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LED Calculations

lehighlinelehighlineOGR Forum Member DIGITAL SUBSCRIBER
Would someone please check my math and confirm that I have not slipped a decimal point or worse.

I have a alterating flashing circuit that I purchased from Dallee. It is rated for 8-20 vac. I plan to power it with my Z4K 14 vac acc terminals. I bought T1 red LEDs from Dallee to use in the circuit. The package says they are rated at 30 ma max, and have a 1.85Vfwd drop.

I come up with 405 ohm resistors for each LED. I think the closest I have is 470ohms. Does this make sense? If I drive the LEDs in parallel, 2 on each side of the flasher, is 470ohms still correct? i.e. 2LEDs on one side of the flasher in parallel, and 2 LEDS on the other side of the flasher in parallel.

Chris
LVHR
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rrmanrrmanOGR Forum Member
And to add on to gunrunerjohn, small resistance differences won't affect light output significantly. So using a 470 or 680 changes the current somewhat, but unless you have a calibrated photometer and measure light exactly the same, the light change would be un noticeable to the eye, and certainly not in train applications! Wink

And two LEDs in paralell is not good, as one will "hog" the current so it lights brightly and other less bright or not at all.
lehighlinelehighlineOGR Forum Member DIGITAL SUBSCRIBER
Guys,

Thanks for the help. I wired things up yesterday using 680 ohms resistors. Worked like a charm!

One question, though. The Dallee flasher circuit I am using appears to be for cross bucks, which is fine. However, I'd like the LEDs to decay a little before they wink out. I'm assuming a small cap on each input might help here. Would this work? Any idea on where to start?

Chris
LVHR
GE Gary EOGR Forum Member
First - You don't have to run an LED at it's rated current. Try larger resistors to see what the light output looks like. I designed a board for one of the train manufacturers that has a small T1 3/4 red LED power on indicator. The LED uses 5 volts and has a 2200 ohm resistor. That's only 2 ma! But you can see it across the room. One large layout has over a hundred of these boards so they're saving almost 3 amps just for the power on indicators.
Second - Go with Custom Signals GCF - Grade Crossing Flasher board. It has the wink out plus many other features.
www.customsignals.com
S stan2004OGR Forum Member
quote:
Originally posted by lehighline:
I'd like the LEDs to decay a little before they wink out. I'm assuming a small cap on each input might help here. Would this work? Any idea on where to start?


Here's a start. Based on your description, I assume each side is as shown. Add a resistor and capacitor.



The larger the capacitor value, the longer the simulated "filament" decay. But there is a sweet spot. 10uF is likely too small and will have no visible effect, while 1000uF is likely too large and the LEDs may not fully extinguish on each cycle.

The 100 ohm resistor is probably not needed but I have no knowledge of the Dallee circuit and this limits the current surge each time the capacitor is charged. Some blinking circuits may choke.

If you need exact part numbers just ask. And if Radio Shack is your preferred supplier, please state such.
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