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Help with Circuitron FL-3 flasher

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  • Member since
    February 2008
  • From: Osage City, Kansas
  • 94 posts
Help with Circuitron FL-3 flasher
Posted by MOPACnut on Thursday, August 29, 2019 8:32 AM

 I'm planning on using an FL-3 to flash a set of red LEDs (each rated 1.8-2.0 volt, 20ma) with 6 on each side. If my calculations are correct, each side would consume 240ma, which is just below the rating of 250ma for each of the 3 outputs on the module. Would it be possible to connect 2 or all 3 of the output terminals together so the load is shared by all of them? Or will one do the trick? The instructions only mention connecting the control terminals together when using it as a heavy duty flasher.

 And for the resistor: A LED resistor calculator I use online says for a 12 volt supply I'd need a 100 ohm, 2 watt resistor for the LEDs. Does that sound right?

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  • Member since
    December 2004
  • From: Bedford, MA, USA
  • 18,774 posts
Posted by MisterBeasley on Thursday, August 29, 2019 10:05 AM

Depending on the LEDs, running them at full power will probably be too bright, so I would start with 1000 ohm resistors.  Resistors are cheaper than LEDs.

It takes an iron man to play with a toy iron horse. 

  • Member since
    February 2002
  • From: Reading, PA
  • 27,332 posts
Posted by rrinker on Thursday, August 29, 2019 10:21 AM

 Not that the info in the catlog is detailed, but I don't read that as you hook the outputs together. There's still a 250ma limit per side of each of the 3 outputs, BUT if you connect all 3 control lines together, then all 3 outputs flash together, so you have 3 sets at 250ma each, or the stated 750ma total capacity.

 Your LEDs won't use anywhere near 240ma. Use a larger value resistor. The 20ma rating for the LED is an absolut emaximum you can supply withotu blowing up the LED. In actual operation, calculate a resistor for around 10ma per LED. 

 How are you planning to wire them up? 6 in parallel for one side, and another 6 in parallel for the other side? The FL3 is an alternating flasher, so at most, only half the LEDs would be lit up, so no more than 6. If running at the maximum current of 20ma, that's only 120ma per side, but you should run them at less. With a resistor ON EACH LED for a limit of around 10ma, you would need a 1K 1/4 watt resistor. 6 LEDs at 10ma is only 60ma, not even close to the 250ma limit.

 In fact, unless you really need flashers for 3 different locations all you need is the FL2. If indeed you are using the FL3 to set up 3 sets of flashers, with 2 LEDs per side per location (ie, 3 grade crassings, with a flashing crossbuck facing each direction), then that's only 2 LEDs per output per side, with a 1K resistor on each one, that's a mere 20ma per output. 

 You can theoretically get away with a single resistor per pair of LEDs - to work with the FL2/FL3, the pairs of LEDs need to be wired common anode. The common connection goes to the output, one LED goes to the H, and the other LED goes to the L. One resistor on the common line would work - but still, one resistor per pair of LEDs, not one resistor for entire strings.


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