Subscriber & Member Login

Login, or register today to interact in our online community, comment on articles, receive our newsletter, manage your account online and more!

What voltage should I expect when decoder set to LED lighting

339 views
6 replies
1 rating 2 rating 3 rating 4 rating 5 rating
  • Member since
    October, 2015
  • 131 posts
What voltage should I expect when decoder set to LED lighting
Posted by passenger1955 on Friday, November 10, 2017 11:59 AM

What voltage should I expect to see on a voltmeter between the 2 leads when the decoder set to LED lighting mode?

Moderator
  • Member since
    June, 2003
  • From: Northeast OH
  • 13,636 posts
Posted by tstage on Friday, November 10, 2017 12:08 PM

Generally, 3mm LEDs are rated at 3V but some are 12V.  So, whatever the LED is rated at, the voltage (plus resistor) should be less than that rating.  And, the lower the voltage, the better for longevity of the bulb.

Tom

http://www.newyorkcentralmodeling.com

Time...It marches on...without ever turning around to see if anyone is even keeping in step.

  • Member since
    October, 2015
  • 131 posts
Posted by passenger1955 on Friday, November 10, 2017 12:12 PM
Great. Thanks.
  • Member since
    August, 2003
  • From: Collinwood, Ohio, USA
  • 4,589 posts
Posted by gmpullman on Friday, November 10, 2017 12:43 PM

Don't assume that because a decoder has an "LED" mode that the output of the function is the correct voltage.

 

In many cases (you really have to dig into the decoder's specific specification) the LED "effect" merely ramps up and ramps down the "on-off" function so that the LED mimics the look more like an incandescent lamp.

 

If there are on-board pads for LED's or traces to jumper, then you might be OK. TCS has a motherboard with solder pads for LEDs.

https://www.tcsdcc.com/Customer_Content/Literature/Motherboards/AS-MB2.pdf

 

For instance, from the ESU Loksound select manual:

LED mode: The LokSound Select’s outputs are pre-set to be used with incandescent bulbs. However, because of their differences in brightness characteristics, some lighting effects will not look prototypical if you use LEDs. If you wire LEDs on the output, you should set the LED compensation mode bit in order to get satisfying characteristics.

It would be helpful to know exactly which decoder you have.

Regards, Ed

  • Member since
    February, 2002
  • From: Reading, PA
  • 22,992 posts
Posted by rrinker on Friday, November 10, 2017 1:33 PM

 If open circuit (no LED connected) and you are using a decent meter with a high input impedence like 10 MOhm or more, and the decoder uses a current source instead of simply including resistors, your voltage reading may be way off. The high impedence of the meter means it puts practically no load on the circuit unde test (usualyl a GOOD thing) but that means a constant current source set for LEDs will eb nowhere near its limit, so the voltage will go as high as it can - I wouldn;t be surprised to see a 12V or more reading. That doesn;t mean you can;t conenct an LED directly. The voltage to an LED doesn;t really matter, it just needs to be more than the junction drop, which for a white LED is about 3.5-3.7V. LEDs ARE sensitive to current, too much and the LED blows up.

 Some meters, like one of the ones I have, have a "Low-Z" volts mode. This is a low impedence input which actually DOES put a noticeable load on the circuit under test - something liek this might give an accurate volt reading froma  constant current source.

 Basically - don't worry about it. If the decoder specifications and instructions say you can hook up an LED directly withotu resistors - you can hook up an LED without resistors. If it says you cna do both, pay careful attention to the wiring diagram. Usually you connect to one terminal for the current limited output and a different location for the full power version of the output. Don't mix them up, or you will either have LEDs that pop like flashbulbs because they were hooke to the fiull current outputs, or you wil have incandescent bulbs that won't light because they are connected to the limited output.

                                  --Randy

 


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

  • Member since
    October, 2006
  • From: Western, MA
  • 7,393 posts
Posted by richg1998 on Friday, November 10, 2017 2:15 PM

My decoders all measured about 12 to maybe 12.5 volts DC. Used 1k resistors.

I do not have any that had LED terminals.

The decoder instructions were very clear.

Rich

N

  • Member since
    April, 2004
  • From: Ontario Canada
  • 3,036 posts
Posted by Mark R. on Friday, November 10, 2017 8:23 PM

The "LED Mode" setting on the decoder has nothing to do with the actual output of the function terminals in terms of voltage / current. It is a setting within the decoder to alter the output for different lighting effects.

Bulbs and LEDs re-act very differently. When using a bulb, the brightness ramps up to full on and dims out to off. An LED is instant on and instant off. To replicate different effects (strobe, mars, etc.) the LED mode allows the output to the LED to re-act more like a filament bulb.

Mark. 

¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ

Subscriber & Member Login

Login, or register today to interact in our online community, comment on articles, receive our newsletter, manage your account online and more!
Popular on ModelRailroader.com
Model Railroader Newsletter See all
Sign up for our FREE e-newsletter and get model railroad news in your inbox!
ADVERTISEMENT
ADVERTISEMENT

Search the Community

Users Online

ADVERTISEMENT
Find us on Facebook