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Here is a handy reference chart for calculating the resistors needed for any light bulb and any voltage. See the picture "series" at left to show how to connect resistors in "series" with the bulbs. Note that most bulbs have a current and voltage rating on the package when you buy them. YOU CAN NOT CALCULATE THE CURRENT BASED ON THE COLD RESISTANCE OF AN INCANDESCENT LIGHT BULB. You must have a variable voltage supply and measure the bulb's current at rated voltage. For GOR (grain of rice) bulbs, this varies greatly because of the size of the filament. The following is a "typical" characteristic of an LED which is 2volts and 20 ma (milliamperes/milliamps). Note that the typical resistor for HO/DCC which is 14 volts RMS (root mean square...don't ask!) is 620 recommended. We often use 470 and this is not ideal but ok because the current through the circuit is really limited somewhat by the LED which is considered a constant current source. At 20 ma with E=IR, E or Volts=.02*470=9.4 volts reduced from the supply by the resistor but since the LED is also a relatively constant voltage also, I=E/R or I=(14-2)/470 or 0.0255 amps or 25.5 ma. This doesn't hurt the LED but will shorten it's life. Note that bright white or super white/yellow LED's are usually 3.4v and 20 to 40 ma. Look on the package. If there are no details then MEASURE FIRST!
Here's what we use now on HO and DCC. Other scales, other voltages (accessory) will vary the resistors used.
You can calculate your own using the following spreadsheet (note: because of licensing restrictions by Microsoft this will only work if you own a current version of MS Excel on your desktop).
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Contact us at 919-562-4378 or e-mail:info@trainbuddy.com (c) Copyright 2002-2008, K.A Parker & Associates, Inc. All rights reserved This page was last updated: 2008-07-26 |
