Engine #12, George Town Loop RR
Using LEDs with DCC decoders!
We prefer to use LEDs when we install a decoder. Why? Several reasons:
- Color - they are now avilable in Golden White and Sunny White colors, which are very close the color shown in the photo to the left. Gone are the days of "blue-white" LEDs!
- Life - the life span of a full brightness LED is about 5000 hours - that is 2-1/2 years of 8 hours a day, 5 days a week! If you are willing to have only 80% brightness - still VERY bright - the life goes up about ten time (25 years).
- Low power - very little power is consumed from your DCC set
- Cool operation - they use very little power, so they don't get hot!
What sizes are available?
Common sizes are:
- T-1 or 3 mm - HO or N scale headlights
- T-1-3/4 or 5 mm - HO and larger headlights
- SMD (surface mount) about 0.06 by 0.03 inches - Z headlights and ditchlights and other effects in N and larger scales
How do I connect them?
The answer is very simple, but, if you do it wrong, they won't light or will give you a splendid light show for a few seconds, sometimes taking the driver transistor in the decoder south with them.
Current Limitation
LEDs have a fixed voltage drop, based on their color. White LEDs have a voltage drop of about 4 volts. Since the output of your decoder is track voltage (about 12 to 15 volts), you need to provide a "dropping resistor" to absorb the extra voltage. No fancy calculations are required, though. If you want the brightest light and are willing to settle for the shorter life span (about 5000 power on hours), select a 750 ohm 1/8 watt resistor. If you are happy with less light than the maximum but want the longest life, select a 1000 ohm (1.0 kOhm) or larger resistor.
While the LED is polarity sensitive, the resistor is not. It can be placed in either direction connected to either lead of the LED. Bruce prefers to put the resistor between the blue lead and the LED, as this provides the greatest margin of safety for the decoder, in the event of a short.
If you leave out the resistor, the LED's life is measured in seconds, but the light show is amazing! If the LED fails by shorting out, you will probably toast the transistor in the decoder, too!
Polarity
LEDs are sensitive to the polarity of the applied voltage. The longest lead as they come from the factory, or the "straight" lead as shown in this drawing, goes to the positive voltage supply (usually the blue lead). If you connect them backwards, they will NOT LIGHT, but no harm will come to them!
The other (shorter) lead goes to the function wire (white, yellow, etc.)
REMEMBER ONE of the leads needs a resistor in it.
What about using LEDs with
SoundTraxx' LC series decoders?
in t he LC data, it says that LEDs are not recommended. Here's why - the rear bulb, when off, will occasionally glow slightly, due to a design issue. The decoder was designed before white LEDs were commonplace and the decoder occasionally sends a few quick pulses of current to the rear bulb. These pulses are too fast to light an incandescent bulb, but the (faster) LED will glow slightly.
By adding a few components to the lighting circuit, LEDs CAN be used.
Click here for more information.