Snap track with lights

[Jim Banner]

I came up with some numbers for you.  The Bachmann turnout I tried drew .65 amps at 12 volts.  On the other hand, the Atlas Snap Relay drew 3.4 amps or as much as 5 Bachmann turnouts all being activated at once.

To see what some small train set power packs can produce from their ac accessory terminals, I measured the short circuit current of 4 of them.  This is the maximum current a supply can deliver and basically occurs when the output voltage is dropped to zero by the load.

supply                          rating            short circuit current

OLD Bachmann                 8 watts          2.1 Amps ac
trainset pack

Old Lionel                        6 VA              2.4 Amps ac
trainset pack

MRC Tech II                    13 VA              8.5 Amps ac

Bachmann                      18 VA ¹           .34 Amps ac²
44212 pack

notes:
¹ "wall wart" power pack rated 18 volts, 1 amp.
² this modern, electronic pack uses a PTC (Positive Temperature Coefficient thermistor) to provide foldback current limiting.  With foldback current limiting, an overload will cause the thermistor to heat up, reducing the current to a value below its maximum allowed level and will hold the reduced level until the overload is removed.  This is much quicker acting than the thermal circuit breakers used in the other power packs.  The response of the meter used in this testing was too slow to measure the maximum current produced before foldback and the value shown is the reduced level after the PTC current limiter kicked in.


The only one of these packs that would throw the Snap Relay reliably was the MRC Tech II which was able to put 3.7 amps through it while still maintaining its output at 13.5 volts.

Bottom line, the Atlas Snap Relay requires more current than small power packs can produce.

One solution is a larger power pack.

Another solution is a capacitor discharge machine as recommended by Jeffery.  A capacitor discharge machine is like a bucket that is slowly filled with water from a small pipe.  When you need a large gush of water, you the tip the bucket.  In a capacitor discharge machine, a small power pack fills a capacitor with electrons at a slow rate, that is, at a small current.  When you need a lot of electrons at once, that is, a large current, you dump the electrons out in a hurry.  Sounds very complicated but in its simplest form, you need only three electronic components in addition to a small power pack.  One is a bridge rectifier to change the ac from the power pack to dc.  The second is a resistor to limit the current going into a capacitor.  And the third is a capacitor to store the electricity until needed.  Typically, it takes 5 to 10 seconds to charge the capacitor but only 1/10 of a second to discharge it into the switch machine plus Snap Relay.  So the current coming out of the capacitor can be much higher than what goes in.  You do not get any more energy out than what you put in but the power (the energy per amount of time) can be much, much higher.  Cost wise, putting together a capacitor discharge machine is much cheaper than buying a large power pack.  And you only need one for all your switch machines/Snap Relays as long as you are willing to wait that 5 to 10 seconds between throwing switches.  If you are NOT willing to wait, there are somewhat more complex designs that reduce the wait to less than a second.  If this approach seems interesting to you, just let us know and Jeffery or I can walk you through it.

The third solution would be to use latching relays as recommended by Donaldon.  These can do exactly what the Snap Relays do but require only a small operating current to do it.  If you did not already have the Snap Relays, this would definitely be the way to go as the latching relays can be cheaper.  But with the relays already on hand, I believe the capacitor discharge machine would be the cheapest solution.

Jim

p.s. a drawing showing how to use LED's instead of light bulbs will be forth coming.

[wayles3]

Good news---
I upgraded my snap track with lights, I bought (2) more power packs (17 va), took out all of the incandescent lamps replaced them with LED's with resistors, wired it all up, it works great, just what I wanted. 

Thanks for all of your support on this problem.

[Jim Banner]

Good news!  Thanks for letting us all know.