DCC Impulses Column - 8
From Mr. DCC’s workbench
How do I switch more current with my
decoder?
Occasionally there is a need to switch a load that isn’t
compatible with the output of a DCC decoder. Whether this
need is due to the voltage or current needed, or if a contact
closure (or opening) is needed, it can frequently be filled by
using a relay.
A relay is an electro-mechanical device that takes some
electrical current and uses the resulting magnetic field to
move some contacts, just like throwing a switch. There is a
mechanical lag time while the contacts physically move, so
they are not well suited for items that get turned on and off
quickly. Likewise, they don’t do dimming, just on and off, so no
fancy lighting effects, either.
Sounds easy, just connect the relay to the blue and green (or
other function) wire and be done with it, right? Uh, no, not
actually.
Think about the spark coil that generates thousands of volts
to ignite the gasoline in our car engines. They work on the
concept of turning the current on and off through a coil.
Similarly, a relay will generate a large spike of voltage when the
current is turned off. This spike is enough to blow the output
transistor (and, perhaps, more circuitry) inside the decoder if
the relay is hooked up directly to the decoder.
A simple diode will fix the problem. It is put across (in parallel
with) the relay coil in a direction where the normal current will
MRH-Oct 2013
