I understand that if you drive a loco onto a turntable, turn the loco 180 degrees and drive it back off onto the same track you’re going to get a short. I’ve heard that by using a DPDT switch this can be prevented. If this is so, how do you wire the switch into the turntable and how is it this works?
Thanks,
JaRRell
If you’re running DCC Digitrax makes an auto reversing unit just for this.
if you have DC you wire the DPDT like this
Wires:
R1 - Red wire from throttle
B1 - Black wire from throttle
R2 - Red wire from DPDT to TT
B2 - Black wire from DPDT to TT
DPDT Switch:
T1 0 0 T2
T3 0 0 T4
T5 0 0 T6
you would wire the DPDT like this:
JaRRell, there are six posts under the switch. The centre ones are for the rails on the TT bridge. Doesn’t matter which post to which rail, just that there should be one post wired to one rail.
Your power from your DCC system bus will enter the switch on one end, again pick one pair. The two posts that are left, on the other end, will be wired in reverse to the other end posts to which your DCC power comes. Or, instead, you can splice into the wires leading to the first set of end posts somewhere along their lenghts, but just be sure to reverse them. Do NOT wire them so that left end is wired to left other end. Cross over to the opposite wire of the incoming current.
Thank you Dan and Philip. I apolgise for not stating I use Digitrax DCC, … Dan, I appreciate you taking the time to write that out, there’ll probably be 5 or 6 other viewers of the thread, DC users, that will benefit from it, and I find it interesting also.
Philip, I check the Digitrax link out. Thanks.
JaRRell
It also depends on the turntable. Some turntables supply power to the bridge rails via a split rail around the ring - when you turn 180 degrees this automatically reverses the polarity. The BEST way with DCC though is if the bridge track power is continuous, and us an autoreverser to handle the polarity. That way when you turn that nice sound-equipped steamer, the sound won’t momentarily cut out as the bridge swings past the gaps in the split ring rail. And those gaps typically are relatively large because if the metal pickup wheel or wiper contacted both halves of the split rail it would create a short. So the dead section is enough to cause sound decoders to restart.
–Randy
Ok, so you CAN use a DPDT switch as a manual method of reversing track polarity while a dcc equipped engine is on the turntable and the engine is selected by the throttle and is on, and it has no effect on the engine and does no damage. Just wire the DPDT switch in a cross wired fashion, not straight through.
OR… buy a Digitrax reversing thingamajig and it’ll be done automatically.
Have I got it right?
JaRRell
yep.
the digitrax would be better in that you couldn’t forget about the polarity and torch a decoder, though with DC, forgetting wouldn’t be THAT big of an issue (the powerpack would throw its breaker before anything has a chance to go too bad… don’t ask how I know this…)
You know, every time I read something like this it reintrenches DC into my psyche. The idea of torching anything, or using anything other than a 99 cent DPDT switch never entered my mind.
You shouldn’t have to fear the decoder frying for the same reason you don’t normally fear an open frame DC motor frying…circuit breakers. Every DCC system has them, or the populace that would like to pay for their use would quickly insist that manufacturers include them…which they have, long since.
To me, lining up blocks of switches vs. a silent digital switch on a reverse loop is not even close. Four solders, $40 for the PSRev, and grab that throttle for fun.
You know, the Digitrax AR-1 actually works way better than I ever thought it would. I’m still curious to hear a sound loco go over one but on a friend’s layout he has a couple, he scratchbuilds small N scale steamers, camelbacks and otherwise, they they crawl insanely slow. Makes a lot of my HO stuff look bad. Of course he does run them with $40+ Keughn and Zimo decoders… but even at 1 tie per minute crawl speeds they don’t hesistate in the east crossing the gaps at a reverseing section, there’s just the gentle click of the AR-1 and it keeps going.
I still prefer the idea of a solid-state switch like the Tony’s though.
–Randy
BTW, if you’re going to have more then two reversing areas on your DCC layout, a more cost efficiant way to do it is this unit. I have one and it works great!