How Do I Switch "High-Current"
Devices With My Decoder?
Frequently folks want to connect an item which draws a lot of current (like a smoke generator) to their DCC decoder so that they can turn it on and off with their DCC set. Or they want to control some item on their train which needs a switch, not a voltage.
Using A Relay
What is a Relay?
It is an electromechanical switch. Current passing through a coil moves
a switch.
Can I Just Connect
a Relay to a Function Output?
Unfortunately, no. There are two considerations: what voltage is the
coil designed for and how to keep the relay from killing the decoder.
Let's take them one at a time.
Relay Coil Voltage:
Many
DCC sets run with 12 to 16 volts on the track. That puts 11 to 15 volts
on the function leads. This is perfect for relays with 12 volt coils.
Alas, they are frequently large and the coils may draw as much power
as the smoke generator, or what ever. Small, low power consumption relays
frequently have a 5 volt coil. If you connect this relay directly to
the function output, it will fry either the decoder, or the relay or
both! You need a dropping resistor. What value, you ask. The quick way
is to measure the resistance of the coil with your ohm meter. What ever
your ohm meter reads for the coil resistance of your 5 volt relay, just
double it for the resistor value and select the next SMALLER standard
value. For example, you read 425 ohms on your relay. Twice that is
850 ohms. The next smaller resistor is 810 ohms. For any resistor larger
than 470 ohms with 16 track volts, 1/8 watt or higher will suffice.
What Resistor Should I Use?
If you don't have an ohm
meter, you can calculate the resistor you need. First you need to figure
out how much voltage the resistor needs to drop. Subtract the relay coil
voltage from your track voltage:
With a 5 volt relay and 15 volts on the track, the resistor must drop
10 volts.
Then calculate the resistor value by dividing the volts dropped by the
current rating of the relay coil:
A 100 mA coil (0.1 amp) 5 volt relay would need:
10 volts / 0.1 amp or 1000 ohms,
which just happens to be
a standard resistor value.
How do I Protect the Decoder?
When a relay is de-energized, the coil releases a HUGE spike of voltage
(remember spark coils and points in your car - same technology - gets
thousands of volts from a 12 volt car battery). Without protection,
this spike will KILL your decoder. Protection is in the form of a diode
across the relay coil. Some relays have these diodes built it. Some
require that you add them. The + terminal of a relay with the integral
diode, or the band of on the diode, goes to the blue terminal.
NOTE: Make certain that the relay coil current rating is less than the decoder function maximum rating or you will damage the decoder.