Using a USB phone charger to run LED's

Discussion in 'The Projects Forum' started by tralam, Aug 8, 2016.

  1. tralam

    Thread Starter New Member

    Aug 8, 2016
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    Hi All,

    I'm new to the forum and I've been trying to get my head around some basic electronics stuff but I'm having a hard time figuring out what I need for my specific projects.

    What I'm trying to do is use atmospheric, low light LED's in some art/craft projects I'm doing. Basically, I'm trying to integrate LED's in wood and other projects. The light does not have to bright, and in most cases rather dim and soft. I'm thinking a single LED bulb (or short strip) would suffice.

    The other requirement is that I would be able to use as a power supply the typical USB style charger as may come with your smartphone or tablet or what have you.

    It looks like most of these chargers are 5v output, and range between 750a and 1.5a, although I have a Samsung charger that lists 5.0v -- 2a.

    My plan was to integrate microUSB ports into the wooden craft pieces, so that you could just plug in a microUSB cable into the piece and plug the other end into one of those phone chargers (some charges have permanently attached wires with microUSB ends). I would also have to integrate an on/off switch into the piece so that the usb wire could remain hooked up.

    In trying to figure out what I would need specifically for this. Here are some questions I have:

    a) Since the various phone chargers have different amp specifications for the output (750ma, 1a, even 2a) does this mean I cannot regulate the light output of the LED(s)?

    b) I have read about resistors and the calculations regarding Ohm's law and the LED(s) used, but would this be thrown off if different phone chargers would be used like 750ma or 1.5a?

    c) I have read about one way to dim LED's by means of a variable resistor or potentiometer. Is this a safe and reliable method? I read some people consider it wasteful as it results in waster energy as heat (and heat is always considered bad for longevity).

    d) Since I'm looking to dim the LED's brightness to atmospheric levels (like a candle light, basically) is this bad for the LED as far as longevity?

    e) Would it be best to use a single LED with prongs or would it be better to use a partial strip of 3 LED's?

    I won't make this too long to start but hopefully you guys got an idea of what I'm trying to do and if this is possible or not. Basically those USB chargers would be my power supply of choice because it would allow you to plug in the art pieces easily, anywhere, since those USB chargers are everywhere. One other consideration I had was ability to change out LED's in case they burn out or go kaput. So I was thinking this would be easiest with those single LED's with prongs. I guess that would be another question, which would be the most reliable LED's.

    Thanks for any input!
     
  2. tcmtech

    Well-Known Member

    Nov 4, 2013
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    The LED does not care what the amperage capacity of the power supply is. They only care about how much current they are getting.
    As for the resistor in series with the LED that exactly what you want and to figure that out just subtract your LED's working voltage from the power supply voltage, in your case 5 volts, and use Ohms law to calculate what current you want based on the remain voltage difference between the LED and the power supply.

    For example, if you have a white LED that has 3.6-volt forward drop and your power supply is 5 volts you have a 1.4-volt difference.

    Given that if your LED can handle up to 30 milliamps you just divide your 1.4 volt differential by .030 and you come up with a resistance value of 46.66 ohms which you can round up to the next common higher resistor value you can find like a 50 ohm.

    As for your power supply amp ratings that just the maximum current they can supply so if you have a 1.5 amp supply you could run up to 1500/30 = 50 LEDS at 30 milliamps each off of it without overloading it.
     
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  3. hp1729

    Well-Known Member

    Nov 23, 2015
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    Running the LED at lower than the "rated value" just makes them last longer. No harm at all. Some LEDs may give you sufficient brightness at 2 mA. Test the LEDs you are going to use. Working voltage may vary with lower current.
     
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  4. tralam

    Thread Starter New Member

    Aug 8, 2016
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    Thanks, tcmtech, that is very helpful to know about the amperage.

    About the resistor in series with the LED: if, like in your example, the resistor value would be 50 ohm, this would put the respective LED at its full brightness I assume? So what if I wanted to dim that LED, would I merely use a larger resistor to achieve this?

    I read about people using potentiometers or trim pots, also called variable resistors by some. Is that a good way to dim the LED or would you think it is better to use a regular resistor and just set the brightness statically that way? (I think if I would use a variable resistor, I would do this in addition to a regular resistor, not instead of one. Does that sound right?)

    On Amazon I saw some potentiometers by different makers, including some by Panasonic. Does it matter which ones I get or do you recommend a certain brand?

    Thanks for the good info so far.
     
  5. tralam

    Thread Starter New Member

    Aug 8, 2016
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    Thanks for the info. I'm going to order some different LEDs and test them out. I saw they did have some soft white single LEDs (not only the white-white ones) so that may work for me.

    What did you mean by "working voltage may vary with lower current"?
     
  6. hp1729

    Well-Known Member

    Nov 23, 2015
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    Any given LED will have a certain voltage across it at some given current. A white LED may say 3.6 V at 20 mA. At 10 mA it may only drop 3.0 Volts across it. So take a sample of your LEDs and try different resistors to get different voltages and brightness.
     
    John Berry likes this.
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Unless you are going to vary the brightness frequently, trial and error with a few fixed resistors, as suggested above, to settle on a brightness you like will be a cheaper option than using potentiometers. Inexpensive potentiometers can handle only low currents.
     
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  8. hp1729

    Well-Known Member

    Nov 23, 2015
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    Design 805 dim an LED.PNG

    If you really want to have the LED dimmable use a transistor to take the load off the pot. But this may be a lot to go through. You could dim more than one LED at a time this way. Select the ballast resistor for the maximum current level.
     
  9. tralam

    Thread Starter New Member

    Aug 8, 2016
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    Thanks, this is what I plan to do. I found some different style LED's from the brand "Chanzon" on Amazon. They have round, straw hat, and flat top style LED's so I can play with different light spread. They make a soft white variety which may work well for what I'm trying to do.

    OK, good to know, I will have to play with that then. HP1729, what did you mean by "ballast resistor" -- just the resistor put in front of a pot?

    I don't have a kit with breadboard and the basic essentials. I noticed when looking for a good beginner's kit on Amazon that almost all kits are Adruino based, and I'm not even sure what that is but I don't think I need it. Since my focus is kind of narrow at this point (using phone chargers, etc.) I also wasn't sure if I need one of those Bench Power Supplies or if I could just feed a bread board with power from a phone charger. I'm kind of noob at all this but want to get some basic stuff to test and play with as to grow my understanding.

    If anyone has any suggestions as to what type of kits or basic equipment I need, please feel free to tell me because I feel I'm kind of groping in the dark right now :)
     
  10. tcmtech

    Well-Known Member

    Nov 4, 2013
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    If you plan to dim them all at once you can just use a larger power rheostat (higher capacity potentiometer) for that and put it between the power supply and the sets of current limiting resistors that go to each LED.

    If you know how many LED's you will have in parallel and the degree of dimming you want we can easily calculate the rheostat's resistance and power rating for you.
     
  11. hp1729

    Well-Known Member

    Nov 23, 2015
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    The ballast resistor is the resistor that drops the rest of the voltage the LED does not and determines the current in the LED.
    An Arduino is not needed for anything you have described so far. Just a breadboard and a phone charger or a battery will work. A set of resistors is handy.
     
  12. John Berry

    New Member

    May 17, 2016
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    Hello this answer is great as I am also a newbie, to get these 50 led's running as above would they need to be in series or parallel please ?
    Many thanks John :)
     
  13. hp1729

    Well-Known Member

    Nov 23, 2015
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    What voltage do you have to work with?
    12 V?
    3.7 V / led at 20 mA means 3 LEDs in series with about a volt left over for a resistor. 1 V at 20 mA means a 47 or 51 ohm resistor per string. So 17 strings of 3 LEDs paralleled. At 20 mA each that totals.340 mA.
     
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  14. tralam

    Thread Starter New Member

    Aug 8, 2016
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    OK, that's what I thought about the resistor then (I got confused because they also have these 'ballast resistors' they use in automotive that are called that).

    I was looking around on a place called Circuit Specialists. I think it's a bit lower end stuff in general but what they have seems to be pretty good for budget stuff. I was looking for a not too expensive yet versatile soldering station and I really liked this one: https://www.circuitspecialists.com/csi-premier-75w-Soldering-Station.html

    Here's a YouTube video for it:


    And:


    I saw they also had some interesting bread boards with back plates and main connectors, here: https://www.circuitspecialists.com/wb-3t5d-bp.html

    I was actually looking for a good combo with soldering iron and hot air (for tube shrinking) but I'm not sure about the combo ones in my price range so I may have to buy a dedicated tube shrink unit.

    Since I'll mostly be working with either single (or otherwise maybe 2 or 3) LEDs, I'm don't know if that rheostat you mention would be suitable for that or not. Would this apply if I had, say, 3 LEDs that I would want to dim to a soft glow (on 5v phone charger)?

    By the way, if I'd be working with low light (and few) LEDs, what type of wire would you recommend? 22ga, or smaller? I think I'd like stranded the best for that (I'd just solder the strands before soldering). I also don't know what type of solder to get. I read lead-based solder is a little easier to use on small stuff. Also, what gauge solder wire would you guys recommend? I wanted to get one of those solder spindle holders and wanted to order some solder along with it, but I have no idea what size/gauge solder to get for this.
     
  15. Kim Sleep

    New Member

    Nov 6, 2014
    2
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    This station is probably good...but you need good soldering techniques as well, Clean your tip with the supplied copper wool, and use a good quality solder as well.
    I would use a separate hot air unit for Heat shrinking...they are cheap as chips on EBay, or Bangood.
    I use a .75mm Kester Rosin Core solder for everyday use.
    Where are you located???..no info in your profile
     
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  16. Kim Sleep

    New Member

    Nov 6, 2014
    2
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    To make things really easy, and to really simplify an answer, I generally use a 330-to 470 ohm 1/4w resistor to illuminate most leds at 5vdc.
     
    John Berry likes this.
  17. hp1729

    Well-Known Member

    Nov 23, 2015
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  18. hp1729

    Well-Known Member

    Nov 23, 2015
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    Oh, right 5 V, I forgot. One resistor per LED is easiest to wire up. 330, 470 or even 1,000 ohms or more depending on the LED and desired brightness.
    Variety of size, color and shape of LEDs? Consider buying them by the pound from places like Jameco. Blinking or flashing LEDs? Electronic Goldmine has a lot of such things. Some even flash in multiple colors.
    You already have the phone charger picked out? What amperage is it rated at?
     
    Last edited: Aug 12, 2016
  19. hp1729

    Well-Known Member

    Nov 23, 2015
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    Just a note on the various LEDs available from Electronic Goldmine.
    Various sizes; T1 (3 mm), T1 3/4 (5 mm), 8 mm, 10 mm
    Flashing, about 80 per minute, regular rate
    Flickering (random rate)
    Rainbow (multiple color) flickering
    how about a spray of many fiber optic strands you can put an LED of your choice behind.
    (Prices not listed. They go in sale so often.)
     
    Last edited: Aug 12, 2016
  20. tralam

    Thread Starter New Member

    Aug 8, 2016
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    Kim, I'm in Texas, US. I was indeed thinking of getting one of those hot air guns for heat shrinking, but most I've seen on Amazon were sort of expensive or otherwise I wasn't sure about their quality. Which cheap one do you think is pretty good? Thanks for the LED info.

    That Goldmine has quite a few different style LEDs, thanks for mentioning it and Jameco. Since I was looking into atmospheric glow and low light "soft white" or "warm white" LEDs would be best, but not all brands/resellers seem to have them. Goldmine does have some, but most of them are already quite bright to begin with. The Chanzon ones I found on Amazon do have lower output so they may be better for what I'll use them for.

    One option would be to cut out the individual LEDs from LED strips as I've seen some do, but I was thinking I may stay away from that because I would like the LEDs to be exchangeable more easily, in case they burn out.

    I think I'm about ready to order some stuff and once that gets in I'm going to have to experiment :) I haven't started yet but this seems fun already. The one thing I wasn't sure about was power supplies. I am going to experiment with 5v phone chargers but thought I might get a Bench Power Supply too, but I don't know if it is very useful to get a cheaper one (like around $60 range).
     
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