jeffers_mz wrote:OK...seems to me, you could just keep wiring LEDs in series till you sopped up all the available voltage drop and limit the current that way, without a resistor.
OK...seems to me, you could just keep wiring LEDs in series till you sopped up all the available voltage drop and limit the current that way, without a resistor.
Theoretically yes
I'm guessing that's a bad idea because of...headroom? The resistor can handle slight fluctuations in potential while the finer tolerance LEDs can't?
Not really. As long as you do not exceed max LED current you are OK. Run them a bit undervoltage. the resistor just limits max current
If true, that leads to the next q. As a fraction or percentage, how much of the total voltage drop does good practice suggest you soak up with the resistor, and how much do you soak up with the LEDs? Half and half? One quarter/three quarters? Which way? In theory, not practice, since I've never done it, the idea of wasting power with a resistor, just making heat doesn't sit well with me. It's inefficient, and I don't like the idea of deliberately creating heat on my mostly wood layout inside my house. I know, I know, it's "on-purpose" heat, precisely what the component is designed to do...I know what I'll just fuse the whole dealio and sleep like a baby. Onward.
Theoretically 100% in LEDs and 0% in resistor. You're correct about wasted heat. Ideally, you feed the series string of LED's from a constant CURRENT supply (not a fixed voltage supply) with no resistor. These are hard but not impossible to find.
Second q...different color, brightness, and design LEDs on one series/parallel circuit like the one at the bottom of your linked page...a whole town's worth. Will minor series leg current variations cause major array problems? Perfectly matching one kind of resistor and it's dependant LEDs with a different series of LEDs and it's resistor won't be possible, not perfectly anyway. I can do simple Kirchoff calculations, but not project how the array will behave with real confidence. How close to I have to match each of the series legs in drop to keep the current from freaking out?
Different colors of LED "drop" (a bad term) different amounts of voltage at similar current and brightness. You need to figure this in or use a constant current supply.
Third q...Simplicity says a high voltage and high current PS is better to drive a whole town's worth of LEDs than several smaller PSs, each with it's own on-off switch, fusible link, etc. Wait...I think I have the answer here...the PS potential needs to be defined by the series leg voltage, and the current needs to be at least the sum of all the series legs in parallel, right?
EUREKA! Also each leg must have the same voltage requirement as the others.
Theory aside, there are also practical considerations. For example, if one LED in a series burns out (opens) then the entire string will go dark. You will then need to find the bad one.
Therefore it's best not to use too high a voltage and too many LEDs in series. Serveral legs with independent switches is a good idea form a controllability standpoint too.
I hope this helps some.
Karl
The mind is like a parachute. It works better when it's open. www.stremy.net
jeffers_mz wrote:What I really need is a simple wiring schematic, ...I can read a simple diagram, but not a complex one.....I THINK the reason I haven't seen such a simple diagram is that the issue is more complex than I wish it was,
But I agree with a prior poster. As long as one considers it complicated it is. When one looks at each piece of a circuit diagram as a separate thing it is actually quite easy. Draw a single LED circuit with the on off switch. Figure out how to add a second simple one to it. Apply that and add a third. Pretty soon one has a complicated circuit by just adding simple things to the simple thing.
We have a similar issue at the club. Some members want the electrical system to do all these wonderous things, but those same members don't want to move out of the elementary school as far as learning how it works. They gripe when it doesn't work as fancy as they want. Change it and then they gripe when it is too complicated (for them) to fix when it breaks. Then they gripe when no one else will fix it because everyone else is tired of listening to all the griping and whining.
an LED is EXACTLY like any diode, and they use LITTLE current. A resistor in series prevent from blowing the diode. (LED) a 1K resistor aughta do you fine. The LED should drop 1.5 volts across it so 8 in series on 12 volts theoretically could work without resistors. An LED is very simple like any light bulb, it just only lights one direction of current. You could light a whole town on one cheapo plug in the wall power supply.
I think you're making things far too complicated. Use a current limiting resistor in line for each LED. If you want to run more LEDs off of one resistor you'll need to remember to increase the wattage of the resistor so that it has greater ability to dissipate the heat it will generate. I think you should also consider have more than one switch to control the lights in the town. This allows you to control the various lights independently of other light circuits You can also vary the ohms of the resistor to control the brightness and intensity of the LED. You also didn't mention what kind of LEDs you'll be using - white, blue white, yelloglow. The characterisitics of each may vary slightly. I'd also consider using smd LEDs. The other thing to consider is the angle of view, which can vary. When used to light a building you would want a small light source with a relatively wide angle of view to avoid a concentration of light.
Also, it is in the year 2525, not 2545.
jeffers_mz wrote: jbinkley60 wrote: Vail and Southwestern RR wrote:That one is pretty cool!One other common topic is whether to use a single resistor for a string of parallel LEDs or a resistor per LED. Unless we are talking about a couple of LEDs then always go with a resistor per LED and even with just a couple a resistor per LED is preferred. There are numerous reasons for this. Wanna be crystal clear on this...
jbinkley60 wrote: Vail and Southwestern RR wrote:That one is pretty cool!One other common topic is whether to use a single resistor for a string of parallel LEDs or a resistor per LED. Unless we are talking about a couple of LEDs then always go with a resistor per LED and even with just a couple a resistor per LED is preferred. There are numerous reasons for this.
Vail and Southwestern RR wrote:That one is pretty cool!
One other common topic is whether to use a single resistor for a string of parallel LEDs or a resistor per LED. Unless we are talking about a couple of LEDs then always go with a resistor per LED and even with just a couple a resistor per LED is preferred. There are numerous reasons for this.
Wanna be crystal clear on this...
Two LEDs in series, one resistor limiting current for both, ok, yes?
Yes, Ok. In series two resistors would add no value except to reduce the power dissapation by each resistor.
Two LEDs in series, times ten legs in parallel, using a single large resistor in-line with the PS on the main trunk, to control current across the whole array, bad idea, yes?
Yes, very bad idea
Engineer Jeff NS Nut Visit my layout at: http://www.thebinks.com/trains/
Hyun wrote: Jeffers,I've found this website highly useful in figuring out resistor requirements:http://metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng
Jeffers,
I've found this website highly useful in figuring out resistor requirements:
http://metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng
I pull that up, look at it, and I'm like jeeze...I'm learning to crawl and Hyun's handing me the flight manual for an F-22, but you know what?
I looked at it again and I actually understand it.
Good stuff, thanks.
Vail and Southwestern RR wrote: OK. As you suspected, there isn't one answer to the question. And, I'm going to start with a little text, to try to explain why, hopefully in an understandable way. The simple cases are pretty easy, and I did find some drawings for those, so that will be a place to start. We can go from there to more interesting, though someone else may have to pick it up since I'm going on vacation for a week and may not be posting. The first point is that LEDs are diodes. In general that means that they pass current in one direction, with a more or less fixed voltage drop across them. There is a region between 'off' and 'on' where things are not quite that simple, but as a rule we don't try to operate them there, so it isn't an issue. Given that they will pass current in one direction and not in the other, they like DC. AC can be used, but they will only be on half the time (usually that is plenty to see), and they are not rated to like even moderately high reverse voltages so they may need some protection from that. That can be gotten into later.Every LED has two ratings that we care about in our kinds of applications, the forward voltage, and the forward current. We don't plan to operate with reverse voltage on them (for now) so none of that is relevant.Here is an example datasheet:http://www.rohm.com/products/databook/led/pdf/slr-342.pdfWe'll use the red one for this example, the SLR-342VR. We don't want to push the maximum forward current, nicely this datasheet shows a typical value of 10 mA, with a maximum of 25 mA. (Ignore the peak value. that is referring to a pulse and we don't want to worry about that.) Often, only the max rating is given. Usually half of that is a reasonable operating state. The typical forward voltage is 2.0 V. Remember, that no matter how much voltage you try to put across the LED, it is only going to drop 2 V. If your supply is greater than that the voltage has to go somewhere, and that is where the current limiting resistor comes in. (I've seen cases of using a power supply about equal to the forward voltage with no resistor. It will work, but in my opinion it is a bad practice. Others may disagree.)Here is a set of diagrams for hooking up a simple LED circuit:http://white-leds.co.uk/led-wiring-guide.htmThe first diagram is the simple case, but if you understand it you are well on the way to understanding how to use LEDs in general.To save you calculating the resistance value, here is a handy tool:http://led.linear1.org/1led.wizRemember, you need to know three things, power supply voltage, forward voltage, and forward current.Further down the page of diagrams there are ways of hooking up more and more LEDs. And here:http://led.linear1.org/led.wizis a calculator that will figure out resistor values for a multi LED array, and give you a diagram. I don't totally agree with the array idea in all cases, put when the LEDs function as a group it makes sense. In other cases, figure out individual resistors, and wire them that way, in my opinion.
OK. As you suspected, there isn't one answer to the question. And, I'm going to start with a little text, to try to explain why, hopefully in an understandable way. The simple cases are pretty easy, and I did find some drawings for those, so that will be a place to start. We can go from there to more interesting, though someone else may have to pick it up since I'm going on vacation for a week and may not be posting.
The first point is that LEDs are diodes. In general that means that they pass current in one direction, with a more or less fixed voltage drop across them. There is a region between 'off' and 'on' where things are not quite that simple, but as a rule we don't try to operate them there, so it isn't an issue. Given that they will pass current in one direction and not in the other, they like DC. AC can be used, but they will only be on half the time (usually that is plenty to see), and they are not rated to like even moderately high reverse voltages so they may need some protection from that. That can be gotten into later.
Every LED has two ratings that we care about in our kinds of applications, the forward voltage, and the forward current. We don't plan to operate with reverse voltage on them (for now) so none of that is relevant.
Here is an example datasheet:
http://www.rohm.com/products/databook/led/pdf/slr-342.pdf
We'll use the red one for this example, the SLR-342VR. We don't want to push the maximum forward current, nicely this datasheet shows a typical value of 10 mA, with a maximum of 25 mA. (Ignore the peak value. that is referring to a pulse and we don't want to worry about that.) Often, only the max rating is given. Usually half of that is a reasonable operating state. The typical forward voltage is 2.0 V. Remember, that no matter how much voltage you try to put across the LED, it is only going to drop 2 V. If your supply is greater than that the voltage has to go somewhere, and that is where the current limiting resistor comes in. (I've seen cases of using a power supply about equal to the forward voltage with no resistor. It will work, but in my opinion it is a bad practice. Others may disagree.)
Here is a set of diagrams for hooking up a simple LED circuit:
http://white-leds.co.uk/led-wiring-guide.htm
The first diagram is the simple case, but if you understand it you are well on the way to understanding how to use LEDs in general.
To save you calculating the resistance value, here is a handy tool:
http://led.linear1.org/1led.wiz
Remember, you need to know three things, power supply voltage, forward voltage, and forward current.
Further down the page of diagrams there are ways of hooking up more and more LEDs. And here:
http://led.linear1.org/led.wiz
is a calculator that will figure out resistor values for a multi LED array, and give you a diagram. I don't totally agree with the array idea in all cases, put when the LEDs function as a group it makes sense. In other cases, figure out individual resistors, and wire them that way, in my opinion.
Believe it or not, I understand that strange region between on and off a LOT better than simple wiring practice. That's just semiconductor theory, holes, doping, PNP junctions all that stuff.
I have a real problem translating text descriptions into useful schematics. I can take directions over the phone once, rattle them back to the giver and have three shortcuts figured out before they finish, but...that's only because I have the NSEW frame of reference to work with. In electrical circuits, there's no starting point, no anchor, and as soon as you mix series and parallel circuits together, I'm hopelessly lost.
Give me a simple diagram to start with and I'm right back in business. You did, and I am.
OK...seems to me, you could just keep wiring LEDs in series till you sopped up all the available voltage drop and limit the current that way, without a resistor. I'm guessing that's a bad idea because of...headroom? The resistor can handle slight fluctuations in potential while the finer tolerance LEDs can't?
Last q, for now anyway...physically, where does good practice locate the resistor, in what type of mount? Just flopping around under the table inline with the LED? Back near the switch at the control panel? Special circuit card under the table somewhere? Where does a non-circuit card burner guy get such a card, if that's the way to go?
Ooops...one more...the whole point of using LEDs, in my opinion, is to set it and forget it, to never have to replace bulbs, etc, a one time effort to light the layout that is still working when aliens unearth the layout in the year 2545, if man is still alive...if woman can survive...ooo...
Is that realistic? Can I HARDWIRE this mess and reasonably expect it not to require disassembly for MX? Is LED QC good enough to get away with that, or should I plan on replacing LEDs even if they work on initial test?
Thanks...You're helping me breach one of the last barriers into the unknown of MR. It's greatly appreciated.
Jeff But it's a dry heat!
If I can't find one, I'll make one. Or several. There isn't only one answer, and there will be a little text!
I've looked and found diagrams...of complicated setups I don't understand, that may or may not apply to LEDs and resistors and power supplies I can find locally.
I've looked and found text descriptions by the quadrillions.
What I really need is a simple wiring schematic, a drawing that includes the type (AC, DC, variable voltage variable current) and rating (how many amps, volts, mhos, whatever is necessary) of the power supply, a simple on-off switch, so that ALL the structure lighting is either on or off, whatever resistors are needed, where they go in the circuit, what their ratings are, how many of what type and rating LEDs I can string together with given power supplies and resistors, and where the wires go, with dots where wires connect, and little semi-circles where wires cross without connecting.
I can read a simple diagram, but not a complex one.
Text descriptions, electrical circuits and my brain just don't mesh very well.
I THINK the reason I haven't seen such a simple diagram is that the issue is more complex than I wish it was, that maybe the LEDs types depend on the voltage of the power supply, and maybe the resistors depend on some other variable, but if such a drawing existed, one solution for one set of components, say, a 12 volt toy train transformer and X LEDs maximum (and Y LEDs minimum if need be), and Z value resistors, then I wouldn't need to understand all the theory and could light the layout using the monkey see, monkey do principle.
Anybody know of a simple drawing like that?