Custom LED Circuit - Wire Gauge?

[Al Skierkiewicz]

Larry,
If I understand you right, you are saying the LEDs you are planning on using drop 3.1 volt per LED? Can you let us know the part number for these? If you connect 4 3.1 volt drops in series, that will be 12.4 volts. If the battery is fully charged when you turn them on, they will draw lot's of current. When the battery voltage drops, they will shut off or be very dim. What you want is two or three in series with an appropriate sized resistor in series. It then sound like you are going to wire two or three of these series strings in parallel, is that correct? How big are these displays going to be? Are the LEDs mounted to something? A circuit board perhaps?

[ratdude747]

they were cheapies from ebay in china... the price was irrestably low and a mentor's grown up son had bought some and had no issues.

they are not here yet. for now, i made a test array out of 4 ultra brights with 3.1v. they are powered by the 22 gauge wire. it has been working fine.

the size has not been determined yet. each display will be about 5-6" diagonally. they are meant to be visible half a field away.

the led's would be mounted in a plastic panel, with either hot glue or a second panel to insulate the solder joints.

i was trying to avoid resistors... i figured that the correct number of LED's in series would be the best solution.

[Al Skierkiewicz]

Originally Posted by ratdude747

i was trying to avoid resistors... i figured that the correct number of LED's in series would be the best solution.

I question the 3.1 volt spec. Normally I see a drop of less than that, but these may have a built in resistor or current limit built in. Working with LEDs it is good practice to add the series resistor to prevent over current. Can you provide a link to where you bought them.

[ratdude747]

i tried to find the listing to no avail. lots of copycats though.

they were discrete led's. no built in resistor. they were rated for up to 3.6 v, but dissapate 3.1v.

the blues were a similar story.

i think they are bright LEDs. hence the high voltage drop.

[Al Skierkiewicz]

OK,
You are buying "Super Bright" LEDs. They do have a higher voltage drop but the same rule applies for current limits. You do want to have a resistor in the string to keep the current at specified level, usually in the 20-40 ma range. If you are making each shape with parallel strings, then the wiring within each shape can be #22 with the resistors wired right at the string. If you wire each shape back to a Spike, make that wiring #18. Each shape is then the custom circuit and all rules are satisfied.
The only caveat I can supply at this point is the relative brightness will be a subjective thing. If the inspector, ref or FTA think they are too bright or if a team complains they are too bright, it will be up to you to correct the problem. That will mean, make them dimmer, or remove the breakers that feed them.

[DonRotolo]

Originally Posted by ratdude747

i was trying to avoid resistors... i figured that the correct number of LED's in series would be the best solution.

Do not try to avoid using a resistor. LEDs are not linear devices, meaning that when they get a little warm, their current consumption increases. More current leads to more heat leading to more current - this is known as thermal runaway. It ends in smoke.

Go ahead and use your four LEDs in series, but then add a small-value resistor - say, 20 Ohms or so (do the math) - in series. This will prevent thermal runaway.

The voltage drop across an LED is determined by its chemistry (and therefore the color it produces). Blue & White run at about 3.5 volts, red runs at about 1.8 volts. This is a physical quantity, not generally subject to change. Keep this in mind.

My advice is to use a milliammeter to measure the circuit current, and adjust your circuit to reach the necessary current, ignoring voltage.

Oh, and if you want, you can use ONE spike to power 2 colors. Hint: what's the "D" in LED stand for??

[ratdude747]

i know that. I also don't like backfeeding LED's.

if the LEDs were not ordered yet, i will see about getting the kind with the resistors pre-installed (ebay has ones like that for 12v). if they were ordered, i will toss in a resistor.

I forgot about diodes not being linear... the flu must have rotted my brain.