Bit of advice needed for more powerful LED’s 1, 3 & 5 watt

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Bit of advice for more powerful LED’s 1, 3 & 5 watt. I will have 24v running around the garden & wanted some bright led’s on some signals “let's say 3 watt” Now a red 3w led with 2.2 FV & a FC 450ma @ 24v wants a “56 ohm 9 watt resistor.” A big devil.
If i do the same at 5v I need a “6.8 ohm 1 watt resistor”. A lot smaller.
Q&A is. Am I better off putting a 24v to 5v 3 amp buck convertor’s in, as these are a lot cheaper to fit. Any down sides welcome.
PS: please check my calculations. Thanks

CORECTED "I think"
 
Last edited:
So, math first:

you want to "drop" 21.8 volts in the resistor.

So by ohms law: V=IR, or in this case we are solving for R:
R= V/ I ok R= 21.8 / .45 = 48.4 ohms

Did you assume 50% duty cycle, i.e. connecting the LED across the rails? There is an even bigger possible mistake there. Has to do with reverse breakdown voltage of the LED, it better be about 50 volts or more. (won't be)

Whatever, you might want to see where your calculations went wrong or talk to an engineer about this.

Power is VI = 9.81 watts, and the rule of thumb on power resistors is double, so 20 watt resistor is what is needed.

Realize that no matter how high a wattage resistor you pick, you are STILL dissipating 10 watts as heat... wasteful and will most likely warp something, and have to be isolated in free air not to melt plastic.

And to answer your question if it's not painfully obvious, yes buck it down to 5v or even lower, get an adjustable one, and while it is a bit dangerous, you could adjust it down to your 2.2 v, the closer down the smaller the resistor, both in wattage and value.

Greg
 
PhilP said:

Current limiting Resistor calculator for leds

ledcalc.com ledcalc.com

Is your friend..
Even dissipating 4W will mean you will melt a plastic signal!

Do you really, really need to go that bright?
Check out high intensity LEDs, and current driver IC's.

PhilP
Click to expand...
I am looking to be able look around the garden in the day time & have possibly a semaphore or even a beacon post i can see in the day time to indicate which way the points are facing. I am also looking at having a fix red & green led & mounting a magnifying glass on the semaphore arm that will increase the light. This is all on a dead rail, but i will have 24v in every place needed.
PS: couldn't make out that calculator. Then realised i am using chrome & google adds was covering the input voltage. Solved when i switched to IE.
 
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I have just bog standard 5mm LEDs in my signals and they are certainly visible across a small garden in summer daylight, and very much so at night. 3mm tend to be OK at night, less so in daylight. For simplicity, my signals are prewired with resistors and the circuit around the garden is 10-12V, as they are controlled from a central panel.
My suggestion would be to have a low cost trial first, before committing to something which might possibly be over the top. Perhaps go to a good supplier like Railwayscenics who are happy to supply single LEDs, resistors etc and buy a couple of examples and some resistors (all pretty cheap) then test them out in your garden with a temporary low voltage power source, then you can know exactly what is visible before a major investment in kit and effort. That supplier has an LED resistor calculator on its website too, which is handy. If you do go for buck converters to bring the voltage down before supplying the lamps, I've found these good value and the seller posts first class for free: eBay Step Down Converter
 
Greg Elmassian said:
So, math first:

you want to "drop" 21.8 volts in the resistor.

So by ohms law: V=IR, or in this case we are solving for R:
R= V/ I ok R= 21.8 / .45 = 48.4 ohms

Did you assume 50% duty cycle, i.e. connecting the LED across the rails? There is an even bigger possible mistake there. Has to do with reverse breakdown voltage of the LED, it better be about 50 volts or more. (won't be)

Whatever, you might want to see where your calculations went wrong or talk to an engineer about this.

Power is VI = 9.81 watts, and the rule of thumb on power resistors is double, so 20 watt resistor is what is needed.

Realize that no matter how high a wattage resistor you pick, you are STILL dissipating 10 watts as heat... wasteful and will most likely warp something, and have to be isolated in free air not to melt plastic.

And to answer your question if it's not painfully obvious, yes buck it down to 5v or even lower, get an adjustable one, and while it is a bit dangerous, you could adjust it down to your 2.2 v, the closer down the smaller the resistor, both in wattage and value.

Greg
Click to expand...
i am hopping it was the calculator i was using. Have edited the 1st post. Thanks
 
viaEstrecha said:
I have just bog standard 5mm LEDs in my signals and they are certainly visible across a small garden in summer daylight, and very much so at night. 3mm tend to be OK at night, less so in daylight. For simplicity, my signals are prewired with resistors and the circuit around the garden is 10-12V, as they are controlled from a central panel.
My suggestion would be to have a low cost trial first, before committing to something which might possibly be over the top. Perhaps go to a good supplier like Railwayscenics who are happy to supply single LEDs, resistors etc and buy a couple of examples and some resistors (all pretty cheap) then test them out in your garden with a temporary low voltage power source, then you can know exactly what is visible before a major investment in kit and effort. That supplier has an LED resistor calculator on its website too, which is handy. If you do go for buck converters to bring the voltage down before supplying the lamps, I've found these good value and the seller posts first class for free: eBay Step Down Converter
Click to expand...
All in the planning at the moment & lots of experimenting to do yet. The layout will be 22m X 16m & it was more the 22m points i needed to see. It will be RC battery trains with Wi-Fi controlling the signals & points, plus general lighting. The size is part the reason for 24v to limit volt drop. I have already purchased a lot of track & buildings & have just put another router in the garage next to the garden layout. I am hoping for rain to soften the ground as i am hiring a turf/sod cutter to start a trench. Where i live, the ground went like iron this summer as we have a lot of sand in the area. But, ha'ho we will get there. But glad you said you ca see your led's.
 
I must admit a 3W 450mA LED sounds like overkill for a signal. They're intended more for hydroponics and for lighting spaces aren't they?
 
ge_rik said:
Another alternative, if all you want is an indicator as opposed to a scale colour light signal, might be these.


They say they are for 12-24v and are waterproof (for use on boats)

Rik
Click to expand...
Must admit i have been looking at loads of ideas. A good one was the image below, but to far out of scale. Its about 10". But i will try it & maybe paint the casing black or dark green to tone it down. They are about £9 each & 24v led.
beacon.GIF
 
viaEstrecha said:
I have just bog standard 5mm LEDs in my signals and they are certainly visible across a small garden in summer daylight, and very much so at night.
Click to expand...
Agree - I have used 5mm LED's powered from a 5V controller and they are visible during the day (these draw around 10mA each) - in fact they were a bit too bright at night - hoping to add some PWM control to my controllers and run a day/night mode.
 
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