45. MRH13-11-Nov2013-L - page 84

DCC Short detection - 1
ike many other model railroaders, I find that trouble-
shooting short circuits on a layout, even a modest one
such as my own, can be one of the most frustrating
things we ever do. The good news is, I discovered that it is pos-
sible to use an inexpensive clamp-on amp meter (ammeter)
and a pair of automotive taillight lamps to rapidly track down
short circuits on a DCC layout by actually measuring the cur-
rent flowing in each wire. (This is yet another reason to switch
to DCC, as this fast and easy troubleshooting method will not
work for conventional DC layouts.)
A few years ago, when our local Charlotte, NC round robin group
met at my house, a gremlin appeared on my Little Mountain and
DCC short circuit detection
Finding those
pesky shorts ...
– Dick Bronson
Model photos by author
1: Schematic of DPDT switch and #1156 lamp wiring.
In one position, the power goes directly from the DCC
booster to the layout power bus lines. In the other po-
sition, the lamps are connected in series with each
bus line to limit the short circuit current to a safe value
while the trouble is tracked down.
Possum Hollow Railroad just as we were ready to run trains.
After an hour of troubleshooting with flashlights, screwdrivers,
and test lamps, we finally located one branch of the DCC power
bus that, when disconnected, would allow the entire main line
to operate once again.
After this disheartening situation, I began to wonder if there
was some easy way to trace the current flowing in a short circuit
without breaking any wires to insert an ammeter. I wondered
if a clamp-on ammeter would measure or indicate the current
flowing through a DCC track circuit. I knew that these meters are
designed to measure normal 60Hz. sine wave household cur-
rent, not the high-frequency square-wave used by DCC. Low-cost
clamp-on ammeters have become available from outlets such as
eBay and Harbor Freight for as little as $10. With such a low ini-
tial investment, I thought it was worth a try.
MRH-Nov 2013
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