To answer the original question - connecting in reverse won't harm an LED as long as the voltage is not higher than the Vrev max.
Connecting in correct polarity but without a *current limiting* resistor will ALWAYS blow it. Its not the voltage that drives the LED, its the current. If there's no resistor, hence, there's nothing to limit the current, hence...
Its easy to find out the correct polarity, its usually marked, and I'm sure you have enough advice by now. I would bother troubleshooting it. Just replace with a new one and good luck!
A LED is a Light Emitting Diode. If you connect it without a resistor, you will have a Darkness Emitting Diode, or DED. The bad news is that creating a DED is irreversible. The good news is that LEDs are cheap.
It takes an iron man to play with a toy iron horse.
RR_MelI’m lucky to remember who I am let alone Kirchoff.
i remember the equations and principles. i rarely remember their names; Newton's 2nd Law
greg - Philadelphia & Reading / Reading
gregc peahrens My problem is that when I went to school, Ohm's law had long been around, but Kirchoff had not yet come along. Gustav Kirchoff (1845) -- wow, you must have been in school a long time ago
peahrens My problem is that when I went to school, Ohm's law had long been around, but Kirchoff had not yet come along.
Gustav Kirchoff (1845) -- wow, you must have been in school a long time ago
peahrensMy problem is that when I went to school, Ohm's law had long been around, but Kirchoff had not yet come along.
Randy, thanks for the through explanation.
My problem is that when I went to school, Ohm's law had long been around, but Kirchoff had not yet come along. Either that, or I forgot a few things!
Paul
Modeling HO with a transition era UP bent
When two things are in series, like the LED and the Tortoise, the same current flows therough both. Not a sum total of current. The Tortoise draws LESS when moving, a LOT less, so the LED gets a lot less current, and is dimmer. When the Tortoise stalls, it draws about 15ma, so the LED also gets 15ma, and glows fairly brightly.
Voltage in series adds - that's why when you apply 12V tot he circuit, the LED drops 2, and the Tortoise gets the remaining 10.
Those two concepts are Kichoff's Current Law and Kirchoff's Voltage Law.
Always remember - current is DRAWN by a load, it cannot be PUSHED by the supply - a Tortoise at a max of 15ma will work just fine on a 100 amp power supply. It won't melt (assuming it's a 12 volt 100 amp power supply). The danger is if something shorts out. A short circuit is extremely low resistance. That means it draws more current. If there truly was a 0 resistance short (there's always some, even if it's miniscule, resistance in the wire) the current draw would be infinite.
If voltage is constant, but the resitance goes down, the current goes up, and vice-versa. If you know any 2 of the three, resistance, current, or voltage,, you can calculate the missing one - this is Ohm's Law. E (voltage) = I (current) x R (resistance). Plug in the two you know, you can calculate the other. By combining Kichoff's Laws and Ohm's Law, this is how the typical use of a 1K resistor with a white LED on a DCC decoder is arrived at.
The function outputs on the decoder are at a little below track voltage. Let's say 12V. A typical white LED is around 3V. By Kichoff, if the supply is 15V, and the LED drops 3V, the resistor must drop 9V, because it is in series with the LED.
Now, the MAXIMUM current of the LED is typically 20ma. One thing about electrical and electronic devices is, you almost NEVER want to run them at their absolute maximum values.
Kichoff also says the current in the resistor is equal to the current in the LED. SO let's say we want to run the LED in the middle, 10ma. That gives plenty of room to go lower and it still light up, and plenty of room for it to go higher without blowing up. So back to Ohm's law, if we know we want 10ma (.010 amps) and we know there is 9V across the resistor, what value in ohms do we need? If V=I *R, then R = V/I. 9V/.010 Amps = 900 ohms. Resistors don't come in every possible numerical value - they have tolerances and so they come in values that only otuside the tolerance range different from each other. It's usually best to go with the next highest standard value, which in this case would be 1K.
If we plug that back in, and solve for what the actual current would be, we have I=V/R, or 9V/1000 Ohms, which is .009 Amps, or 9ma. A perfectly safe value for our LED.
Now, let's say someone goofs up and flips the DCC system to the O/G scale setting where the track voltage goes 4 volts higher. So now we have 16V, not 12V. Solve again with 16V, minus the 3 volts the LED drops, we have 13V. I=13V/1000, or 13ma - still safe for the LED.
If instead we tried to run the LED closer to 20ma, we have R=9V/.020A, or 450 ohms. 470 ohms happens to be the closest standard value. So we are actually running the LED at 9V/470 Ohms, or 19ma. Sounds fine so far. Now that switch gets bumped again and the voltage shoots up by 4V. Now the current in the LED is 13V/470 Ohms, 27.7ma. Oops. Bye bye LED.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
ndbprrI assume that connecting an led with reversed polarity does no harm since it is a diode and just requires the polarity to be reversed.
As long as the voltage is not too high it just does not light w/reverse polarity.
I have bi-color LEDs in the DPDT output to my Tortoise machines. The available circuit voltage is 12v DC and the LED (whichever one is lit) uses about 2v IIRC and the Tortoise (when not moving, at stall) about 10v. When the DPDT is thrown, it just reverses the outgoing polarity to the (in series) LED and Tortoise. Only red or green lights up, depending on the polarity. The one not lit is not harmed by the reverse polarity, and does not burn up at that reverse voltage.
When the DPDT is thrown and the Tortoise is moving (before stall, with the points closed), the LED is a bit dimmer. It then brightens when the Tortoise is finished moving / at stall. Perhaps someone can explain what's happening then. The circuit voltage drop is constant (the power supply is regulated) at 12v. Something is going on with the current and voltage drops I presume the Tortoise is drawing more current to move than when at stall, so it has a higher voltage drop, leaving less voltge drop for the LED, so it is dimmer??? I'm getting confused!
i often put a led and 1k resistor across DCC to check for power or frog polarity. (diode do have a reverse voltage limit)
This link should help you for measurements. I have four because I misplace them. One in the car. They are accurate. I made longer leads. I have more expensive meter but these are rugged.
http://www.trainelectronics.com/Meter_Workshop/index.htm
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
ndbprr I assume that connecting an led with reversed polarity does no harm since it is a diode and just requires the polarity to be reversed. What happens if an led is connected with no resistance? Does it blow the diode or is it reusable? Thank you
I assume that connecting an led with reversed polarity does no harm since it is a diode and just requires the polarity to be reversed. What happens if an led is connected with no resistance? Does it blow the diode or is it reusable? Thank you
You should Google LED and then Google diode.
Two different devices.
Some have reported the LED blew apart. When you move into DCC, you better know the basics.
Some have smoked a pricey decoder because they did not know what they were doing.
http://www.members.optusnet.com.au/nswmn1/Lights_in_DCC.htm
use a led calcultor to determine resistor value .. they vary depending on led color, and -most- run at 2 volt to 3 volt ....
if you run a current dropping resistor then they can be switched side to side with no damage , if you don't run a dropping resistor they -may- flash once, but then it's time for the garbage bin
http://led.linear1.org/led.wiz
Brent
"All of the world's problems are the result of the difference between how we think and how the world works."
If you are lucky it briefly lights for the final time. If not, wear safety glasses, people have posted that they have had them explode.
Henry
COB Potomac & Northern
Shenandoah Valley