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LEDs and resistors

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  • Member since
    July, 2017
  • 9 posts
LEDs and resistors
Posted by dbhone on Thursday, September 07, 2017 10:08 AM

I am putting surface mount LEDs in the head and tender lights of an old HO Bowser die cast 2-8-0 and am used to using #603s, but not sure what surface mount resistors to use.  I am running straight DC 6-14V and want bright but not too bright.  Will wire them for directional lighting which I've done with a couple of diesels.

Can anyone give me the resistor numbers to get? And wattage?

Thanks, Dan

  • Member since
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  • From: Reading, PA
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Posted by rrinker on Thursday, September 07, 2017 10:48 AM

 SMD resistors come in all sorts of sizes, like the LEDs. Any should work - wattage you can calculate. For regualr LEDs, a 1K resistor is usually the starting point, but the SMD LEDs are usually much brighter and can use significantly larger resistors and still be plenty bright. If you have detailed specs on the LED you cna claculate it all - but the LED spec is usually something like lumens per watt and unless you work with light measurement (I don't), the lumen value will be next to meaningless. Instead I would just wire up an LED and use some regular leaded resistors and see what value generates a pleasing level of light. Running on DC, it's iikely that there is no resistor that will be ideal across the entre voltage range. Nothing less than 1K, that will keep the current under the limit at full speed, but will probably be too bright. 

 You need to know the forward drop of your LEDs to calculate anything (Vfwd in the data sheet, typically). 3.5V is common for white LEDs but CHECK, don;t assume, and substitute the actual value in the equations below:

At full throttle, 14V DC. Subtract the LED voltage, 3.5.  10.5V left over

You want 10ma or less. Probably much less, but most any 603 LED can handle 10ma without popping. Lower current is less bright. 10ma is 0.010 amps. 

So, the resistor value is the voltage divided by the current, in this case 10.5/0.010 or 1050 ohms. About 1K.

Now, if you take the 1K resistor and slow the loco down to 4 volts, now you have 4 volts minus the 3.5 volts of the LED, or just .5 volt. Current is the voltage divided by the resistence, so .5 volts/1000 ohms or 0.0005 amps, 0.5 ma. The LED may still light, but it won't be very bright. All you can do is tweak the resistor value, you can go slightly lower than 1K and still be ok at the top end, but even a 1K will be nice and bright. You can (maybe) aso make the assumption that you might not run much at full throttle and go down to as low as a 560 ohm resistor, at the full 14V that would put a bit over 19ma through the LED which may still be safe - even the little ones can usually handle 20ma peak. But it will certainly be very bright. The difference is, at slow speed it will be nearly 1ma, which might be bright enough.

 Really - the best way to figure this out is to just test, just respect the maximum limits of the LED.

 There's an alternative way using a vltage regulator to provide a constant current over a wide voltage range, the problem is, such a circuit consumes nearly 3V, so in addition to the 3.5V of the LED, it wouldn't even light up until you have over 6.5V on the rails. However, it would light at the configured brightness and then stay there all the way to full throttle.

                                       --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
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Posted by richg1998 on Thursday, September 07, 2017 10:58 AM

Most LED installations, I have used 1k, 1/4 watt resistors.

For some LED's, I used a 2.5 k pot set at max resistance and lowered the pot resistance until I saw the brightness I preferred. Measure resistance with ohmmeter and selected close standard resistor out of the junk box.

With super bright LED’s a 5k pot might be needed.

Rich

N

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Posted by BATMAN on Thursday, September 07, 2017 11:03 AM

Here is a handy dandy little wiz you might find helpful.

http://led.linear1.org/led.wiz 

 

Brent

It's not the age honey, it's the mileage.

https://www.youtube.com/user/BATTRAIN1

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Posted by shahomy on Thursday, September 07, 2017 1:26 PM

are you soldering to a board? if not, maybe use resistor with leads?(trimmable)

Probably about a 680 ohm 1/4watt

14v-2v(for led)=12v

current = volts/resistance...so 12/680 = 17.6 ma

power = current x voltage...so.0176 x 12 = 211mWatts

do you know the current ratings(max) for your LED?

it may need more current to make it bright enough for you

Am i ever gonna be able to lay any track???

  • Member since
    September, 2013
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Posted by shahomy on Thursday, September 07, 2017 1:54 PM

Like Mr Rinker said, constant current source would be best...i did a quick look...if your led can handle 20ma constant this looks ideal http://www.mouser.com/ProductDetail/Microchip-Technology/CL2N3-G/?qs=sGAEpiMZZMsE420DPIasPsmbGP%2fDsdbv7CfbaiDKVaw%3d

2 terminal device, like a resistor, looks like transistor(3rd leg/middle is no connection) 5-90vdc input constant 20ma out

have no personal experience with these(yet) but i`m gonna have to try these as i wanting to do the same thing u are...i want the headlighton as soon as it starts moving and constant brightness

maybe look to a different led that can work with this device

edit: heres a 15ma device, 2.5-60v input

https://www.digikey.com/product-detail/en/diodes-incorporated/AL5809-15P1-7/AL5809-15P1-7DICT-ND/5030226

 

Am i ever gonna be able to lay any track???

  • Member since
    February, 2002
  • From: Reading, PA
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Posted by rrinker on Friday, September 08, 2017 6:53 AM

 That 15ma one would be perfect. 2.5V minimum is nice and low, which means if the LED is 3.5V, it will need a total of 6V to light the LED. A few back to back sets of regular diodes will reduce the motor voltage a bit if needed so the lights can come on when or before the loco moves, at the expense of some top speed.

                           --Randy

 


Modeling the Reading Railroad in the 1950's

 

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

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