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Turntable Reverse Polarity
Tony models OO and asks:
“Having got my Arduino-controlled Turntable working at last, will I have to reverse the polarity of the complete railway setup when the engine does a 180-degree turn on the turntable? And, if so, how do I do it? Ta in advance for your help.”
4 Responses to Turntable Reverse Polarity
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I have had the same query.. I have been advised to get a SPDT switch to manually change the polarity.
You would need a DOUBLE pole double throw switch (DPDT), a SIngle pole double throw (SPDT) isn’t going to work. If you have DCC sound, there will be a slight pause as you throw the switch (unless you have a stay alive), but if you have DCC then use an auto reversing switch.
I’m guessing you have scratch built the turntable? (in which case, well done!) All commercially available turntables and kits that I have seen have a built in way to reverse the polarity as it turns, usually by having the power supplied through a set of contacts that run over two fixed strips, each strip going only halfway around, so that when the table is turned the contacts “change strips” and thus change polarity.
If you have DC, yes, forwards becomes reverse if the loco is turned, but this is just a matter of using the controller direction switch (which is a DPDT switch) anyway, so there are no wiring or electrical problems. (The loco goes forward when the right hand wheels get positive voltage.)
If the system is DCC, polarity is irrelevant, as the track voltage is AC and the loco goes in the direction is is told by the commands sent to it.
Electrical polarity (DC or DCC) switching issues only occur when you have triangles or balloon loops in your track.
There is a slight inaccuracy in one of the answers above.
For DC, as correctly stated, the solutions are either a DPDT switch or the contacts built in to some commercially available turntables which basically have two semicircular static tracks onto which spring loaded contacts attached to the turntable bridge make contact. When the turntable gets halfway round, contact is momentarily cut and then reversed as the spring loaded contacts connect with the opposite semicircular track.
For DCC, the polarity still needs to be switched to avoid a short circuit when part of the locomotive is on the turntable bridge and part on the surrounding track. Again, a DPDT switch can be used for this but it will affect sound if fitted. A better solution is to use a reverse loop module or two frog juicers between the main power bus and the turntable bridge which will deal with it automatically without interrupting sound.