Ok so I have 4 LED's I am trying to hook up for for my cars dome light.. I'm not an electronics wizard here so I could use some help with what I should be doing here and the resistor(s) that I may need for this project... because I already almost blew my car up once... So i've got 4 LED's I want to hook up, 2 LEDs in each dome.. There are two dome lights in the car, so one white and one red LED for each dome light. The white LED's are rated at 10000mcd - 45* viewing angle 5 Volts max 25mA max. The red LED's are 6500mcd - 45* viewing angle 4 Volts max 25mA max. The car is a 14.5Volt system when it's on and running, so i'm going to use 14.5Volts for my reference. Now I wired these in parallell, so I took that 5Volt + 4 Volt (1 white + 1 red) and 25ma + 25ma = 50ma. So now i've got a total of 9 Volts and 50mA. So now if I punch everything in the calculator correctly, its showing me I need a 220 ohm resistor at 1 watt? Does this sound correct? I calculated wrong last time and got a 33 ohm resistor at 1/2 a watt and fried the leds last time... Thanks for anyone who can just check my math here and make sure i'm doing it right.. Or should I really be hooking both of these LED's up individually? I do have a small space constraint though. :banana:E
Resistors are color coded. Its been a while since Iv done stuff like this, but a quick google: http://www.csgnetwork.com/resistcolcalc.html Im confused at what your trying to do. You know they make LEDs that will replace the bulbs in cars? Pretty much every size and shape. Be much easier just to get one that will fit your system: www.superbrightleds.com I replaced almost all the lights in my car with LEDs from that site. They have all you will need LED wise.
Yes i'm aware I can get direct fit replacements, but I have a nice little (actually huge) electronics store down the street that has anything you could ever think of, and I want to do this as a project.. Just to say I did it.. And I did use the resistor calculator, this is why I want my work checked. I'm wiring 2 LED's in parallel, but the LED's are not the same.. As you can see mentioned above, one is white and one is red... they both require a maximum of 25mA current draw, but one is 4 volts max, the other is 5.... That gives me a total of 9 volts... but I don't know if that would be corrrect or if I should just be going off 4 Volts since it's the lowest rated of the two. And again the car itself is a 14.5 volt system when on.
I think they will both get whatever voltage the car gives them....which will most likely be 12v+ . So you would have to put a resistor between them and the power wire going to them. So what you wanna do is reduce the 14.5v incoming to 9 volts. What size resistor to use for that, Im not sure. Why you make it so complicated using 2 different LEDs? You like a challenge or something?
I've already stated this cambria. We know the LED's will get the full 14 Volts without a resistor in this circuit, and would make them pop instantly. Using the wrong resistor can also give this problem, or not give the LED enough power. Now these LED's are also $4 a pack so i'm not gonna go blowing led's like a nutcase experimenting, but I really need to know if I want to add my voltages 4v+5v to = 9v? or if I should be going off the voltage of the lowest LED (4volts) and drop from there?
is that something like this that you would like to implement? Bat 14.5V---------|-----------| oooooooooooooooo|oooooooooo| oooooooooooooooLED1ooooooLED2 ------------------|-----------| in that case the voltage seen by each LED is 14V since they are in // you have to use a resistor to reduce the voltage on each LED to avoid any damage. Considering LED1 requiring 5V and using 25ma, it means that a resistor of 14.5-5= R *25ma => R=9.5/25e-3 = 380 ohm must be used in serial with LED1. but this rely on the max A of the LED. Is that close to the normal operation amp? same type of operation for LED2: 14.5-4=R*25ma => R=10.5/25e-3= 420 ohm
Your diagram is correct, just two leds in parallel. I'm not getting what your saying though. It looks to me as though you haven't even considered LED #2? If you go 14.5-5=r *25 you'll end up with 380ohm resistor at 1/2 watt. I guess i'm confused myself on should I be adding the LED1 + LED2 Voltage? I would assume I should be as everytime you would add an LED to the circuit it's going to require it's own voltage. so i.e. one led would be a draw of 1.5Volts, 2 - 3Volts, 3 - 4.5Volts, 4- 6Volts, 5 - 7.5Volts, 6 - 9Volts Or am I assuming wrong, and I that all I am increase is the amperage since they are in parallel, and the voltage remains the same?
exactly in // the voltage does not change and amp is added at the source. http://www.furryelephant.com/content/electricity/parallel-circuits/ it would be interesting to know the typical amp consumed by each LED to make that operation more accurate but I'm almost sure the LED could work normally even if the voltage at their boundary is a bit lower than the max specified. So if you consider something like ~400 ohm for LED1 and ~440 ohm for LED2 that should work. I'm just a bit concerned about what could happen during the first microseconds when you turn the system on. At that time the current is not fully draw and therefore the voltage will not decrease as expected at the resistor boundary. Could we imagine something with a diode to avoid overvoltage during that time? I'm maybe a bit too pessimistic
Uploaded with ImageShack.us I made a little sketch of how I was going to try it. Can I do it like this using the 1 resistor for the the 2 leds or could that be dangerous? When the system is first turned on I doubt there would ever be a spike due to the fact that the starter is consuming such a large amoun of power from the battery, and the alternator takes a second or two to get up to speed.... so in all honesty a spike is out of the question completely. Frying resistors cuz I am an idiot is another story in itself though.
Using only one resistor means that we need to insure that we never have more than 4V at the 2 LED boundaries to avoid any damage on the red one. This means that the white one will never operate at its maximal brightness. (using 4V instead of 5V) so if we consider that LED are pure resistive components (which is far from the reality but otherwise it will be a nightmare to compute the values) we have LED1 R1*Imax=5V => R1=200ohm for LED2 R2=160ohm if we consider red LED running at max => 4V (25ma) at its boundary, it means that the current going through LED1 will be I1= 4/200=20mA so we have at resistor boundary : 220*(20mA+25mA) = 9.9V which is not enough considering that 14.5-9.9 = 4.6V and we were expecting to have 4V max at LED boundary. So maybe the red LED will be able to handle the extra voltage To be safe Resistor should be around (14.5-4)/45mA=233.333 ohm => 240ohm but I'm not feeling very comfortable with my computation since the resistive assumption might be incorrect.
I agree with your analysis that a resistor in each parallel path makes more sense then a single resistor before due to the fact that they will both see 5v.
http://led.linear1.org/led.wiz << That should help you! Solution 0: 2 x 2 array uses 4 LEDs exactly * each 180 ohm resistor dissipates 112.5 mW * the wizard thinks 1/4W resistors are fine for your application Help * together, all resistors dissipate 225 mW * together, the diodes dissipate 500 mW * total power dissipated by the array is 725 mW * the array draws current of 50 mA from the source.
here's the exact site you need. http://led.linear1.org/led.wiz Even tells you how to wire them up and the color code just incase. Keep in mind the max voltage on LED's isn't their normal operating voltage. Take half a volt off for the reds, so 3.5v, and go 4.25v for the white LEDs. Heres the other issue, car voltage kind of jumps a bit. Especially while running vs turned off. LED's are a bit sensitive. If you find yourself having problems, use a voltage regulator.
I think this has been addressed, maybe not spelled out. But you don't add 5V to 4V to get 9V. You need to separate the two and provide resistance (or voltage regulation) to each separately - providing what voltage and current each need on their own.
You could also keep them in parallel 14.5V --> 5V --> 4V. Still two separate resistors but the second one would take the 5V out from the LED and cut it down to 4V. That said, I think in general LEDs can take fluctuating power levels pretty well and should operate fine (ie just as bright) with lower voltages. The specific LEDs you have should list what their tolerances are (beyond MAX it should also list MIN to produce the same brightness) - so I this case you may be able to get away with using only one resistor matching the lower of the two (MAX) voltages and the higher (MAX) LED should work just fine. Again, just check the specs on what you have.
Just make a voltage divider. R2 / R1+R2 so 15v -> 5v would be something like : (example) R1 = 20k ohm R2 = 10k ohm VCC ---- ...........| ..........R1 ...........| ............----- LED ---- ..........R2.................| ...........|..................| GND ------------------ The important thing for getting correct voltage is the resistor ratios, not the resistor absolute values. The resistor absolute values will determine your max amps, and the number of amps bypassing your LED device. Ideally, you want the highest resistance (to limit bypassing current) that still will properly power your device (LED). -scheherazade
as i see it if you put all lds in the same light fixture you can just put the ones with the same voltage in series and use a different resistor for each series group for the 5v led at 25ma you have 14.5-5-5=4.5v 4.5/0.025=180 ohm and for the 4 volt leds you have 14.5-4-4=6.5v 6.5/0.025=260ohm you should preferably use higher value rated resistors as close as u can to the values rather thanlower and you get 2 groups of 2 leds and a resistor in series this way you get less heat disipated on the resistors therefore less power wasted 0.11W for the first resistor and 0.16 for the second
