entropy
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posted March 12, 2003 03:56 PM
electronics expert: hook up LED bulb????
I bought some 12V LED bulbs and I figured, hey, they're 12V so just hook em up to the battery and they'd work...
I want a MUCH brighter oil idiot light and figured I'd use the the OEM pressure sensor, but externally mount a BRIGHT LED instead of that tiny stock light in the dash.
Seems I need something else in the circuit. Putting 12V accross the bulb leads seems to blow em up.
any hints???
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MadMike
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posted March 12, 2003 04:09 PM
that dosent sound right? a 12V light should run on a 12V system, ? weird.
MM
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TedG
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posted March 12, 2003 04:44 PM
 Edited By: TedG on 12 Mar 2003 16:49
LEDs are light emitting diodes and are current sensitive. Usually 20mA. There is an Anode and a Cathode and in the case of an LED the anode goes to positive. So you must limit the current with a resistor. Find out what the current requirement is and use Ohms law to figure it out. But usually a 1000 ohm resistor will work. brown - black - red- (silver or gold for tolerance). Tie the resistor to the LED Cathode and touch the Anode to 12 volts and the other end of the resistor to ground and it should work. By the way you probably fried the LED without the resistor. I it isnt bright enough buy a 720ohm resistor (or something close) and an 520 ohm resistor and try those.
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tbbt
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posted March 12, 2003 04:45 PM
Edited By: tbbt on 12 Mar 2003 17:08
You can't just hook a LED up to a voltage source. You need to put a resistor in series with the LED to limit the current. Check the specs on the LED to see what its maximum forward current is and the how much voltage drop the LED has. Common voltage drops and max forward current specs are 2.2V and 25mA, respectively. Given these parameters you can calculate the size resistor required.
Ex:
(12V - 2.2V)/0.025A = 392 Ohms
Use the next standard sized resistor greater (use the next greater sized resistor to make sure the current is below the max allowed) than 392 Ohms is 470 Ohms.
To figure out what wattage resistor you need - calculate the power dissipated by the Resistor. Use the formula P = (I^2) * R, or (V^2)/R.
Ex: 12V - 2.2V = 9.8V, which is the voltage dropped across the resistor.
So, the power dissipated by the resistor is:
(9.8V^2)/470 Ohms = 0.204 Watts.
So in this case the resistor should be a 1/4 Watts resistor (at minimum).
When hooking up the LED and series resistor, hook the top of the resistor to the + terminal of the voltage source, hook the bottom of the resistor the Anode of the LED, and hook the Cathode of the LED to the - terminal....
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wannabe
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posted March 12, 2003 04:52 PM
quote:
You can't just hook a LED up to a voltage source. You need to put a resistor in series with the LED to limit the current. Check the specs on the LED to see what its maximum forward current is and the how much voltage drop the LED has. Common voltage drops and max forward current specs are 2.2V and 25mA, respectively. Given these parameters you can calculate the size resistor required.
Ex:
(12V - 2.2V)/0.025A = 392 Ohms
Use the next standard sized resistor greater (use the next greater sized resistor to make sure the current is below the max allowed) than 392 Ohms is 470 Ohms.
To figure out what wattage resistor you need - calculate the power dissipated by the LED. Use the formula P = (I^2) * R, or (V^2)/R.
Ex: 12V - 2.2V = 9.8V, which is the voltage dropped across the resistor.
So, the power dissipated by the resistor is:
(9.8V^2)/470 Ohms = 0.204 Watts.
So in this case the resistor should be a 1/4 Watts resistor (at minimum).
When hooking up the LED and series resistor, hook the top of the resistor to the + terminal of the voltage source, hook the bottom of the resistor the Anode of the LED, and hook the Cathode of the LED to the - terminal....
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TedG
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posted March 12, 2003 08:19 PM
I was trying to keep it non technical.
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entropy
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posted March 13, 2003 12:04 AM
WOW!!!!
Thanks much tbbt & TedG!
I suspected that the "SNAP!" noise I heard when I hooked up the bulb to the battery meant something wasn't set up exactly right...
I can do a resistor.
Now how was it in Freshman Physics lab?
Bad Boys Rape Our Young Girls But Violet Gives Willingly... or something like that
Thanks again
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entropy
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posted March 13, 2003 12:07 AM
WOW!!!!
Thanks much tbbt & TedG!
I suspected that the "SNAP!" noise I heard when I hooked up the bulb to the battery meant something wasn't set up exactly right...
I can do a resistor.
Now how was it in Freshman Physics lab?
Bad Boys Rape Our Young Girls But Violet Gives Willingly... or something like that
Thanks again
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MadMike
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posted March 13, 2003 06:24 AM
Yes... LOL I thought our class was the only one... I thought that the design of these LED lights, had the proper resistor inserted? wouldent you think? I guess they sell twice as many. LOL...
Mad Mike
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entropy
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posted March 13, 2003 06:29 AM
the LED's only cost 50-80 cents ("bulb" only), so adding a 50 cent resistor shouldn't break my bank !
On the BBROYGBVGW; I went to school so long ago the phrase started with "black"...
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TedG
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posted March 13, 2003 08:09 AM
One reason they don't come with resistors is they are used in literally thousands of applications. With many voltage ranges which would require different values depending on what intensity one wants. Automotive it probably a minor chunk of the market.
Right now I am working on a light ring that is USB controlled. I had to write a Windows XP driver to control it, along with the interface program and the pattern generators. The LEDs are RGB and can have up to 255 colors. They are really cool.
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entropy
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posted March 14, 2003 05:11 AM
I bought a range of resistors at Radio Shack, 220 - 1K Ohms, tried em all.
MAN O' MAN, that is a BRIGHT little SOB using the 220!!! Next time my oil lite goes on, I'll see it for damn sure!
Thanks TedG & tbbt!! 
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n da zone
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posted March 14, 2003 09:57 AM
I agree! Great post! I am going to hook up an LED for my license plate that is way up under the tail, and it sounds like I was getting ready to blow a few out trying to wire it up.
I thought 12V was 12V, but I guess it's a matter of 'pressure' or amps, too.
Thanks guys!
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tbbt
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posted March 14, 2003 01:05 PM
Edited By: tbbt on 14 Mar 2003 13:08
quote:
I bought a range of resistors at Radio Shack, 220 - 1K Ohms, tried em all.
MAN O' MAN, that is a BRIGHT little SOB using the 220!!! Next time my oil lite goes on, I'll see it for damn sure!
Thanks TedG & tbbt!!
Yes, reducing the size of the resistor will make the LED a bit brighter. However, you may want to be careful with the 220 Ohm resistor. You may be exceeding the maximum current of the LED or the maximum wattage rating of the resistor. If either is the case you run the risk of "letting the smoke out" of either compenent. As we all know, where there is smoke there is Fire!
Use the specifications of the LED and the calculations I showed you above and make sure you are well within the limits of your components....
If you are within specification of the LED, but are drawing too much current for the resistor, then go up to a 1/2 watt resistor....
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TeamSpeed
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posted March 14, 2003 08:50 PM
Edited By: TeamSpeed on 14 Mar 2003 20:54
If you want a good source for some of the brightest LEDs you will ever find, check out this guy's site. http://www.whitelightled.com
I use this guys LEDs when I convert conventional flashlights to LEDs. Amazing to have a flashlight that lasts 20-30 hours on one 9v. 
By the way, you only want to run about 20-40 ma thru these LEDs. You can overdrive them higher, but they will burn out a little sooner than they typically would with the correct amperage.
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