I am needing to reduce my power supply from 12V DC at the source to 2V DC to run my LED indication lights on my control panel.
Am I needing to install some kind of resistor / device on my supply bus to prevent burn out?
Transformer is 12V DC 1000mA. I require a drop to 2V DC 15-30mA Is this possible to use this transformer with the above.
Thanks Chris
Cheers...
Chris from down under...
We're all here because we're not all there...
Yes, you need a resistor in series with each LED. 680 ohms would be a good value to use based on the numbers. One per 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.
Thanks Randy.
have looked at a few of the examples around the threads and websites but it was all foreign language to me. Some say 1080 ohms some say 890ohms. I will just get 3 different resisters and test them at the outlet.
My enthusiasm is no match for experience.
Thanks again
Chris
Which makes me wonder, can you purchase LEDs with built in resistors?
Rich
Alton Junction
You probably can purchase them with resistors, but *WHY* would you do that. Those would be made to some specification that might not be what you really wanted after all.
In a perfect world you would buy LEDs with known specifications, and then run some numbers through some mysterious legerdemain which produces a number equal to the perfect resistor.
We live not in a perfect world, I buy LEDs from Menards during the Christmas season, keep the LEDs and throw away the rest of the light set. Who in the world knows what the actual specs of the LEDs are. LION uses a 1KΩ resistor which works perfectly with these LEDs and the colored LEDs that I buy in bulk for wayside signals from All Electronics.
It works, and what more could you ask from a circuit.. LION sees no point in doing otherwise especially since you can buy the 1KΩ resistors for about $2.,00 per thousand..
ROAR
The Route of the Broadway Lion The Largest Subway Layout in North Dakota.
Here there be cats. LIONS with CAMERAS
Yes, you can buy LEDs with resistors already in them, those are the ones sold as "12 V" LEDs. But like the Lion says, why? If you are building a large panel, or installing LED headlights in dozens of locos, you can buy the resistors in large quantities for a few cents each. Also a good reason to buy the LEDs in quantities also, and not in some big "grab bag" where the LEDs may have various differences in specs which would require different resistors to work and have similar appearance.
Back to Chris, the resistor value needed depends on the voltage of your power supply and the voltage drop of the LED, as well as the current rating of the LED. The LED current rating is a MAXIMUM, not a suggestion, so you want to stay well below that. You can run the LED at less than half the maximum and still get plenty of brightness. So my suggested resistor will put less than 15ma through the LED. Math: (power supply voltgae - LED forward voltage)/current desired = resistor value. Or flipping it, voltage/resistor value = current
So (12V - 2V) = 10V, 10v/680 ohms = .0147 amps or 14.6ma
The other factor to consider is that resistors don't come in every exact number, except very expensive and unecessary precision resistors. SO to runt he formula with 15ma, you get 10v/15ma or 10v/.015 amps = 666.67 ohms. Usually safest to go UP ot the nearest standard value, which for conventional 10% tolerance resistos is 680 ohms
You can easily get away with 1K ohm resistors as well, the result would be 10ma throught he LED, which should still be plenty. 1K also happens to work out well for installing most white LEDs as headlights on DCC decoders, so if you happen to also be doing that sort of thing - see what I mean about just buying a big bulk pack of resistors? 1K for enarly everything if you use those same panel LEDs everywhere plus white ones for locos. In say a pack of 100, the resistors should be a few cents each.
I am pretty sure someone tried to teach me this in school all those years ago but for now the equations have remained lost i my head, along with a pile of second guesses.
I am off to go find some PC board some time today in order to service all the LEDs. Understanding that there is a voltage drop on each bulb I will need to run a BUS to keep it all powered evenly.
I will try the 680ohm first. This will save me some$1****$2farting around.