LED ARRAY - Who is a Guru for calculating resistance needed ???
05-18-2013, 06:47 AM
#1
JK Enthusiast
Thread Starter
I have a 37-LED array that comes from a 3-cell flashlight. The LEDs are wired in parallel on their circuit board, operating on a typical 4.5 volts and drawing about 215 milliamperes.
I want to adapt it for 12-14 VDC to replace the standard 12v automotive bulb in the center hub of my spare tire. It's a custom CHMSL (3rd brake lamp).
So, question is: What spec resistor (ohms/watts) would I need to keep these 37 LEDs operating at their design voltage and amperage?
I want to adapt it for 12-14 VDC to replace the standard 12v automotive bulb in the center hub of my spare tire. It's a custom CHMSL (3rd brake lamp).
So, question is: What spec resistor (ohms/watts) would I need to keep these 37 LEDs operating at their design voltage and amperage?
Last edited by HankFard; 05-18-2013 at 07:14 AM.
05-18-2013, 07:32 PM
#2
JK Enthusiast
Join Date: May 2012
Location: Robinson, Illinois
Posts: 121
Likes: 0
Received 0 Likes
on
0 Posts
I am no guru so I may not be of any real help. I was trying to figure out some LEDs that were installed in the car we bought for our daughter. There are some calculators on the net that should be able to help you. I don't remember the site just that they were out there. I think you could input some criteria and it would figure a circuit that would work. There are many options like wiring some in parallel and adding one or more resistors. Good luck.
05-19-2013, 06:11 PM
#3
JK Enthusiast
Join Date: Aug 2010
Location: Royersford, PA
Posts: 375
Likes: 0
Received 0 Likes
on
0 Posts
Dusting off my old electrical engineering degree, the general rule of thumb is that you need enough voltage to overcome the forward voltage in the diodes and don't push too much current through them and fry them. Given that they worked at 4.5V then you'll have no problem with 12 available. Knowing that you want the current to be 215 mA this means that you'll want to drop 4.5 V across the diodes and the remaining 7.5 V (12-4.5) across your resistor. Since V = I * R, this means that you'll need a resistor of R = V / I = 7.5 / 0.215 = 35 ohms. Generally speaking, you can't find a 35 ohm resistor, it's not one of the standard sizes. But 33 ohm is plenty close enough. This web site will do the math for you LED Resistor Calculator.
Now to worry about the power. Since P = I * V = 0.215 * 7.5 = 1.6 Watts. So you'll need a resistor with this power rating (or better).
Now to worry about the power. Since P = I * V = 0.215 * 7.5 = 1.6 Watts. So you'll need a resistor with this power rating (or better).
05-19-2013, 07:36 PM
#4
JK Enthusiast
Thread Starter
Now that's what I'm talking about!
33 ohms is what I also calculated but I wasn't sure I was using the correct formula. Since I was afraid of blowing up my LEDs, I tried 33 ohms with a new 9 volt battery. That worked fine, so then decided that since the running voltage is 14+ volts, I would adjust up to 50 ohms. I bought a 50 ohm 10watt wire wound resistor at Radio Shack. At 12 volts, the LEDs are bright, and at 14 volts they are nearly brilliant. The resistor gets warm to the touch, but by no means hot. And since it is a stop lamp, it's off more than it's on.
Thanks for the reassurance!
33 ohms is what I also calculated but I wasn't sure I was using the correct formula. Since I was afraid of blowing up my LEDs, I tried 33 ohms with a new 9 volt battery. That worked fine, so then decided that since the running voltage is 14+ volts, I would adjust up to 50 ohms. I bought a 50 ohm 10watt wire wound resistor at Radio Shack. At 12 volts, the LEDs are bright, and at 14 volts they are nearly brilliant. The resistor gets warm to the touch, but by no means hot. And since it is a stop lamp, it's off more than it's on.
Thanks for the reassurance!