LED help November 25, 2006, 04:30:42 PM Just need to know if I wire up multiple led's in parallel can I use just one resistor or do I need to use one for every light.ThanksScott Quote Selected
LED help Reply #1 – November 25, 2006, 05:36:16 PM I would think that you could, but I never can remember how each of the em go... lol. (Series and parallel)But I am not sure. Quote Selected
LED help Reply #2 – November 25, 2006, 10:08:57 PM Yeah i'm not sure either thats why i'm asking, already have the led's glued in so I dont wana screw them up.BumpScott Quote Selected
LED help Reply #3 – November 25, 2006, 10:18:57 PM Hello,Quote from: tbirdscott;115492Just need to know if I wire up multiple led's in parallel can I use just one resistor or do I need to use one for every light.Depending of the amount of leds You have in parralel, the resistance value do not change, BUT the wattage YES!, If You put 10 leds in parralel and use only an 1/4 watt resistor, the resistor heat and blow...For an exemple, if You have 10 leds in parralel, take a 2-3watts resistor, if You put a bigger wattage resistor, this is not bad, but just take more place in Your project....Hope this help... Quote Selected
LED help Reply #4 – November 25, 2006, 11:26:40 PM okay, well I emailed the person I bought them from and i'll see what they say. I dont know what wattage of the resistors are or anything, theyre small though i'd need a magnifying glass to get the colours off of them. Quote Selected
LED help Reply #5 – November 26, 2006, 10:11:39 AM Yes, the resistors values do change with more LED's in parallel. The resistor in an LED circuit is there to limit current and voltage going to the LED. Any LED circuit with a resistor (which means just about all LED circuits) is a series circuit. The resistor uses some of the 12 volts available, leaving about three volts for the LED. If you put two LED's in parallel you still need three volts, but you need double the current. Three LED's in parallel still require three volts but triple the current. A resistor can only reliably limit current, not voltage. To tweak the current a different value of resistor will need to be used.As the current across the LED part of the circuit increases the voltage drop requirements of the resistor will decrease (as more of the available energy is used by the LED's), so the resistance value will go down.I don't know the math involved - when I'm designing a circuit I use a breadboard, potentiometer, and an ammeter. I put the circuit together with the potentiometer set to a high resistance, then turn the potentiometer down until the current showing on the ammeter is 25-30 mA per branch circuit (the LED's I use are at peak brightness at this level). I then remove the potentiometer, measure its resistance, and use a resistor of that value in the circuit.It should be known that resistance values will depend on the color of the LED as well. I have found that in order to maintain 25-30 mA the R-value for an amber or red LED is double that of a white, blue or green.You can also save yourself a lot of work (and a lot of resistors) by using series/parallel circuits. Because cars use 12 volts and LED's generally operate at 2-3 volts (depending on colour) you can put them in series. I've found that you can put no more than three in series before the brightness starts to suffer (you might get away with four red or amber, but that leaves no room for voltage fluctuations). Then you can put several of these series circuits in parallel and use only one resistor for several LED's. The beauty of doing it this way is that each series circuit draws the same current as one LED (in other words, three LED's in series draw the same 25-30mA that one does). A characteristic of a series circuit is that the entire branch circuit sees the same current flow, but different parts of the circuit can see different voltages.Having truly talked myself in circles trying to explain something that I understand in function but not in theory, I'll give you a diagram showing what I mean. This is similar to how I wire the LED lights I sell. Remember, each group of three LED's draws the same current as one LED, but has triple the voltage requirements. Since all are groups of three the voltage can be considered a constant and ignored here, but current cannot. One group of three LED's will draw 30 mA while two will draw 60 and 3 will draw 90. The resistor must limit this current to this number, which is why different values are required. Quote Selected
LED help Reply #6 – November 26, 2006, 10:25:30 AM To find the wattage value is a little more complicated. First you need to know the voltage being absorbed by the resistor, then you need to determine the wattage based on that voltage drop multiplied by the current in the circuit. Let's say you have a branch circuit of two LED's in parallel. The voltage drop in this branch circuit will be about 3 volts. The current will be 60mA. If the LED's are using up 3 volts the resistor must use the other 10.8 volts. At 60 mA * 10.8 volts = .648 watts, so you'd need a 3/4 watt resistor.Now, to really muddy things up, if you had two branch circuits of three LED's in series as shown in my drawing above, you actually need a lower wattage resistor. This seems backward - smaller resistor to feed more LED's, but remember: each branch circuit still only draws 30 mA, but each is now consuming 9 volts (3 per LED). That means the resistor now only has to "use" 4.8 volts instead of 10.8. With the same 60 mA the circuit would be drawing, the wattage value of the resistor would now be .288 watts. Sinec my LED's flash and the resistor is only at a 50% duty cycle I can get away with 1/4 watt, though if the LED's were to be on steady I'd be safer with a 1/2 watt.Have I thoroughly confused you yet? :D Quote Selected
LED help Reply #7 – November 26, 2006, 01:16:10 PM My head hurts... I think i'll just cram 5 resistors behind the housing and call it done.I knew there was a reason I dropped electronics in school.Thanks though, maybe i'll read it a few more times later, its still early.Scott Quote Selected
LED help Reply #8 – November 28, 2006, 07:24:11 AM Hello,In the past I used leds in serial, like 4 or 5 for 12 volts in a vu meters, and the leds are never lights equal, one a bit more another, and one a bit less one other, etc...Past two years I purchased 100 WHITE leds on ebay, putted all on a breadbord, lighted up with a bench supply at 4 volts, and some lighted more than others, except if the voltage is a bit over the specs, but the life of the led's is a bit shorter...And You're right on the resistance value, I was a bit confused with the parralel/serial connection, it's very not the same thing, Quote Selected
LED help Reply #9 – December 04, 2006, 01:20:34 AM I thought i'd update this since I finally got everything done. I got the 5 resistors all put in and jb-welded over everything once more after this pic.And heres a pic of it lit up.I asked the boss' dad a couple electronics questions and now hes sending me home with books to read lol He used to be a university teacher and did all his training in europe so he knows what hes talking about.Scott Quote Selected
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