LED wiring and resistance

[Anonymous]

LED's have no "resistance", unless you want to call a complex equation as a resistance. It's basically a step function, above a certain voltage drop it'll conduct. Below a certain voltage it won't. Same thing as a diode, but a poorly made diode at that.

Mark in Utah

[Anonymous]

Im an MET (fourth year, of five), and am in a nasty Electronics class right now, so all this stuff is still very, very fresh. Actually, just had EE lab a few hours ago.

[rtesta]

all, thanks for the help. one thing i couldnt find or figure our was the likely resistance of the LED's, i'm glad you were able to provide. while all the info was a help, the lesson on the math will get me through it the next time.
you can either build a rr for someone at one time or teach them to build it themself for ever!

i have forgot all this since my college days, + i'm an ME not EE. never liked the electrons much. you just cant touch them, although sometimes they touch you, w/a bang!

[Anonymous]

Remember, V=I*R
V is volts, R is resistance (in ohms), And I is amps (in amps). If you know your voltage and amperage, you can easily figure out your required resistance.

EDIT** Didn't see BP's post. He beat me to it.

Also, your resistance values will change depending on if you have two or more LED's wired in series or parallel.

[bp020897]

LED are polarized. If you hook them up backwards they won't work. To determine polarity the lead wire nearest the flat spot on the led is the Cathode or negative lead. If the leads of the LED have not been cut, the longest lead is usually the Anode or positive lead.
To calculate the resistance use Ohms Law.
Resistance = the supply voltage minus the voltage of the LED(s) and divide that number by the current of the LED. The current of your LEDs are most likely 20 ma.
So 12v - (1.4 +1.4) = 9.2v divided by 20 (.02A) ma = 460 ohm1/4 watt resistor.
A 460 ohm resisitor is yellow-blue-black.

The lead of the LED which you hook-up your resistor is optional.

[Anonymous]

I'm sure there must be others, but the Bad Boys is the one I know. The Bonjoes thing would be impossible to remember......

[oleirish]

I use 680ohm (red blue sliver or gold)1/4watt on my tortoise switchs and 5MM red and green LEDS.
JIM

[Medina1128]

BBROYGBVGW - Big Brothers Really Offer You Great Banjoes Very Good Water...[:)]

[cacole]

Randy, you mean the 'Bad Boys' routine? I haven't heard that one used in some time now.

[rrinker]

If you put the LEDs in series with the Tortoise motor, you do not need a resistor. The resistence of the Tortoise motor limits current in the LED.

I'd post one of the mnemonics I use to remember the color code, but they are all too off-color to post in a family forum such as this.

--Randy

[Anonymous]

The brightness of an LED is proportional to the current flowing through it. Most LED's can handle up to 30 mA, and I shoot for about 20 mA.

Put the LED's in series with the tortoise switch machines (the two in parallel but reversed polarity). No resistor needed. I have done this AND wired a 1000 ohm resistor across the tortoise to increase the current through the LED, but I was using a 10.5 volt power supply and my LED's had a 2 volt drop across them. I was seeing 15 mA without the additional resistor.

If you want to wire the LED's across the tortoise, I'd use a 560 ohm resistor, as a larger value will leave the LED dimmer. For me, brighter is better.

Mark in Utah

[nbrodar]

I use 470 ohm resistors - yellow, purple, brown. Some prefer 560 ohm ones - green, blue, brown. Read the bands from left to right, with the silver or gold band to the right (which for MR purposes we can ignore).

What are you using to control the LEDs? The contacts on the switch machine, or the polarity change of the panel switch? The easiest is to use the panel switch, in which case you can wire the LEDs back to back, and only need one resistor per pair.

Nick Brodar

[cacole]

A 1K Ohm resistor should be sufficient, and it doesn't matter which lead it is connected to. The color code for 1K is Brown-Black-Red. A fourth band designates the tolerance, with Silver for 10% or Gold for 5%.

[rtesta]

I just got some 1.4V LEDs for wiring to my dpdt tortoise switch machines. I plan to use red and green on the panel for each direction of the turnout etc. what I cant seem to find is the right resistor, which I beleive I DO need, to wire in series with the negative (short arm) LED lead?

I have a 12V power supply for the switch machines and know I have to bump the voltage down, any help?

finally, can you tell me the right color bands for thr proper resistor? theres a bunch of this stuff laying around here but nothing marked, I'll have to rely on the bands, and of course, all the *** smart guys here cant answer my questions, have to rely on all you smart guys on the forum!

thanks bobt