tegwin
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| posted on 5/2/16 at 06:35 PM |
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Powering 40 small LEDs
Im having a bit of a brain fart and can't figure this out..
I have 40 LEDs current of 35ma, voltage of 3.4 being powered by a car battery. If I put three in series with a resistor several times it would work
but I would be loosing a lot of power into the resistors. Does anyone know of a more elegant way of powering such a large array? (ideally dimmable).
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HowardB
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| posted on 5/2/16 at 06:39 PM |
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Current limited pwm.
..
Pulse width modulation
Howard
Fisher Fury was 2000 Zetec - now a 1600 (it Lives again  and goes zoom)
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hizzi
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| posted on 5/2/16 at 06:55 PM |
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could you not use a string of them? you can buy a 15ft strip on ebay sticky backed that you cut to length
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tegwin
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| posted on 5/2/16 at 06:56 PM |
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quote:
Originally posted by hizzi
could you not use a string of them? you can buy a 15ft strip on ebay sticky backed that you cut to length
I could but they would still need to be powered somehow?
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Would the last person who leaves the country please switch off the lights and close the door!
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britishtrident
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| posted on 5/2/16 at 07:02 PM |
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12v LEDs either indivual or strips.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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theconrodkid
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| posted on 5/2/16 at 07:41 PM |
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http://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2050601.m570.l2632.R2.TR5.TRC1.A0.H0.Xbuck+converter.TRS0&_nkw=buck+converter&_sacat=9
2074
who cares who wins
pass the pork pies
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hizzi
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| posted on 5/2/16 at 08:15 PM |
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what are you trying to do with the leds? are they for a car or other use. i use the 15ft strings cut to length and powered witha. wall wart for a
model trainset, i use the same in the lockers on my landrover again easy as they are 12v
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tegwin
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| posted on 5/2/16 at 08:20 PM |
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They are going in the headlining of my van. Plan was to use single leds as stars rather than strips
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02GF74
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| posted on 5/2/16 at 08:21 PM |
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You need to have constant current for 3 leds in series for 13 lots in parallel (39 in total). Plus the one extra ked
See here http://www.edn.com/design/led/4424539/Overcome-the-challenges-of-driving-parallel-LED-strings
A transistor for each series switched so the average current is within spec, get that wrong and all they will go pop.
You can buy led drivers but these are for higher current led emitters.
Do you need 40 leds or can you use brighter leds?
I suspect drop resistor will work out cheapest and simpkest but not the most efficient?
Re. 12 v leds, correct me but arent they regular leds with built in resistor?
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coyoteboy
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| posted on 5/2/16 at 09:46 PM |
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Yep, no such thing as a 12V LED (ok strictly some of the 100W LED chips can be driven at 12V but only for millisecond pulses)
There's no elegant way of driving that many LEDs in parallel (if you run constant current on a parallel bunch there's no control over
which gets what current). By far the easiest and most cheap way is strings of 3 series with a current limiting resistor. You're dropping losing
about 900mW on the whole system if you do it that way.
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tegwin
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| posted on 5/2/16 at 09:54 PM |
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Hmm ok. Well I'm sat on the floor wiring in the leds with resistors in sets of three. Decided on 30 as wiring 40 just seemed too much like
effort on a Friday night!
Might get a voltage regulator and a pwm to change the brightness
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Would the last person who leaves the country please switch off the lights and close the door!
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02GF74
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| posted on 6/2/16 at 08:06 AM |
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quote:
Originally posted by tegwin
Might get a voltage regulator and a pwm to change the brightness
Or use pwm to a switching device (e.g. transistor or open collector buffer) to have best of both worlds.... but it is extra complexity, get the pulse
with wrong and you say bye byes to all the LEDs.
The switch could be more sophisticated and be current limiting but pushes complexity and cost so youve gone for the besy compromise.
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