Topics

reverse loop

starvingartist49 <starvingartist49@...>
 

I am new to the list. My layout is still basically at the end of the
design phase although I have purchased some materials and started
construction. I plan to use DCC although I have little experience
with it. I am hoping to learn from this list so that by the time I
get around to wiring I will have a basic understanding of how things
work.
I thought I was "getting it" but a recent post left me rather
confused. It was my understanding that one of the benefits of DCC is
that the decoder will figure out the polarity of the track on its
own.
For example say I am going around a loop and the left rail
changes from positive to negative after an insulated joiner. Doesn't
the decoder adjust for this automatically to keep the locomotive
moving forward?
I guess I don't understand the need for a reverser unit. I
thought this was a feature of the decoder unit.
Perhaps I misunderstood the question which may have been posted
before I joined the list.
I would appreciate it if anyone could clear this up for me.
Thank you,
Jawno

Earl T. Hackett <hackete1@...>
 

The DCC signal is a square wave with equal + and - voltage cycles. The decoder doesn't care about the polarity, or more correctly the phase of the signal. The boosters however have sensing circuitry that will detect a short, even if it lasts only a microsecond, and shut down. There's enough power in most boosters that without short detection they could start a fire. That's why there is frequent talk about 'DCC friendly' turnouts that have rails electrified in a way than minimizes the chance of a short, auto reversing boosters that switch phase quicker than the power booster can sense a short, and various other schemes to handle reverse loops, etc.

----- Original Message -----
From: starvingartist49
To: WiringForDCC@...
Sent: Saturday, January 21, 2006 1:15 PM
Subject: [WiringForDCC] reverse loop


I am new to the list. My layout is still basically at the end of the
design phase although I have purchased some materials and started
construction. I plan to use DCC although I have little experience
with it. I am hoping to learn from this list so that by the time I
get around to wiring I will have a basic understanding of how things
work.
I thought I was "getting it" but a recent post left me rather
confused. It was my understanding that one of the benefits of DCC is
that the decoder will figure out the polarity of the track on its
own.
For example say I am going around a loop and the left rail
changes from positive to negative after an insulated joiner. Doesn't
the decoder adjust for this automatically to keep the locomotive
moving forward?
I guess I don't understand the need for a reverser unit. I
thought this was a feature of the decoder unit.
Perhaps I misunderstood the question which may have been posted
before I joined the list.
I would appreciate it if anyone could clear this up for me.
Thank you,
Jawno







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Doug Stuard <dstuard@...>
 

--- In WiringForDCC@..., "starvingartist49"
<starvingartist49@g...> wrote:

For example say I am going around a loop and the left rail
changes from positive to negative after an insulated joiner. Doesn't
the decoder adjust for this automatically to keep the locomotive
moving forward?
I guess I don't understand the need for a reverser unit. I
thought this was a feature of the decoder unit.

You are almost right. Once over the joint, the loco will keep going,
except that as the loco crosses the gap, the wheels will bridge the
gap and create a short. What the automatic reverser does is detect
this short and, rather than disconnecting the load as a normal circuit
breaker would, it reverses the phase (polarity) in the loop, right
under the loco. The loco (or more correctly, the decoder) is
oblivious to this, but the short is thus removed and the train
continues forward. The same happens again as the loco exits the
reverse loop if the polarity/phase across the exit gap is again
mismatched.

Note that if there is a REAL short that is not corrected by phase
reversal, you could be in trouble without a actul circuit breaker.
That's why the current detection point in the reversers is less than
the trip point of the booster.