LED's off 9V

drache · Post by **drache** » Thu May 31, 2007 10:45 am

How many LED's can be lit off one 9V? White 5mms

Scott Hasty · Post by **Scott Hasty** » Thu May 31, 2007 5:35 pm

A question that screams for other questions. How are the hooked up, series or parallel? Are you using a current limiting resistor? Technically, you could hook up bunches of them wired in parallel with a current limiting resistor, but on a 9V battery they wouldn't last too long (9V batteries are the worst choice for these circuits IMHO because of they're current capacity).

Whites generally have a 3.6V forward voltage, so you could wire three in series to a 9V battery. They wouldn't be at full intensity, but would light. Additionally, if one goes bad, or you lose a conection, the circuit would open and you'd lose the entire string.

Scottie

robiwon · Post by **robiwon** » Sat Jun 02, 2007 12:56 pm

I have 10 LEDs running off one 9v in my Martian War Machine. I don't leave them on but it's the same battery since I built it several months ago.

BERT aka MODEL MAKER · Sun Jun 03, 2007 5:25 pm

And fantastic lighting work you do, robiwon !

robiwon · Post by **robiwon** » Tue Jun 05, 2007 6:59 pm

Thanks Bert!

BERT aka MODEL MAKER · Wed Jun 06, 2007 5:07 am

Your welcome my friend

Gonna start on the seaview tomorrow, my hand & arm want to start modeling again, i had to surrender my commercial driver license along with all of my endorsements and special certificates yesterday due to not being able to pass the DOT physical

oh well, some extra time for modeling.

robiwon · Post by **robiwon** » Sat Jun 09, 2007 6:31 am

Well, there is a purpose to everything I say. Hang in there my friend!

Madman Lighting · Post by **Madman Lighting** » Sun Jun 10, 2007 1:44 pm

Back to the original question.....

"How many 5mm white LEDs can be driven from a 9V battery"

The simplest answer is to hook them in series and then you can get three. You should also use a current limiting resistor.

Lesse: 9V - 3* 2.6V = 2.0V and, 2.0V @ 20mA, R = V/I so we have 0.1K ohms aka 100 ohms.

Final answer: Three LEDs plus a 100 ohm 1/4 watt resistor will do ya.

Or, you can buy my kit and have 12 white LEDs and not have to know electronics math.

Scott Hasty · Post by **Scott Hasty** » Sun Jun 10, 2007 2:46 pm

Actually it's best to wire the LEDs in parallel and wire your limiter in series to the power supply and LED's

Scottie

Madman Lighting · Post by **Madman Lighting** » Sun Jun 10, 2007 3:10 pm

Actually it's best to wire the LEDs in parallel and wire your limiter in series to the power supply and LED's

How you figure? If you put diodes of any kind in parallel you should have ballast resistors on each one to make them share the current. Otherwise you'll have small variations in diode voltage causing the one with the lowest voltage to hog the current and overload.

Scott Hasty · Post by **Scott Hasty** » Sun Jun 10, 2007 7:05 pm

Madman Lighting wrote:
Actually it's best to wire the LEDs in parallel and wire your limiter in series to the power supply and LED's
How you figure? If you put diodes of any kind in parallel you should have ballast resistors on each one to make them share the current. Otherwise you'll have small variations in diode voltage causing the one with the lowest voltage to hog the current and overload.

NEVER and I mean never have I had a problem with "variations" in diode voltage (that's 26+ years). Today's manufacturing of diodes (and other electronics) has EXTREME tolerances and various strict quality standards (Six Sigma, ISO, etc).

You wire them in series and you lose a resistor, an LED or have a bad connection open, and you loose everything. I've wired my LED's using the method I mention and have never had the problem you speak of.

I hate using the word never, but in this case, it is true!

Scottie

Madman Lighting · Post by **Madman Lighting** » Mon Jun 11, 2007 3:36 pm

Yikes! I stand corrected!

Scott Hasty · Post by **Scott Hasty** » Mon Jun 11, 2007 9:52 pm

Madman Lighting wrote:Yikes! I stand corrected!

Dude!! Sorry, seriously, if I came off as a hard a$$! I just hate to see someone cram miles of wire and electronics into a model to have it work till the weakest link fails.

To give you an example, I'm working on a PL 1/1000 TOS E. In the saucer is 12 white LED's. All wired in parallel with one resistor controlling the current in series. It went strong for nearly a month, till the AA's died.

Again, don't feel intimidated; that was my mistake for not making my message more light-hearted. Anywho..hope your projects go well!

Scottie