Led Q

myp71 · Post by **myp71** » Sun Aug 08, 2004 12:20 am

What resistance would I need to run two leds off of 12VDc?
Thanks Ryan

bodgy · Post by **bodgy** » Sun Aug 08, 2004 1:05 am

Without further facts, I can only give you the general formula, which isVsupply - Vled / currentVled is sometimes referred to as the voltage drop and depends on the type and colour the led is, anything from 1.1v - 3.6v.The lower voltage is normally required by those defined as high brightness types and 3mm or so in diameter. White leds are normally 3.6v with true blue around 2.8 - 3.0vThe current required will be in the led's data sheet. A good average value is 10mA led.Your calculation will depend on the color/type of your leds and if they are connected in series or parallel.So using these guesstimate numbers -12 - 1.2 / 0.010A = 10.8v / 0.010A = 1080 ohms.Use a 1K or 1K2 as a nearest preferred value.Colin[ August 08, 2004: Message edited by: bodgy ][ August 08, 2004: Message edited by: bodgy ]

cato · Post by **cato** » Sun Aug 08, 2004 3:19 am

The discussion above is correct for one LED with a voltage drop on the low end. However, I read the question to imply the two LEDs are in series. If that's the case. the 1.2 volts (or what ever Vforward is) in the exemplary calculation, should be doubled.[ August 08, 2004: Message edited by: cato ]

myp71 · Post by **myp71** » Sun Aug 08, 2004 10:06 am

Yes it is for two leds.I didn't know the specs on these Leds because they are coming out of something I bought.I just opened the unit up.The unit has two brighteness settings so that means it has two resistors: Black ,Violet,Yellow2nd: BLack,GReen,Violet
I can't seem to find my color wheel so thats why I'm posting the color.
Thanks for your help again Ryan

Edd · Post by **Edd** » Sun Aug 08, 2004 1:38 pm

I tend towards an individual dropping resistor for each LED.Otherwise, in a series arrangement there is going to be a tolerance differential and a current hog unit.
Using reverse Polish logic on your color codings leaves you with a 47 and 75 ohm units.
73’s de Edd

Chris Smith · Post by **Chris Smith** » Sun Aug 08, 2004 2:02 pm

Caps do a better job, less current and heat loss.

myp71 · Post by **myp71** » Sun Aug 08, 2004 2:37 pm

So 680 ohm should do it per led?? 
Thanks RY@n

dyarker · Post by **dyarker** » Sun Aug 08, 2004 7:18 pm

<blockquote>code:<hr><pre>
Mem Crutch Color Number X10 Magnitude
Bad ------- Black - 0 10 to 99 (zero zeros added)
Boys ------ Brown - 1 100 to 999 (one zero added)
Rape -------- Red - 2 1K to 9K
Our ------ Orange - 3 10K to 99K
Young ---- Yellow - 4 100 to 999K
Girls ----- Green - 5 1M to 9M
But -------- Blue - 6 10M to 99M
Violet --- Violet - 7 100M to 999M (never seen)
Goes ------ Gray - 8
Willingly - White - 9 ?? 1 to 9 ??
</pre><hr></blockquote>
The resistor markings can't start with zero (black), so you read them backwards. The yellow (1st resistor) and violet (2nd resistor) bands should be closer to one end of resistor body than black bands are to the other end.No fourth band +-20%, silver +-10%, gold +-5%.1% resistors have 3 number bands (XX.X), a x10 magnitude band, and tolerance band (white I think).If the LEDs will be in series with each other, then one resistor in series with the LEDs is okay. Since the current thru them is the same, the brightnesses will be very close.If the LEDs will be parallel, don't. Use a separate resistor in series with each LED. With two, or more, parallel with one resistor in series, one LED will be a current hog and be brighter the other(s). (like Edd said)((same color and type))Red? 1.3V? Two LEDs in series, one 680 resistor:
12V - 2.6V = 9.4V
9.4V / 680 Ohms = 13.8mA, okayLEDs, each with a 680 resistor:
12V - 1.3V = 10.7V
10.7V / 680 Ohms = 15.7mA, okay, brighter; but you could use a lower value resistor with series LEDs for same brightness.A capacitor WILL NOT work with 12VDC!Cheers,[ August 08, 2004: Message edited by: Dale Y ]

myp71 · Post by **myp71** » Sun Aug 08, 2004 9:30 pm

<blockquote>quote:<hr>The resistor markings can't start with zero (black), so you read them backwards <hr></blockquote>
OOPs

Ry@n

bodgy · Post by **bodgy** » Mon Aug 09, 2004 1:56 am

<blockquote>quote:<hr>Originally posted by Dale Y:
<blockquote>code:<hr><pre>
Mem Crutch Color Number X10 Magnitude
Bad ------- Black - 0 10 to 99 (zero zeros added)
Boys ------ Brown - 1 100 to 999 (one zero added)
</pre><hr></blockquote><hr></blockquote>Well now Dale with that mnenomic, you and I may be of the same vintage or there abouts.A major discussion some 7 years ago in a UK electronics mag, got all conflummeried over the non PC'ness of that little ditty, as secondary schools (many have electronics as a subject these days), felt it too much for the delicate flowers in their charge.I always added on - God Shall Persevere for the tolerance - perhaps I should copyright that bit.COlin

dyarker · Post by **dyarker** » Mon Aug 09, 2004 3:42 am

Picked that one in Electronics Technology at Community College '69 - '71. There was a girl in the course, said she wasn't Violet but suspected who was

level headed that oneThey've gone over board with political correctness. It says Bad Boys not cool boys. Just something else for busy bodies to _itch about.What comes after silver that starts with "P"?

myp71 · Post by **myp71** » Mon Aug 09, 2004 10:43 am

Ok now I'm

Ry@n

toejam · Post by **toejam** » Mon Aug 09, 2004 11:30 am

a 1000 ohm pot and a meter capable of reading millamps will help. Most leds seem to work uaing about twenty ma.

dyarker · Post by **dyarker** » Mon Aug 09, 2004 6:07 pm

120 OhmsLonger explanation later if you want. I need to get ready to go to work right now.[ August 09, 2004: Message edited by: Dale Y ]

myp71 · Post by **myp71** » Mon Aug 09, 2004 9:44 pm

Thanks Nah,I like to learn but I don't think I need it

Thanks again for the help RY@n

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