series with the output/relay.

Thanks Dale.  I'll wire it in between the rectifier and the capacitor.

Bob

Well yes Dale, I guess I assumed that was a given.

Well gentlemen, I have failed in my efforts.  I have been able to hook up the #415 Lionel signal bridge using 14vac and a MTR-12T relay from Scott's Odds & Ends.  It works perfectly.  But of course as soon as the train leaves the isolated track section the lights change back.  Starting over - can either of you describe a circuit that will allow me to delay the light switch back for even a few more seconds after the train exits the block? I can only assume that I have hooked up the capacitor and the small initial resistor the wrong way. But now I'm stumped.

Bob

Not many ways to do it wrong, maybe the relay just consumes too much power in the coil to hold in for any length of time after the power is removed.

If the capacitor is backwards it would explode.  It should be hooked directly across the relay coil in proper polarity. 6800 uf if charged should hold the relay for at least 5 seconds  if it charges .  Not sure of the circuit used by the relay in question. If there is a bridge rectifier in front of the coil it should not bleed back.  You can use an additional relay and timer module and delay it as long as you wish. 

Dale H

Should the control wire from the isolated track connect directly to the relay or to the bridge rectifier - maybe that's the problem.

B

To one of the bridge terminals marked ~

Dale H