LED electrical question

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LED electrical question

Postby tink2011 » Tue May 08, 2012 8:00 am

I have a mutant vehicle and for the last few years had battery powered (4.5v) LED strands as part of the lighting. This meant having tons of little battery packs hidden all over the car, and a huge pain to turn them all off at night, in the dark and the dust. This year I would like to solder the light strings together and wire them all to a bus bar and 12V car battery. Then I could just flip some switches to turn on or off!
My question is this: The LEDs in the light string appear to be wired in parallel. They are rated at .048mA (I got that using Ohms law with 4.5 voltage and the resistor that was factory installed) and an online Ohms calculator.
The existing set-up that works well is a string of 35 lights with one 2.6 resister strung between the batteries and the light string using 4.5V (three AA batteries). If I switch over to a 12V system Ohms Law is giving me a resistance of 2.38 at 60 Watts power. What resistor do I need? It seems they are rated for watts (1/4 up to 25 at Fry's Electronics) and resistance, but I'm not sure how to pick the right one(s). I don't want to blow $60 worth of lights without having a good idea of what I'm doing.
I plan to string 105 LEDs (3 of my current strands) in one continuous chain, and more, if I can. Can I just use one resistor for the entire chain or do I need to put them every few lights? These lights are covered by a nice shiny fabric sleeve that gives them a really cool look, so I can't get to the wiring except at the beginning and the end of the string, or every 35 LEDs.
Can anyone help? THANK YOU!!!
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Re: LED electrical question

Postby junglesmacks » Tue May 08, 2012 8:16 am

Check this out: http://led.linear1.org/led.wiz - Much easier!

Now then. If they are all wired in parallel, then essentially what you're doing is figuring out what it would take to run just one LED via that source voltage.. since basically that's all that all of them are seeing is that straight 12V. The downside to this is that you're going to be sucking all of that current through one resistor.

The biggest question before using the wizard is what color are they? If they are blue, green, white or pink.. then they will typically want 3.2v. If they are red, yellow, amber or purple.. then they want 2.2v.

Using the wizard and assuming 1 LED running at 20mA which will give you plenty of brightness.. and assuming a 3.2v model.. and a 12v source voltage.. we can see that what you need is a 470 ohm, 1/2w resistor (1w if you can since you're pulling juice). If you're using a 2.2v LED, then you will need a 560 ohm resistor. This can be wired up inline in either the positive or negative lead of the chain but typically it's favorable to wire it on the ground side of things.

To determine what rating of resistor you're needing, you need to figure out how many watts you'll be burning. Remember watts = amps * volts and 1ma = .001amps. So.. a 35 LED string running @ 20mA per LED would be W = .7 * 12 which equals 8.4 W. 105 lights? That would be 25.2 W. Meaning that in order for you to not turn that resistor into a toaster heating element.. that you need to be using a resistor rated for at least the amount of watts as you're pulling through it. This is why for most large LED arrays, people use small chains of LEDs connected in a series instead of one long chain.

For last year's fully body LED lightsuit, I had a total of 1200 LEDs just on the pants.. but divided into 240 chains of 5 LEDs per chain with a 150 ohm / 1/4W resistor on each.


Ta-dah! That wasn't so hard, was it?
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Re: LED electrical question

Postby 5280MeV » Tue May 08, 2012 8:44 am

tink2011 wrote: My question is this: The LEDs in the light string appear to be wired in parallel. They are rated at .048mA (I got that using Ohms law with 4.5 voltage and the resistor that was factory installed) and an online Ohms calculator.


Are you sure that there is not one resistor per light?

The conventional wisdom that I always hear is that when you do LEDs in parallel, then each LED needs its own resistor. I have no experience "doing it wrong" so I don't know how big a deal this really is in practice.

One thing I can say is that if you just put another resistor on before the string to get the correct amount of current flow, then you will be wasting a lot of power. As JS said, the voltage drop across your LEDs may be something like 3.2V, that means that the other 1.3V drop across the resistor is just wasted power.

If you take an LED with a 3.2V drop and use the right resistor to connect it to a 12V source, then you will have an 8.8V drop and be running at about 35% efficiency, rather than about 70% efficiency with the 4.5V source. So you will consume twice as much power to get the same amount of light.

You might instead think about getting a DC-DC converter to go from 12V to 5V. The converter can be up to 90% efficient, so you would need less power to get the same amount of light.
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Re: LED electrical question

Postby junglesmacks » Tue May 08, 2012 8:51 am

5280MeV wrote:You might instead think about getting a DC-DC converter to go from 12V to 5V. The converter can be up to 90% efficient, so you would need less power to get the same amount of light.


This is a very accurate statement. All resistors do is burn up/dissipate electricity in order to lower the current. You're losing it.. not just choking the flow.

Check out a SBEC converter which would wire inline to your 12v in order to scale it down to 5V. The one show can handle up to 5A which is way more than you will ever be pushing. http://www.hobbyking.com/hobbyking/stor ... Lipo_.html

35 LEDs @ 5v = 3.5 W
105 LEDs @ 5v = 10.5 W

compare that to 8.4W and 25.2W respectively.. to obtain the same amount of light output.



Dammit.. he/she got me all focused on answering the question at hand rather than thinking logically about the overall solution to the problem. I think I need to switch to de-caf for a while.
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Re: LED electrical question

Postby tink2011 » Tue May 08, 2012 1:37 pm

Thanks to both of you! I've been wracking my brains for over a week, visiting websites and local electrical shops and had almost given up. First of all, let me say that I've never soldered or calculated electrical circuits, but I am very excited about this! Although each of you gave great descriptions, I am still a little confused regarding the wattage rating on the resistors. Can you calculate the math on these scenarios for me and tell me which resistor to use??? PLEASE? :-)

140 LED string wired in parallel (four strands soldered together)
Amber (or yellowish) 2.2v
20mA
5 Volt source (using the DC/DC converter)

70 LED string wired parallel
Red 2.2v
20mA
5 volt source

35 LED string parallel
2.2v
20mA
5 volt source


Now, should I use the resistor in the ground/negative line instead of on the positive wire between the bus bar and first LED? I plan to wire the 12v to a bus bar. Should I put the DC/DC coverter between the 12v car battery and the bus bar? I guess that's how it will have to be because I will have four different runs coming off the bus bar that will each need the 5 volt. Then, can I use the same bus bar to run the 12 volt stuff? I have already gotten the bus bar and it has well over 10 attachment points. Yikes! This is getting more difficult!

The other items I am using are CCFLs and EL wire that run off the 12v. My understanding is the CCFLs wire straight to the 12v without resistor/inverter (Is that correct?) and then the EL wire runs with the inverter (I've done the EL before, so that's a positive! :). THANK YOU SO MUCH! BTW, my vehicle is a golf cart based space ship carrying a 12 foot tall alien who is holding the world...you may have seen it on the playa with mediocre lighting...hoping to fix that this year!
-Alice :D
P.S. JungleSmacks...I saw your light suit online when doing research last week....VERY IMPRESSIVE!
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Re: LED electrical question

Postby junglesmacks » Tue May 08, 2012 2:09 pm

Well.. it's very simple math. Let's go over this again so that you can do this all yourself and OWN this!

A wattage rating of a resistor is just that.. a rating. Since resistors reduce the amount of current going through them by dissipating power, they also can act as a burner.. as in.. that orange glowy thing on your stove or in your toasters. All those are, are wires that are acting as resistors. You don't want that resistor that you just soldered in to act as a stove burner, right? So.. you need to figure out how many watts you'll be pulling through it and then make sure to use a resistor that's rated to pull that much power through it.

To figure out how many watts you're burning, you use the formula of: watts = amps * volts

In this case, each of those LEDs is running at 20mA.. or .02A (A=amps). 35 of those LEDs all running at .02A would be a total of amperage draw of 0.7A. So.. with a 5v source voltage, your total watts would be 3.5W. (.02*35) * 5 = 3.5. Thus.. you would need a resistor that is rated for at least 3.5W.

You should be able to do the rest of the wattage calculations, right?

I have to tell you though.. that I would recommend against putting any more than one of those 35 LED strings in all in parallel. You're starting to draw way too much current through things.. and you start dealing with things like voltage drop. Are you absolutely sure that each LED is run in parallel? Most of those strings have the LEDs with a small resistor inline that you don't even know is there. They're all in a parallel.. but each with it's own resistor. There's a reason why large arrays of LEDs are usually done in clusters of series connections (like I did my suit) or in parallel but with an individual resistor for each LED. But.. that's not saying that it can't be done. You can do anything that you want to..

Have you checked into using something like the 5 meter sections of waterproof RGB lights that are made to run off of 12v? They're direct cheap these days, SUPER bright, you can completely change the colors all through the rainbow, they have brightness control along with a remote and they're totally easy to just plug and play. They're now under $30 shipped..

If you're going to do this using the 5v step down converter, you need to pay attention to how much power you're drawing through it. The one that I showed you is rated for up to 5A. We know that one of those strings of 35 LEDs is 0.7A.. so that means that you could run about 7 of those strings through there. If you wanted the bus bar to put out 5V, then yes.. you would put it in between the battery and the bus bar. The bus bar would then be 5V though. If you wanted to run other 12V accessories, then you would put it after the bus bar.

Honestly.. electrical systems and LEDs are really very simple things and most of it comes in the planning. It's all logic. Draw out on paper your wiring diagram and where everything needs to go.. what you want to connect.. and figure out what it will take to get you there.
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Re: LED electrical question

Postby tink2011 » Tue May 08, 2012 2:16 pm

http://www.flickr.com/photos/nyceone/4987108059/

Here's the link to a photo of my vehicle 2009 playa. EL wire only, little on the alien...but they still gave me a night license! I haven't read you last post, but THANK YOU SO MUCH!!!
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Re: LED electrical question

Postby junglesmacks » Tue May 08, 2012 2:37 pm

Very nice! I'll find you on the playa when you least expect it.. watch.. :wink:

Or.. just look for me on the horizon every night walking. I'm going for 2000+ LEDs on the lightsuit this year.. you should be able to find me.. :D
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Re: LED electrical question

Postby tink2011 » Tue May 08, 2012 2:39 pm

So, if I ran each one individually (35 LED per string run back to the bus bar with speaker wire to reduce the amount of power -- I already bought these so I'd like to use them, plus they look awesome!), then I would need a 7.14 ohm resistor that is rated to 3.5 watts, right? If I got this right I'll be really excited and headed to Frys! I understand that I should go higher wattage rating, rather than lower if given the option (i.e. choose the 4 watt rating over a 3 watt in this scenario). But, what about the resistance? Err on the side of higher resistance too? Am I correct that a 100K ohm resistor is rated for 100,000 ohms? And I will only need 7.14 ohms? You have been more than helpful...if I ask too many questions...just let me know! The man burns in 116 days. )'(
Your 2012 playa suit sounds crazy!!! Crazy in a good way!!! I will look for you for sure... I camp on the 3:00 side of town, how about you?
-Alice
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Re: LED electrical question

Postby tink2011 » Tue May 08, 2012 2:43 pm

I plan to heat shrink tube all of my EL wire solders. Can I also heat shrink right over the top of the resistors. That makes sense to me... but...don't want an alien on fire!
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Re: LED electrical question

Postby 5280MeV » Tue May 08, 2012 3:43 pm

tink2011 wrote:Err on the side of higher resistance too? Am I correct that a 100K ohm resistor is rated for 100,000 ohms? And I will only need 7.14 ohms?


Err on the side of higher resistance and your LEDs will be a bit dimmer. Err on the lower side and they will be brighter but potentially with shorter lifespan or in danger of breaking. The 100k does mean 100,000.

The resistance is not so much a rating as a value. When a resistor is rated for up to 1 Watt, it means that it can dissipate up to one watt of power without overheating. However, the resistance is a constant and pretty close to exactly what the resistor says.

The amount of resistance in a circuit limits the current that goes through. If you are just using a resistor and a battery, it will follow Ohm's law, which says that the current is the voltage divided by the resistance.

So if I get a 10 Ohm resistor and wire it up to a 12V lantern battery, then 12 V/10 Ohm = 1.2 Amps will flow through the resistor. This will dissipate about 14 Watts of power (Power = Voltage * Current). So my resistor needs to be rated for at least 14 Watts or it will overheat.


If you wire up the 100k Ohm resistor, only 0.12 mA of current will flow through the resistor, and the power dissipated will be just 0.0014 Watts, so you can easily use a resistor rated to only 1/4 watt for this.

The problem with calculating circuits involving LEDs is that they do not follow Ohm's law, and therefore require more complicated mathematics to figure out. This is why an online calculator like the one JS linked to is really useful.

tink2011 wrote:Can I also heat shrink right over the top of the resistors.


Yes, but you will reduce their ability to dissipate heat. You can test the circuit before you do the soldering and see if the resistor gets warm over time. If it doesn't heat up much, then covering it with the tubing should be fine.



I am still kind of surprised/confused about the 35 LED string itself. Are you sure that there is not a resistor on each LED? Have you tried using it yet, and does it just plug into a battery pack?
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Re: LED electrical question

Postby EspressoDude » Tue May 08, 2012 4:21 pm

what about connecting each of 3 strings in series? 4.5 volts x 3 =13.5 damn close to the average voltage of an automobile electrical system. ( 12.2 battery only to 14.4 charging)

no changes, no new resistors.

string 1 negative to ground
string 1 positive to string 2 negative
string 2 positive to string 3 negative
string 3 positive to +12 ~ 14v via a switch

assumption is that all 3 strings are the same
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Re: LED electrical question

Postby tink2011 » Tue May 08, 2012 7:01 pm

Thanks everyone for your great answers. The string series wiring may be a tad over my abilities since this is the first time I've soldered! I have held the strand up to the light and I don't see any resistors. Here's the link to where I purchased them, and it doesn't give any specifics:

http://www.amazon.com/gp/product/B000VV ... 00_s00_i00

I used them last year on the playa and they worked great all week long using the 4.5v battery pack, so I ordered more a few days ago to add more lighting to the vehicle. When I opened up the battery pack/on and off switch pack (cheapo white one) there was a 2.7 ohm (5%) resistor inside. Red, blue, gold, gold. That's how I figured (maybe wrong) that the lights were 48mA each, but now I see I should use 20mA for LEDs.

Can someone please explain how to wire a grounding bus bar? Is it the returning negative wire that goes to the ground screw on the "grounding bus bar?" Can I use something labeled "grounding bus bar" to connect the positive wires and run them out the the lights? I have two of them and thought I could use one for positive and one for grounding.

THANKS! I'm soaking this up and loving it.
-A
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Re: LED electrical question

Postby 5280MeV » Wed May 09, 2012 9:38 am

tink2011 wrote:When I opened up the battery pack/on and off switch pack (cheapo white one) there was a 2.7 ohm (5%) resistor inside. Red, blue, gold, gold. That's how I figured (maybe wrong) that the lights were 48mA each, but now I see I should use 20mA for LEDs.


I'll be damned! I have always been told to use one resistor per branch with LEDs, but I guess either it doesn't really make that big a difference or the manufacturer was being cheap (or both!).

Red, purple, gold, gold is 2.7 Ohms, maybe the purple looks kinda blue on this one.

The only problem with your calculation was that you neglected the voltage drop across the LED. If I guess at a forward voltage of 2.2V, then there is a 4.5-2.2 = 2.3V drop across the battery. With a resistance of 2.7 Ohms, this means that there is a current of 2.3/2.7 = 851 mA running through the battery, or 24 mA through each LED branch - which sounds about right. Must be a fairly good sized resistor as about 2W of power are being dissipated when you turn the string on.

The LED also has a resistance and drop in voltage, the funny thing about an LED is that the resistance changes with the applied voltage. If the voltage is low, the resistance becomes very high, and no current flows. If the voltage is high, then the resistance becomes low and too much current flows through the LED and destroys the component.

tink2011 wrote:Can someone please explain how to wire a grounding bus bar? Is it the returning negative wire that goes to the ground screw on the "grounding bus bar?" Can I use something labeled "grounding bus bar" to connect the positive wires and run them out the the lights? I have two of them and thought I could use one for positive and one for grounding.


To ground the circuit, you would connect a wire to negative that went to a copper rod pounded into the dirt. This will prevent your car from moving, so you should probably skip it. :)

Bus bars are generally just thick pieces of metal with screws or holes drilled in that can handle huge amounts of current. For your application, which sounds like well under 10 Amps total, they are absolute overkill and it sounds like what you are thinking is just fine, using them to branch out from the positive and negative leads of the battery. You may also want to get something like a few 12VDC 10A or 20A fuses to place inline between the battery and your busbar. That way you will be protected against an accidental short.
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Re: LED electrical question

Postby tink2011 » Wed May 09, 2012 11:00 am

Ah, yes, a fuse! That'd be bad to forget! I will be running lots of EL wire with inverters, 10 CCFL 12" and 9 of the 35 light LED starings, each on their own strand from the bus bar. So, should I get one 12v fuse / 20 amp inline? Or more than one? And then take some extras out the the playa? Also, I am wondering about the size of battery to use, as I am wiring a stereo system into the starting battery with the alternator and plan to use a separate 12V battery for the lights. Would you suggest a regular 12V car starter battery, or do I need to pop for the deep cycle marine? Those are very heavy and large, and if I can get away with a regular car battery that'd be awesome. Or maybe a golf car battery? Even smaller. Thanks in advance, you are all helping so much I'm a happy girl! Getting this all squared away!
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Re: LED electrical question

Postby some seeing eye » Wed May 09, 2012 1:01 pm

LED circuits in series to run off a higher voltage, like 9 or 12 is a common strategy. Sounds like each individual string has all the LED's in parallel, so an individual one failing won't affect the string.

Putting the strings in series with each other does mean a connector failure between the strings will kill everything. You can down regulate 12 to a lower with an analog voltage regulator, but more efficient is an adjustable switching regulator http://www.amazon.com/Step-adjustable-s ... B001GQ9RWO. Even better would be an adjustable PWM LED driver. Then you could run the strings in parallel for better reliability.
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Re: LED electrical question

Postby oscillator » Wed May 09, 2012 3:13 pm

Hey, check out a constant current driver and say bye-bye to resistors, formulas, and the headaches that go with them.

I have used these BuckToots (and the larger Buck Pucks) for years. Very efficient devices, very forgiving, have not destroyed one yet despite much abuse.

http://www.ledsupply.com/bucktoot.php

Easy-Peasy.
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Re: LED electrical question

Postby BBadger » Wed May 09, 2012 4:30 pm

Good to see people learning DIY electronics!

You should use a deep cycle battery, not those car batteries. Car batteries are designed for providing a very high peak current, but not sustained current, which is what you'll be using them for. Plus, they're only like 85AH in capacity, compared to 220AH or so you can get in those deep cycle batteries. Costco sells 220AH 6V deep cycle batteries for about $85 each. Wire two together and you'll have your 12V. I would also invest in a smart charger, as those batteries shouldn't be discharged below 50%.

On the lights you're using, I'm really not sure how they're wired up. There are 35 LEDs there, and you can't have 35 LEDs in series and run them off a 4.5V battery. That means either: 1) there's a resistor on each light, or 2) there's a different means of powering those LEDs like a simple voltage converter (that's what EL wire power supplies have). I also don't know if that 2.7ohm resistor is the actual current regulating resistor or for some other purpose, and it may be a waste of power to modify that to use a different power supply.

Do you have a multimeter? They're really cheap and very handy. If you can measure how much current is being used by each strand (measure the voltage across that 2.7ohm resistor) you can weigh your options. For example, if they're not drawing much current, it might be easy enough to just put some diodes in series with the 6V supply to drop the voltage down to 4.5V--assuming you can run your power from only one of the batteries instead of from the pair for the 12V source. If there's more current drawn, something else like DC-DC converters (the link One Seeing Eye provided) may be in order.

Other things you should consider:

  • How to switch your system off. You need a decent switch for this, especially if you are switching off a high current load at a single point. Make sure your switches have the proper DC current
  • How are you going to seal the power and electronics from the elements? Screwing to a grounding bar is one thing, sealing it is another.
  • If you're centralizing your power, do you want to do more with it? For example, you can do lighting sequencing, fading, etc. with transistors and a microcontroller that would be hard at the individual point lights.

For the last point, you can also put some transistors in line with your LED strands and electronically switch the strands on and off individually from the control point. This also means you can hook the connections to a microcontroller to sequence, fade, etc. which could be fun. Using transistors as the switches will also drop the voltage, so you may not need as many components. Just something to consider.
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Re: LED electrical question

Postby tink2011 » Thu May 10, 2012 10:57 am

Lots of amazing information here. I am a newbie to wiring, soldering and not able to process what everyone is saying, as most of it sounds like a foreign language. So, I am going to read all the posts a few times and put together a wiring plan and then post it here for your critique. I will try to have it posted by late next week. A BIG THANK YOU to everyone who's posted. I've learned so much! SpaceGrab is gonna be glowing this year for sure. :-)

For clarification: the LEDs in the 35 LED strings are strung with the LED sitting between the two wires. Each LED is encased in plastic on top of the soldered connections. It looks like a ladder with the LEDs as the steps and the wire as the legs. Or, you could also say it looks like a bunch of letter H stacked on top of each other. I believe this is parallel, right? I hooked a 4.5v source to a strand without the resistor and the first two LEDs fried, but the rest of the string kept working (one I put the resistor in!) so it must be parallel? Everyone is saying this is not possible, but it seems to have worked at BM all week long last year! Maybe ignorance really is bliss?
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Re: LED electrical question

Postby tink2011 » Thu May 10, 2012 11:05 am

@B Badger:
I dissected (literally because I am a medical person!) the battery pack assembly that came with the LED strings. It is composed of 3 AA batteries --- one 2.7 ohm resistor --- on/off switch -- out to 35 LEDs strung in parallel --- back to ground. There are no resistors in the string.
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Re: LED electrical question

Postby BBadger » Fri May 11, 2012 1:06 am

tink2011 wrote:Lots of amazing information here. I am a newbie to wiring, soldering and not able to process what everyone is saying, as most of it sounds like a foreign language. So, I am going to read all the posts a few times and put together a wiring plan and then post it here for your critique. I will try to have it posted by late next week. A BIG THANK YOU to everyone who's posted. I've learned so much! SpaceGrab is gonna be glowing this year for sure. :-)

For clarification: the LEDs in the 35 LED strings are strung with the LED sitting between the two wires. Each LED is encased in plastic on top of the soldered connections. It looks like a ladder with the LEDs as the steps and the wire as the legs. Or, you could also say it looks like a bunch of letter H stacked on top of each other. I believe this is parallel, right? I hooked a 4.5v source to a strand without the resistor and the first two LEDs fried, but the rest of the string kept working (one I put the resistor in!) so it must be parallel? Everyone is saying this is not possible, but it seems to have worked at BM all week long last year! Maybe ignorance really is bliss?


Ah, that's good information. It sounds like it's using the bad--but inexpensive--method of distributing the current. If any of those LEDs fail, more current will go through the others and shorten the lifespan. At that price though, it's probably expected. At least it's not those incandescent Christmas bulbs that really start to burn up.

If they're red LEDs, they'll probably drop 2.0V, so the strand is using:

Code: Select all
Total current = (4.5V - 2.0V) / 2.7 Ohm = 0.93Amp
Current per LED = 0.93A / 35 = 0.026Amp
Watts per strand = 4.5V * 0.93Amp = 4.18W


The earlier replies' ~20mA estimates above should be roughly correct. Each strand will use about an amp of power, which is actually bad for Alkaline battery lifespan. You'll actually get more lifetime from your deep cycle batteries because they can tolerate higher discharge rates. Car batteries are not good though.

Wasted power

If you use a 12V power supply and simply replace the resistor, to get the same amount of current (1Amp) you would need to replace that 2.7Ohm resistor with a higher valued resistor:

Code: Select all
Resistor = (12V - 2.0V) / 0.93A = 10.75 ohm


The problem? Now you'll be using about 12V * 0.93A = 11W per strand--and 9W worth being just useless heat! That sucks! At about 1 Amp per strand, and 3 strands, your 220AH batteries will only last 220AH / 0.93A = 73 hours if you completely drain them to death (bad to do!), and for only three strands. Even if you use a 6V battery, you'll still be wasting energy as heat.

A better solution is to buy one of those step-down converters One Seeing Eye suggested, and set them for a voltage just above the forward voltage of your LED strands (if they're red, that is 2.0V, for other colors it'll be different). For example, get one and set it for 3V, and replace that 2.7 ohm resistor with a 1.1 ohm resistor (1.075 is the exact value, but that'd be hard to obtain). If you're going to buy resistors anyway, why not save some power? The math:

Code: Select all
(3V-2V) / 1.1ohm = 0.90Amp
Current per LED = 0.90 / 35 = 25mA = probably longer lifetime for your LEDs anyway.


You could also try connecting the strands together end to end. However, if the way the current regulation is done now is a bad idea, it's a worse idea to put those parallel LEDs in series with each other. The different voltages, currents, LEDs manufacturing, etc. will all affect each other and if some of the lights burn out, all your strands could die.

Multimeters

Something you should also obtain is a multimeter. Here's a cheap one at Sparkfun (though before you buy it or something else, see if there's anything else to buy to combine shipping). The reason you'll want one is so that you can check voltages, and also troubleshoot things if things go wrong. It is also for safety reasons: you want to make sure that you have no shorts (metal connections) between the battery power and ground. You can literally burn metal if you accidentally connect the terminals, especially on those deep cycle batteries. Use the multimeter to make sure that your grounding block and all the other electronics are properly wired.

If you get a multimeter you should also check the forward voltage on those LEDs. Reds are usually 2.0V, but it might be different. If you get other strands and colors you might as well check those too.

Sequencing and control

But wait, before you buy anything, see if you want to do some sequencing with your lights. For example, you can easily hook a transistor (MOSFET) in series with the strands and switch them on and off. They're pretty cheap and you can use kits for them too if you pay a little more ($4).

How to sequence them? Connect the control (gate) pin of the MOSFET to a switch (see top figure of this) or to something like the output of a microcontroller (like an Arduino board). If the voltage is high the light will turn on, if low, the light will be off. Also note that the MOSFET drops some voltage too (1.5V in the above kit's case), so that needs to be factored into your chosen resistor value and power supply. Example:

MOSFET source-to-drain voltage = 1.5V, LED forward voltage = 2.0V, choose a cheap 0.5ohm resistor ($0.14) for current regulation

Code: Select all
(PowerSupplyVoltage - 1.5V - 2.0V) / 0.9A = 0.5 ohm 
PowerSupplyVoltage = 0.9A * 0.5ohm + 1.5V + 2.0V = 3.45V  or about ~ 3.5V


Even easier is to just measure the voltage across the resistor to determine the current, and adjust it until you get what you want. The setup above uses 0.9A * 3.5V = 3.15W, which is less than the original strand--and you can make them blink.

And finally...

It's your project. You're not obligated to use any of the above, but it'll give you an idea of what can be done if you want to.
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Re: LED electrical question

Postby SnowBlind » Fri May 11, 2012 2:30 am

5280MeV wrote:
tink2011 wrote:When I opened up the battery pack/on and off switch pack (cheapo white one) there was a 2.7 ohm (5%) resistor inside. Red, blue, gold, gold. That's how I figured (maybe wrong) that the lights were 48mA each, but now I see I should use 20mA for LEDs.


I'll be damned! I have always been told to use one resistor per branch with LEDs, but I guess either it doesn't really make that big a difference or the manufacturer was being cheap (or both!).


Here is why you've been told not to do it this way:

LEDs have manufacturing tolerances, which means they are not all the same. That's true for all electronics components, but the curve on which LEDs operate is non linear, so small changes have large effects (as opposed to, say, resistors, where small deviations have small effects).

If you connect a bunch of LEDs in parallel and run them with a single resistor, you could get one, or a few, that have lower resistance, so all the power will run thru those, and the rest will stay dark. Or at the very least, they could run at uneven brightness. Which may or may not be a big deal, depending on the application. If you build a sign or video display with it, different brightness would stand out badly.

If a manufacturer builds LED lights and makes sure they are all from the same batch, the risk is somewhat smaller than if you were getting random LEDs. So maybe that's why they go this route, or maybe they just don't care. They are definitely saving money this way.
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Re: LED electrical question

Postby CornMan » Fri May 11, 2012 8:35 am

Can't you just run a power adapter to all those little battery/switch boxes? You can have all the LED strings switched on and control them by your one power source.
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Re: LED electrical question

Postby tink2011 » Wed May 16, 2012 7:05 pm

Hi Everyone,
So much for me having a wiring plan posted here by late this week. Believe it or not, I ended up having emergency surgery for an appendicitis yesterday! I am home and on the mend, but won't be up to posting any plans for awhile. Glad I got started early so I will have time after this little hicup! I look forward to getting back on my journey to light up SpaceGrab in another few weeks...thanks to everyone.
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Re: LED electrical question

Postby gyre » Wed May 16, 2012 8:36 pm

I would consider the LED/fibre optic approach.

There's a thread.
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Re: LED electrical question

Postby BBadger » Thu May 17, 2012 11:27 am

Zeke Chaparral wrote:Can't you just run a power adapter to all those little battery/switch boxes? You can have all the LED strings switched on and control them by your one power source.


That's essentially the same thing as above, with the problem that you need to have exactly 4.5V going to each box. Yes, a step-down converter can be used for that, but if you're going through all the trouble of wiring all this stuff up to a central point, you might as well do a little more to get more out of what you have.
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Re: LED electrical question

Postby tink2011 » Tue May 29, 2012 8:56 am

I am down to final calculations before purchasing items later today. Am I correct on the following?

If I run the 35 LED all wired in parallel (red or amber) from 12v deep cycle battery through DC/DC converter step down to 5v, then I will need a 7.14 ohm, 3.5W resistor. Is this correct? I understand that parallel isn't the best way to connect them, but I have already purchased these lights and they are really great looking with a silver ribbon cover that is perfect for my application.
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Re: LED electrical question

Postby 5280MeV » Tue May 29, 2012 1:23 pm

Hi Tink,

I am not sure how you got 7.14 Ohms - maybe you could post your calculation or assumptions?

I get the following:

After the DC/DC conversion, your voltage source is 5V. The voltage drop across a red LED is typically 2.2V. You would like 20 mA of current through each LED, or 700 mA of current for the whole array. So the voltage drop across the resistor should be 2.8 Volts, and the resistance should be (5-2.2)/0.7 = 4 Ohms according to Ohm's Law.

For the power rating, you would need a resistor with a tolerance of *at least* 2.8V * 0.7 A = 2W.

You cannot actually buy a 4 Ohm resistor.

I think that what you want is a "wire wound" resistor with a 10W power rating. This would definitely not overheat, and they cost on the order of 15 cents each from digikey or another supplier. You cannot actually buy 4 Ohm resistors, but you can just chain 4 of the 1 Ohm resistors in series to get 4 Ohms total. This will also reduce heating to the point where they may not even get warm.
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Re: LED electrical question

Postby gyre » Tue May 29, 2012 1:29 pm

He still needs to now the native LED voltage.
They vary a lot now.

And there are sound reasons for not using a single resistor.

You could test them to find the voltage they light up on.
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Re: LED electrical question

Postby SnowBlind » Tue May 29, 2012 11:16 pm

I would put one resistor in front of each LED if you can, that way you can run them parallel with no issue. Plus the resistor would only have to be 1/4 or 1/8 Watt rated (Less maybe, depending on the exact voltage and mA that you pick).
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