I have a 10vdc bipolar supply to run my tortoise switch motors.Being the power supply has 3 wires(+), (–) and(+/–)would I be able to use just the + and-- leads(not using the +/-- at all ) to operate LED signals?
Yes but the voltage will be 20 volts. (+) to (+/-) is +10 volts. (-) to (+/-) -10 volts. (+) to (-) is 20 volts. Size the bias resistor using 20 volts.
Or add a regulator circuit for the LED power.
While waiting for a reply,I messed around up in the train room.*** those LEDs blow out easily.Ned you are right it is 20 volts…It’s a s l o w learning process for us “older” folk.I will get it after awhile,but it’s costing me a bundle in Oregon,Tomar,NJ International and just stand alone red and green LEDs and assorted resistors.
ISN’T the (+/-) a Common or ground wire? If so,
A (+) or (-) to ground should produce RED or GREEN in a 2 conductor BI polar LED with the appropriate resistor.
The AMBER is the ‘toughie’, requiring either AC (2 wire) or Combining (3 wire) electronic’s to get. On most home layout’s distances aren’t long enough for 3 aspect signalling, or the increase in cost’s. Your Tortoise machine’s contact’s would suffice with proper wiring and resistor’s.
Best Advice is to skip 3 Color indication , and use 3 conductor LED’s.
Someone recently posted a diagram (from Tony’s?) showing wiring for a Tortoise.
(K.I.S.S.)
Hold on, it OUGHT to work exactly the same as using a DPDT toggle wired for reversing and a single power supply. You shouldn’t be getting 20 volts to the Tortoise. One pin of the Tortoise should go to the power supply common. The +10 should go to one side of a SPDT toggle, and the -10 should go to the other side. The center terminal on the switch goes to the other Tortoise terminal. If you put a 2-lead bicolor LED, or two regular LEDs back to back (anti-parallel) in series with the Tortoise motor lead you will have an indicator that sees 10V one way or the other, protected by the resistence of the Tortoise motor to a safe current level.
–Randy
Rinker,Yes that works,but instead of unsoldering the connections,I thought that I might have been able to connect something up to the 2 (or in this case 3) wires that I use to drive the tortoises.It would be easier to strip the 18g wires and solder to them directly.
Nothing is easy…I just figured out how to use my rotary phone!
The LED’s are probably designed for 5 volts so you have to figure out which combination of wires you’re going to use and then adjust resistor(s) to ensure the LED’s see no more than 5 volts in any direction. If you’re using a bicolor LED one color will be driven by +5 volts, one color will be driven by -5 volts, and “yellow” is the result of rapidly switching between +5 and -5 volts such as AC voltage. Exactly what you need to do is specific to the LED’s you’re using and I’m not an electrical engineer but you canNOT exceed 5 volts at the LED itself without blowing it (by the way an LED is a semiconductor diode that gives out light of certain color(s) when voltage is applied so think little diode or transistor when you’re dealing with it).
the power supply you have is called a center tapped P/S. The +/- is nothing more than the center tap. That is why you will get +20 from +- +10 from ++/- and you should get +10 from +/-_- if you keep the plarities going the same direction. if you want or need any negative voltage simply reverse the wires. As for the LED colors you get RED and Green from using the center and one outside lead. (remember LEDs are diodes and will only work if the polarity is going the correct direction) and you get AMBER by using both RED & Green at the same time. You do not need an A/C signal to get amber that will only cause the LED to flicker. Most LEDs are rated at 1.5V, 2.5V and 5V DC and the mA varies depending on how they were designed. if you need to know how big of a resistor you need to get the voltage down to the right size you can use a variable resistor (Potentiometer or Pot) and a multimeter and turn the pot up or down till you reach the desired voltage then take a reading of the pot and get a resistor that is the same approx value going a little higher in the resistor to be safe. Or you could just leave the pot in the circuit. Remember that the rated voltages on a LED are the maximun safe voltage ratings going a little (1/4 to 1/2 volt) less will increase the life of the LED by alot.
There is a relatively simple formula that will allow you to get it right the first time and not create the “it sure was bright for a little while” light show. Most have probably heard of Ohm’s law but here is a perfect place to use it. The basic law goes E(electromotve force or voltage) = I (don’t ask me why “they” use “I” for amperage) x R ( resistance). The most important value in the circuit is the amperage. Regardless of the voltage rating, if you exceed the amperage, you have a burned out LED. Voltage comes more into play if you expect to use the diode action of the LED to prevent current flow. Let’s say we have an LED that is good for 60ma (.060 amps) and a 10 volt dc power supply. If we juggle the equation to seperate out the resistance,the item we are looking for, we get R=E/I. Substituting our known values gives R = 10/.060 or 166.6667. A 180 ohm resistor will do the trick. In actual fact the current will be somewhat less than .060 amps because we assumed no voltage drop across the LED which is not true. But, using this formula, you won’t burn out any diodes either!
Not quite correct, if the power supply is 10v, you have to figure in the voltage drop across the LED, typically around 1.9-2.1 volts for a regualr color LED and most white LEDs are closer to 3 volts. Then use that to figure out how much current the resistor must dissipate.
Not too many LEDs can handle 60ma. 20ma is usually the max for standard LEDs, so I usually calculate for 10ma, which gives some room to play with without blowing the LED.
So, assuming a bi-color LED with a 2.1v drop, a 510 ohm resistors results in about 16ma, a 680 ohm resistor about 12ma. (510 and 480 are standard values, even if Radio Shack might not have them). Put the resistor in series with the LED, and connect this assembly across pins 1 and 8 on the Tortoise and you will have an indicator that shows green for one direction and red for the other (other color mixes of bi-color LEDs are available, in some cases). Or pick two LEDs of whatever color you want and conenct them anti-parallel (anode of one to cathode of the other, and vice-versa) and put the resistor in series with this assembly and connect as above - you just made your own bi-color LED using the colors you want.
–Randy
I did acknowledge the fact that I ignored the voltage drop across the LED. Technically, a resistor does not dissipate current. In a series circuit, which this is, the current is the same throughout the entire circuit. The voltages vary across each component of a series circuit. Wattage or heat is what is dissipated. Again to be technical, what we are actually calculating is the amount of resistance we need at the given current to drop the excessive voltage across the resistor instead of across the LED. This having been said, ignoring the voltage drop across the LED would cause the amperage to be less than calculated and hence “safer” for the LED. The calaculations I had made, if one takes the voltage drop to be 2 volts( in the middle of the range you suggested for colored LEDs) the actual current would be more like 44ma (.044 amps). As to the amount of current for a particular diode, they vary all over the map. Each LED has its own parameters and must be treated accordingly. Admittedly you probably won’t see a 10amp LED with heatsink as a headlight on the front end of an “N” scale loco, but they do exist. The older LED’s that I started playing with were rated for 60ma. As much as some would have us believe that electronics is an exact science, it is not. The very fact that you gave a range of 1.9-2.1 volts drop across the LED is proof of that. If you are going to use one of the extremes of the voltage range, use the lower end as this will result in a cooler running LED and longer life. My suggestion was mearly that, a suggestion to take out some of the pure guess work suggeted in one of the earlier posts. By the way, if one figured 44ma through a 180 ohm resistor, one would need a 1/2 watt resistor. The formula for that is I x I x R= W. Your much lower currents could use a 1/4 watt in the case of th 16 ma draw or 1/8 watt in the case of the 12 ma draw. Do NOT use the E x I = W to calculate resistor wattage as this will often result in smoke being released from the resistor.
Mike,
I just finished wiring a bunch of signals using the set up you describe. The difference is that I was using switchmaster motors rather than tortises. I believe that tortise includes an explanation of how to use their contacts for running LED signals. Since switchmasters are very spartan (no contacts) I used a different method. I used Sunrise Signals bi color LED target signals. These signals have three leads: One is a common ground and there is one wire for red and one for green. I hooked the common ground from the signal to the appropriate resistor and then hooked it up to the common ground from the power supply. I then tested the signal to see which pole (neg. or positve) made the signal light. I then hooked it to the middle pole of a DPDT switch. I hooked the appropriate green and red wires from the signal to the other poles. I have wired several signals to show positions of hidden switches. They look cool and actually serve a purpose. I have also used tripple throw DP switches to do this with one set of poles for frog power one for the switch machine and one for the signal.
Hi,
I come perhaps a little bit late but you have to take care when handling led, never use a current greater than 20 mA (standard manufacturer working value for single chip led) otherwise, you decrease life expectancy.
Just go there http://dccflash.free.fr/DC&C_Station_Eng.htm and click on the “Miscellaneous” button and scrolling down you’ll get information and formulas to calculate resistors for led application.
In the April 2000 MR page 83, there is a led killing machine with a lot of wrong information. The December 2004 page 98 is a good issue.
Patrick [:)]