45. MRH13-11-Nov2013-L - page 85

Keeping the current flowing
To measure current, current must be flowing. But a short circuit
trips the DCC booster's protective circuit, shutting off the cur-
rent. I needed a way to keep current flowing through a short
circuit without tripping the booster. The solution was to wire
a #1156 automotive taillight lamp in series with each booster
bus wire. Allan Gartner discusses this in his Wiring For DCC Web
pages. Caution! See
using taillights for short cir-
cuit protection.
I wired a DPDT (Double
Pole Double Throw) switch
such that in one position,
the booster's output is con-
nected directly to the DCC
bus wires for normal rail-
road operation position.
In the other position, each
booster output terminal is
wired in series with a #1156
automotive taillight lamp.
Sure enough, when I recon-
nected the section of my
layout we had isolated dur-
ing the round-robin short
circuit, both lamps lit up.
This indicated to me that the
short circuit was between
the two bus wires, and not
from one wire to ground, or
to another booster district.
2: A typical inexpensive
clamp-on ammeter. This
one is available from Harbor
Freight. We will use a meter
similar to this to see if we
can detect the DCC current
flowing in our layout wiring.
DCC Short detection - 2
3: The yellow arrow in this image points to a nearly
invisible feeder wire connection that caused this entire
section of the layout to short-out during open houses.
The portion of this feeder under the layout had been
covered with a lamp fixture, making it impossible to
see this extraneous feeder from below.
If only one lamp lights, that indicates that the current is flow-
ing from that booster terminal through the short circuit, and is
returning via the system ground wire, not through the other DCC
bus wire.
Initial test
Now we can allow a safe current to flow through our short cir-
cuit long enough to use the clamp-on ammeter to check the
Will it read DCC?
It did! I immediately got a reading of 0.95A when clamping the
meter around either bus connection at the booster. Now I was
MRH-Nov 2013
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