leds circuit - help planning

Discussion in 'General Electronics Chat' started by zohara, Nov 1, 2018.

  1. zohara

    Thread Starter New Member

    Oct 29, 2018
    9
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    Hi,
    I'm planning to create a backlit batman logo for my kids room.
    At first I thought that I will use led string as light source but I gave up on that since it was very bulky (wires are too long).

    I decided to create it myself but have little knowledge in electricity...

    I bought red/orange/blue leds (100 each color) + some resistors.
    the plan is to frame the batman sign with mix of red and orange colors (total of ~100 leds) and for the eyes - 1 blue led in each eye (attached illustration + the batman sign that I created). the leds to be bright as possible but without damaging them.

    I spend the last few days reading about parallel and series circuits and the mixture of them but couldn't decide which way to go so it will be safe enough, efficient, and bright.
    The plan is to use mains to power the leds (220V) using some kind of power supply. I have few at home but I guess I will buy one if needed once I have plan for the circuit and I'll know the spec needed.

    I would appreciate any help in designing it ;-)

    Tnx!
     
  2. oz93666

    Distinguished Member

    Sep 7, 2010
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    These leds can be wired up in such a way to suit the power supply you say you already have ...

    If you give details of the power supply/s ... the details of the leds .. and what equipment you have (multi-meter resitors , soldering iron ) we can offer relevant advice
     
  3. zohara

    Thread Starter New Member

    Oct 29, 2018
    9
    2
    Thank you.
    regarding the leds (as published in the online store):
    red and orange: DC 2.0V-2.2V (IF=20mA) / 0.06 Watts
    resistors: 470ohm (For DC 6-12V) 1/4 Watt Metal Film Resistors Included / ±1% Tolerance

    blue: DC 3.0V-3.2V (IF=20mA) / 0.06 Watts
    resistors: 430ohm (For DC 6-12V) 1/4 Watt Metal Film Resistors Included / ±1% Tolerance

    the leds and resistors came in a bundle of 100 each.

    I ordered the multimeter in the link for these kind of projects: https://www.amazon.com/gp/product/B071JL6LLL/ref=od_aui_detailpages00?ie=UTF8&psc=1

    I have soldering iron, etc. I will probably need to buy wires...
    regarding the power supply - I will need to be at home to check the spec (have few of them).

    Thank you again for offering your advice!
     
  4. ebeowulf17

    AAC Fanatic!

    Aug 12, 2014
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    The included resistors are sized such that they only make sense in a parallel circuit arrangement, with one resistor per LED. I'd say they're pushing it just a little on their 12V maximum spec for those LED/resistor combos - they'd draw more than 20mA each in that scenario, and it's generally better to run at or below the rated current, not above it... although there's a separate "absolute maximum" current rating for any given LED (not listed in your specs,) and they're almost certainly not violating that, so they're in a grey area there.

    Anyway, using a 9V supply with the parts you've got would yield 14-15mA per LED, which is probably just about right. So that would be the cheapest (since you already own the parts,) and easiest to understand solution, but it uses the most parts and is somewhat inefficient. If you're using just over 100 lights total, your 9V power supply would need to be capable of at least 1.5A (probably safer to say 2A and leave a little overhead.)

    If you want to maximize efficiency and use fewer parts, you can create series/parallel combos, but you'll most likely need to buy different sized resistors and/or use a higher voltage power supply.
     
  5. zohara

    Thread Starter New Member

    Oct 29, 2018
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    2
    Thanks. I'm looking at it as learning project - so I don't mind buying more parts if needed in order to be able to build it better..
    The most important thing for is to keep it safe since it is going to be in my kids room.

    I would love to get recommendation for additional parts needed and what to do with them :) and also pointers on how do I keep this project save..
     
  6. BobTPH

    Senior Member

    Jun 5, 2013
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    If these LEDs are at all efficient, running them near 20ma will be very bright, especially if uses as a night light. You might want to try running them at about 2-5 ma each.

    Bob
     
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  7. Audioguru

    Expert

    Dec 20, 2007
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    You should seal the wiring so that the kids cannot put a paperclip or metal hangar on the wiring and cause a fire.
     
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  8. gerty

    AAC Fanatic!

    Aug 30, 2007
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  9. ebeowulf17

    AAC Fanatic!

    Aug 12, 2014
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    There are a million ways to do this, depending on how high of voltages you want to deal with, but here's an example that's 4 times as efficient as running individual LEDs per resistor.
    003B559C-46B3-44EC-AB55-316052A6BE16.jpeg
    This image was created with a design wizard at the following website:
    http://led.linear1.org/led.wiz

    It's a pretty cool way to quickly visualize these combos and get some ballpark figures, but after playing with it for a few minutes, I wouldn't trust it completely. In three scenario above, with 12V supply and roughly 15mA target current, it also suggested strings of 5 LEDs. At first glance that seems good because it's even more efficient, but at that point, any variability in LED forward voltage values changes the current dramatically.

    Here's an example of why I'm wary of its recommendations: if these LEDs have forward voltages from 2-2.2V, then the total forward voltage for a string of 5 is 10-11V. On a 12V supply, that would leave 1-2V to drop across the resistor. For any given resistor choice, the current if all forward voltages are 2V ends up being double what the current would be if all the forward voltages were 2.2V. Too unpredictable for my tastes.

    With 4 LEDs per string and 12V supply, it's much more predictable:
    Vf of string is 8-8.8V
    Voltage across resistor is 3.2-4V.

    So instead of max current being double the minimum current like in the first example, now it's only 25% more. That's much more reasonable.

    This is all a long winded way of saying you need to make sure you're dropping enough voltage across the resistor to ensure reasonable stability in spite of parts tolerances.
     
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  10. ebeowulf17

    AAC Fanatic!

    Aug 12, 2014
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    The short version:
    • 12V supply
    • 4 LEDs per string (except blue)
    • 270 ohm resistors
    • Assuming 25 strings, you need roughly 1/2 Amp power supply (that's minimum requirement, but it's fine to have more ampacity.)
    You can choose different voltages with different numbers of LEDs per string, but the concept and the mathematical approach stay the same.
     
  11. zohara

    Thread Starter New Member

    Oct 29, 2018
    9
    2
    Wow, thank you for all the information. I'm learning a lot.

    I ordered kit with different resistors..

    So, 4 LEDs in series each string, 25 strings connected in parallel?
    Regarding the power supply - I have several from old laptops. But now I think that I will not use it since it is too heavy. I will order new one from Amazon, so i can buy exactly what's needed. If you can recommend one it will be great!

    Thanks again for all the info, it is very helpful.
     
  12. zohara

    Thread Starter New Member

    Oct 29, 2018
    9
    2
  13. zohara

    Thread Starter New Member

    Oct 29, 2018
    9
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    One more question - what is your recommendation for wire thinkness?
     
  14. Audioguru

    Expert

    Dec 20, 2007
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    It is very cheap and is Chinese. Will it cause a fire? Will it work for only one week?
    It has many voltages and many currents. If it is set to the wrong one then it will blow up your project which might cause a fire.
    Get a locally made wall wart with a local safety certification and with the voltage and current you need.
     
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  15. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    I just came across this hread, but I am suggesting strings of ten for the red and orange LEDs and strings of 8 for the blue ones. Then you can feed them with 24 volts and a smaller series resistor and be able to adjust the relative brightness fairly simply.
    For the wire size I suggest number 24 wire, it will easily handle the current and be strong enough to not break frequently. And it will be a size fairly easy to work with.
     
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  16. ebeowulf17

    AAC Fanatic!

    Aug 12, 2014
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    I disagree with audioguru's implied condemnation of all Chinese products. There's plenty of high quality stuff built there.

    On the other hand, there is also a lot of junk too. Personally I don't like to buy random stuff from unknown sellers on eBay or Amazon, especially if there's a potential safety risk when things fail.

    I'd be looking at power supplies from places I trust, like digikey, mouser, allied, etc. Just as an example, here's a list of possibilities from digikey:

    https://www.digikey.com/products/en...4&pv1120=347&pv1120=382&pv1120=364&pv1120=402
     
  17. zohara

    Thread Starter New Member

    Oct 29, 2018
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    2
    It is been awhile - and I finally got most parts from Amazon.
    I tested the leds and saw that the red and orange are 1.8V and the blue are ~2.8V, little bit lower from the values published by the seller.

    So If I will use 12V 2a power supply and will build it as recommended above - 25 strings of 4 leds (mix of red and orange), 1 string of 2 blue leds (for the eyes):
    Would it be OK (and bright ) if I will use 240 ohm resistor for each red/orange string and 320 ohm resistor for the blue string?

    And one last question - I have this PWM motor that I've used in different project. can I use it with this setup as dimmer?

    https://www.ebay.com/itm/351491759896?ViewItem=&item=351491759896

    Thanks for the explanation and guidance :)
    Zohar
     
  18. dl324

    AAC Fanatic!

    Mar 30, 2015
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    You could put 3 of the blue in series and use a 180 ohm resistor; that would waste less power in resistors.
    Should work.
     
  19. Audioguru

    Expert

    Dec 20, 2007
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    Three blue eyes?
     
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  20. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    @dl324:: Probably the small amount of power wasted in resistors will not matter much.
    For Zohara: using lower values of resistors will certainly work and indeed make the LEDs brighter. There is a trade-off between brightness and lifetime, but probably it will not matter, since the LED lifetime is quite long anyway. The full-power lifetime is still thousands of hours.

    Now for a connent about using a PWM motor speed controller for dimming: Possibly it might work well, but there is also a good chance that it will switch the LEDs off and on at a rate that you can see, which will be quite unpleasant. AND there is that very interesting strobe effect from the rear lights on some cars, where if you turn your head the light sems to be flashing. Of course, it IS actually flashing and by quickly turning your head you see it. So PWM controller might work as a brightness control, but it may not be what you want.. You would need to experiment. But it should not damage anything.
     
    Last edited: Nov 11, 2018
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