Tuesday, September 21, 2010

Resistor Adjustment for SMD Signals

I adjusted the resistors for the SMD signals.  Found out from Serge that the installed resistors were 1000 (green) and 1500 (red) ohms.  After deciding that the result was just too dim, I removed the resistors and just went with the 100 ohm resistors that come with the Digitrax TSMK connectors.  This was way too bright and probably would have greatly shorten the life of the LEDs.  So I used 250 and 320 ohm which put the LEDs at the same brightness as the rest of my Sunrise signals.  Below are a comparison between 1000/1500 and 250/320.

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4 comments:

  1. SputnikSeptember 22, 2010 at 9:19 AM

    Before you go and buy lotsa resistors, you should get the LED's ratings from Serge (volts and milliamps), and do some quick math with Ohm's law, just too make sure that you aren't going to cook the LEDs with even these resistor values.

    ---jps

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  2. genevasubSeptember 22, 2010 at 11:02 AM

    Excellent point! I checked the Digitrax website and found out that the SE8c puts out 5 volts. Serge informed me that the LEDs need to be below 20 milliamps. With the 100 ohms build into the TSMK connectors, the total risistance is 350 for green and 420 for red. Using Ohms law this calculates to 14 milliamps for green and 12 milliamps for red. So, I am okay. When I connected the signals without any additional resistor, just using the built in 100 ohms, the milliamps calculated to 50 milliamps. Not okay! I may try upping the resistance a bit yet as they are just a tiny bit on the bright side. Right now I am waiting on a new roll of ribbonwire which I hope will be in this weekend.
    Daryl

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  3. SputnikSeptember 23, 2010 at 8:27 AM

    Right on, Brother!

    BTW, love the backdrops too!

    ---jps

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  4. UnknownNovember 4, 2015 at 6:17 PM

    I have been building my HO signals for a couple of years and found out that using 12 vac works good and you do not need to worry about polarity except I have been using a general use diode either 4001, 4002, 4003 or 4004 and you must have the polarity stripe end of the diode connected to the return terminal of the power source. It appears the diode which I have been using are 1/2 watt size and seem to help with reducing the current flow but does not effect the brightness of the LED. That is using 1/2 watt 460 ohm resistor which will come with some axial type LEDS.
    The diode number 4001 - 4004 I found at Radio Shack and not sure where else since many Radio Shacks have gone out of business.

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