I got a bunch of 5 mm Green LED's off Ebay.

Now I'm going to make a circular ring of Green LED's.


* Emitted Colour : Green
* Size (mm) : 5mm
* Lens Colour : Water Clear
* Peak Wave Length (nm) : N/A
* Forward Voltage (V) : 3.2 ~ 3.8
* Reverse Current (uA) : <=30
* Luminous Intensity Typ Iv (mcd) : Average in 8000
* Life Rating : 100,000 Hours
* Viewing Angle : 20 ~ 25 Degree
* Max Power Dissipation : 80mw
* Max Continuous Forward Current : 30mA
* Max Peak Forward Current : 75mA

I know you have a pretty good LED tutorial on this site, but maybe this one time you could provide me with the size of the load resister for 30 of the LED's mentioned.

 

12v will not run 30 of these leds in series.

They each have a ~3.2v drop so you would need about 96v to operate this circuit as is.

 

I made some 9 LED strings, maybe a little time consuming but I checked V drop of of all 27 & put them in 5 piles of = V. Estimate average V drop for 3 in series & build up the strings so all have close to same combined V drop. You need 10 strings in parallel of 3 LEDs & a resistor comprising each the strings. Assume strings measure 10.8 V, 12V - 10.8V = 1.2V[ a little skimpy] 1.2V /.025mA=48Ω @ .o3W or 1/4 W & next higher std value is 51Ω. To measure V drops use 12V source, with a diode & 330Ω 1/2W resistor in series with LED under test, cathode to battery neg. Two parallel rods were taped to bench to make a LED socket, with V meter connected between them. Diode is to prevent smoke from reversed LEDs

 

That was quite a wakeup call. I realized while doing the math that I'm too far and too long out of school.
I have a 12V supply a 51-Ohm resister and a Diode to 3 green LED's. That makes one circuit using 90 ma.

I've always wondered if that's 90 ma per second? Hour?

 

Ten strings @ 25 mA each = 250 mA. Value of resistors will depend on measured V drop of strings.

 

I've always wondered if that's 90 ma per second? Hour?

Amps is a measure of current, which is a rate... it's just 90 mA, not per second or per hour.

Amps *is* a rate, like gallons per second.

1 femtoAmp is about 6,000 electrons per second. That helps me to put it in perspective.

One mA is then about 6,000,000,000,000,000 electrons per second.

 

Thanks Sage. I always used the ma as a part of a formula without ever realizing how long a Battery could last at that rate.

BERNARD, you drawing put it all into perspective for me. Thanks for making that clear for me. It's exactly what I asked for.
Dick