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Model Forum / General / Railroads / March 2008LED signals with tortoise swiches
I got my tortoise switches all working great. I even have a double crossover that operates from a single toggle. Now, I would like to make some dwarf yard signals to show how the switches are thrown. I have wired colored LEDs to the switch and they seem to do OK. Wondering about the proper Ohm resistance for the lights. Currently I am using 1/2 watt 220 ohm and it all seems to work good. I want to be sure the bulbs don't burn out too soon - they're a little too bright. Any help welcome. > I got my tortoise switches all working great. I even have a double crossover > that operates from a single toggle. [quoted text clipped - 8 lines] > > Any help welcome. Increase the resistor to decrease the voltage across the led, thus the brightness. If you have them, a potentiometer works great for a test, then just measure the resistance after you disconnect it, and replace with fixed resistor. Big Al (BigAl@nowhere.com) said... >Increase the resistor to decrease the voltage across the led, thus the >brightness. No, it is the current through an LED that determines its brightness; the voltage across it is relatively constant, being a diode. Typically, an LED will have a forward voltage drop of about one volt, and as a rule of thumb, I would light it with 10 mA of current. Though, you are likely safe from burning it out anywhere between 5 and 20 mA. You can experiment to see what brightness you get from a given LED for a given current. To do this, take the voltage you are supplying and subtract 1. Take that value and divide it by 0.01 (multiply it by 100). That will give you the resistance needed to limit the current from that voltage supply to 10 mA. For example, if the supply is 6 volts, then take 5 and multiply by 100 to get 500 ohms. The nearest resistor would be 470 ohms (or 510, if you have 5% tolerance resistors). The only other thing to be aware of is the power rating of the resistor. It must be sized to be able to dissipate the power it will draw when the LED is lit. This is the square of the current multiplied by its resistance. In the case above: 0.01 * 0.01 * 470 gives you 0.047 watts, so a 1/4 watt resistor is more than ample. Power the same LED off of an 30 volt supply and you get the following: Resistor = (voltage - 1) * 100 = 2900 ohms (2k7 is close) Power = 0.01075 * 0.01075 * 2700 = 0.311 watts So, a 1/4 watt resistor is not good enough, 1/2 watt is needed  SignatureCalvin Henry-Cotnam "As I listened to the Leader of the Opposition, it reminded me a little of the professor who goes through our term paper, marks all over it everything he disagrees with and then passes us anyway." - Right Hon. Stephen Harper, October 17, 2007 ------------------------------------------------------------------------- NOTE: if replying by email, remove "remove." and ".invalid" -- Posted via a free Usenet account from http://www.teranews.com > Typically, an LED will have a forward voltage drop of about one volt, No allmost 2 unless it is a white or blue. Klaus SignatureModelbane Europas hjemmeside: http://www.modelbaneeuropa.hadsten.dk Modeltog, internet, gratis spambekæmpelse, elektronik og andet: http://home6.inet.tele.dk/moppe On 3/29/2008 1:50 PM Klaus D. Mikkelsen spake thus: >> Typically, an LED will have a forward voltage drop of about one volt, > > No allmost 2 unless it is a white or blue. It's actually 1-point-something (or 1-comma-something for Yurpeens). Varies from type to type. > It's actually 1-point-something (or 1-comma-something for Yurpeens). > Varies from type to type. Yes but closer to 2 than 1 volt. http://www.kpsec.freeuk.com/components/led.htm Klaus SignatureModelbane Europas hjemmeside: http://www.modelbaneeuropa.hadsten.dk Modeltog, internet, gratis spambekæmpelse, elektronik og andet: http://home6.inet.tele.dk/moppe > > It's actually 1-point-something (or 1-comma-something for Yurpeens). > > Varies from type to type. [quoted text clipped - 3 lines] > > Klaus Voltage drop and current handling ability will vary by LED type. This page http://members.misty.com/don/ledc.html has a pretty good breakdown on the various types/colors and their ratings. And when I said I general use 5 - 12VDC with resistors in the 330 - 470 ohm range for LEDs, maybe I should have clarified a bit. I use 330ohm with 5 - 8VDC, 470ohm for 9 - 12VDC. I'm using older LEDs from a bulk pack I bought years ago that have a max current rating of 50ma. None have burned up on me yet. Len David Nebenzahl (nobody@but.us.chickens) said... >On 3/29/2008 1:50 PM Klaus D. Mikkelsen spake thus: > [quoted text clipped - 4 lines] >It's actually 1-point-something (or 1-comma-something for Yurpeens). >Varies from type to type. I was giving a "rule of thumb" example, so for the most part whether one uses 1 or 2 volts, it will have little effect as far as pushing the current capacity of the LED or power capacity of the resistor. If one wants to do something so close to the limits of the devices, then more accurate calculations using more accurate values for the parts in question will be needed. Generally though, rules of thumb procedures have 100% (or greater) fudge factors. SignatureCalvin Henry-Cotnam "As I listened to the Leader of the Opposition, it reminded me a little of the professor who goes through our term paper, marks all over it everything he disagrees with and then passes us anyway." - Right Hon. Stephen Harper, October 17, 2007 ------------------------------------------------------------------------- NOTE: if replying by email, remove "remove." and ".invalid" -- Posted via a free Usenet account from http://www.teranews.com > Big Al (BigAl@nowhere.com) said... >> Increase the resistor to decrease the voltage across the led, thus the >> brightness. > > No, it is the current through an LED that determines its brightness; the > voltage across it is relatively constant, being a diode. That is a very important point. LED's are not light bulbs. They are rectifiers, diodes, that just happen to glow when they conduct. Once a diode switches on, at acts like a closed switch, a short circuit, and huge amounts of current will flow. Followed by a small cloud of smoke. You must have a current limiting resistor in series with a LED, you cannot bias them by applying just the right voltage. > Typically, an LED will have a forward voltage drop of about one volt, and > as a rule of thumb, I would light it with 10 mA of current. Though, you > are likely safe from burning it out anywhere between 5 and 20 mA. You can > experiment to see what brightness you get from a given LED for a given > current. Also an important point. Always compute your current limit resistor to limit the current to your design value. 10 mA is a good design value, the LED will glow plenty bright enough for model railroading purposes. Don't calculate for voltage, always calculate the current. > To do this, take the voltage you are supplying and subtract 1. Take that > value and divide it by 0.01 (multiply it by 100). That will give you the > resistance needed to limit the current from that voltage supply to 10 mA. That's the right way to do it. Actually you can neglect the LED forward bias voltage completely and get an answer good enough for government work. After measuring the forward bias voltage of many LED's over the years I use 2.2 volts for the forward bias voltage of the typical colored LED, but 1 volt is close enough. > For example, if the supply is 6 volts, then take 5 and multiply by 100 to > get 500 ohms. The nearest resistor would be 470 ohms (or 510, if you have [quoted text clipped - 10 lines] > Power = 0.01075 * 0.01075 * 2700 = 0.311 watts > So, a 1/4 watt resistor is not good enough, 1/2 watt is needed In catalogs and data sheets I have seen many LED's specified to have a forward bias voltage as high as 3.75 volts, but I have never encountered such a high voltage on the bench. The high voltage is a worst case rating. The maker wants to be sure that all his LED's will pass incoming inspection at the customer's site. So he specs a high voltage, which every LED he ever made, going right back to the invention of the LED, will pass. You don't want to design with the specified worst case voltage, stick with 2.2 volts and things will work just fine. SignatureDavid J. Starr Blog: www.newsnorthwoods.blogspot.com > Wondering about the proper Ohm resistance for the lights. Currently I am > using 1/2 watt 220 ohm and it all seems to work good. I want to be sure the > bulbs don't burn out too soon - they're a little too bright. Higer resistance - lower light. You mention the resistor but not the voltage driving the circuit. Klaus SignatureModelbane Europas hjemmeside: http://www.modelbaneeuropa.hadsten.dk Modeltog, internet, gratis spambekæmpelse, elektronik og andet: http://home6.inet.tele.dk/moppe >>Wondering about the proper Ohm resistance for the lights. Currently I am >>using 1/2 watt 220 ohm and it all seems to work good. I want to be sure the [quoted text clipped - 5 lines] > > Klaus The VOLTAGE is what determines the STARTING POINT for resistor selection. You are reasonably close with 220 ohms (or you would be 'blowing' LEDs now. To decrease the intensity, increase the ohms value of the resistor. You will know when you have gone too far, because you won't have much output from the LEDs. Chuck D. > >>Wondering about the proper Ohm resistance for the lights. Currently I am > >>using 1/2 watt 220 ohm and it all seems to work good. I want to be sure the [quoted text clipped - 15 lines] > > Chuck D. On circuits running on 5 - 12VDC, depending on how bright I want them, I generally use 330 - 470 ohm resistors with LEDs and haven't had any problems. Len > On circuits running on 5 - 12VDC, depending on how bright I want > them, > > I generally use 330 - 470 ohm resistors with LEDs and haven't had any > problems. 12 volts 330 ohms and 2 volts for the LED gives you something like 30mA/LED (buuuurn) 12 volts and 1000 ohms usually works for Me. Klaus SignatureModelbane Europas hjemmeside: http://www.modelbaneeuropa.hadsten.dk Modeltog, internet, gratis spambekæmpelse, elektronik og andet: http://home6.inet.tele.dk/moppe Thanks guys, Using 9v 500 mAmp power from old walkman as power supply. As I add more signals I assume I will pull more current from the switch motors and all the lighting will get dimmer with each addition. > > On circuits running on 5 - 12VDC, depending on how bright I want > > them, [quoted text clipped - 8 lines] > > Klaus > Thanks guys, > Using 9v 500 mAmp power from old walkman as power supply. > > As I add more signals I assume I will pull more current from the switch > motors and all the lighting will get dimmer with each addition. Put the lights on a separate power supply. Signaturewolf k. > Thanks guys, > Using 9v 500 mAmp power from old walkman as power supply. 9 volts - 2 volts for the LED = 7 volts. 7 volts / 220 ohms = 32mA for the LED = (for normal LED's) very short lifetime. 550 Ohms should be fine, that gives you 12-13 mA/LED and more than sufficient light. Klaus SignatureModelbane Europas hjemmeside: http://www.modelbaneeuropa.hadsten.dk Modeltog, internet, gratis spambekæmpelse, elektronik og andet: http://home6.inet.tele.dk/moppe > Thanks guys, > Using 9v 500 mAmp power from old walkman as power supply. > > As I add more signals I assume I will pull more current from the switch > motors and all the lighting will get dimmer with each addition. Max LED brightness is achieved at 20 milliamps thru the LED. For model railroading you may find full brightness a bit more than you like. To figure the current in the LED, use a meter to measure the voltage across the dropping resistor. Calculate current with Ohm's law (I = V/R). Current thru the dropping resistor is the same as current thru the LED. Naturally increaseing the resistor will lower the current in the circuit. If memory serves, Tortoises draw about 30 millamps all the time. Your 500 mA power supply is supposed to keep its output at 9 volts until the load exceeds 500 ma . That's about 15-16 tortoises. When the load exceeds 500 mA, the power supply may do any one of number of things. If may continue to furnish current and heat up. It may decide life is too tough and turn itself off. It may allow its output voltage to sag off from 9 volts. SignatureDavid J. Starr Blog: www.newsnorthwoods.blogspot.com |
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