Large Scale Central

A little electrical theory please

Electronics
#1

OK, so there is a lot being thrown around all of a sudden about electronics, mainly LEDs. And it has brought up some questions for me. My understanding of how to properly operate an LED, at least in series, is that you need to calculate in coming voltage, figure out the operating voltage of the LED and then apply and appropriate resistor on the negative side of the LED which “knocks down” the power of the incoming voltage. So for example I have a 14.8 volt supply it goes through my Electronic Speed Control (ESC) board which causes a .7v drop (due to a transistor I believe) giving me 14.1v on my lighting output. Now the LED I am operating is rated at 3.6v so I wired in a resistor on the negative pole of the LED of the appropriate ohm rating (actually a bit larger because I wanted to under power it). The key to this equation is that each power source voltage would require a different resistor to fully power the LED right? The same resistor would not work for 21v or you would plow the LED and 5v would likely not power it at all, correct?

Question 1) So how are people selling prewired LEDs that are covering everything from 9 to 21 volts.; am I missing something? Or are they using a resistor that controls the highest voltage and then just allowing it to be under powered in all other situations?

Question 2) I don’t understand why the resistor goes on the negative side of an LED, could someone explain that? And would that be true of any electrical component that you need to step down source voltage?

#2

Wrong… LEDs are controlled by current, and the function of the current limiting resistor is to limit current.

LEDs - light emitting DIODES are a special varient of an ordinary diode, which is from a practical perspective 0 ohms in one electrical “direction”.

Ohms are units of “resistance” to the flow of electrons. When you get near 0 ohms, a lot of stuff becomes difficult.

So, the “voltage” across a LED is very difficult to ACCURATELY measure, and CONTROL. Unlike an incandescent bulb, a small variation in the voltage to an LED can make a huge difference in the current, i.e. melt down the diode “junction” inside and turn it into an open or dead short.

So you basically add a resistor to limit the CURRENT to the LED, as opposed to trying to apply a CONTROLLED voltage.

This is why the resistor is always referred to as a current limiting resistor, not a voltage reducing resistor.

Now, to answer the questions you posed, modern LEDs can operate on a wide range of CURRENT, often lighting from about 6 milliamps to 20 milliamps, so you can pick a resistor for max current at max circuit voltage, and it will normally run on a lot lower voltage.

The other thing is that there are devices that are a regulated CURRENT supply, the Suntek CL2 is what many of us use, and it is really an integrated circuit that limits the current to 20 ma no matter what the input voltage.

The resistor can go on either side of the LED, all you need is the LED and the resistor in the series. By convention people might always put it in one location or the other.

Current is like the gallons per hour through a pipe, if you have a long garden hose, does it matter where you put a shutoff valve? No, at the beginning, end or middle, it will control the flow the same way.

Electrical circuits are always a closed loop, there is a power source with basically a plus and minus, and electrons migrate from the minus to the plus of the power source through a series of objects, so if you have a battery, a resistor and an LED, as long as they are in a loop the order of components does not matter.

Greg

#3

I believe the wired LEDs that Devon saw had a rectifier in the circuit which dropped it to a working 5 volts. As a battery guy I decided against that option as rectifiers waste juice by making heat. OK for track power. Methinks.

I am not qualified to advise on electrical magic.

John

#4

OK Greg,

Your explanation makes a ton of sense to me. It actually answers (I think) another question I had but didn’t care about til now. If I understand this correctly an LED then is either On or Off; provided min voltage is enough to operate it and max voltage wont burn it up. So these guys are developing the LED/resistor to operate at the high voltage range and the lower voltage still is enough to light it.

Dimming the LED by putting in a larger resistor is not altering voltage but creating more resistance to current flow. That makes sense why a traditional dimmer rheostat isn’t an effective tool for dimming LEDs.

#5

Yes LEDs are current devices, and the CL2 doesn’t waste power quite like a regulator does John.

Devon, if you were to look at a voltage/current curve of a diode, you would see that they aren’t just on and off. And LEDs will run quite well with a lower current then they are rated for, but too much current will shorten their lifespan. (http://largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-wink.gif)And like Greg stated, there is a law called Kirchoff’s Current law, that states that current is the same in all parts of a series circuit. So the current limiting resistor does not have to be on one side of the LED or another. Also if you are running 2 LEDs off of say 12 volts, they could be wired in series with an appropriate resistor and they will both work fine.

#6

I think I have it at least enough to be dangerous. But it does make more sense now. What I guess I really care about knowing is can I buy one type bulb and one type resistor and be able to use it over a wide range of applications? As long as I plan the LED/resistor combination for the highest voltage rating I plan to supply.

Here is what I am thinking about. I am thinking about buying this LED and it is rated at 3.0-3.4V at 20mA and says to use a 620ohm resistor at 15v. The highest voltage I plan to go with at this point is 14.8v. but I would like to mess with smaller locos running 11.1 or even 7.4v. I can even see messing around with the 3v Tamiya motor/gear boxes for little critters. I am assuming that I can’t use the same resistor all the way down to 3v but could you use the 620 ohm resistor say down to 11.1 v (which states a 430 ohm is needed) and then maybe a 270 ohm resistor which is stated for 7.4 down to say 4v, and then just leave the resistor off once I get to 3.4v.

I mean resistors are cheap and honestly I was always figuring I would have one LED and then just buy several different resistors and wire them up for the situation. But seeing these wide ranging applications with the same LED intrigued me.

#7

OH and David you answered another question. So in series you only use one resistor, I new this, but that resistor needs to be sized differently than if multiple LEDs were wired with their own individual resistor in parallel right? In parallel you wire each LED/resistor as it if it were individual right? But in series you have to take into count all the LED’s in the series and how much current they rob cumulatively and then size the single resistor to match don’t you? If an LED is a 20mA led and it is wired in series with another don’t you size the resistor as if it were a 40mA?

#8

Devon close, but the not quite. The current in a series circuit is the same in all parts of the series circuit. So we have 2 LEDs and a resistor, wired in series, the current is still 20ma. Each LED, buy your example, drops 3.4 volts. So the 2 LEDs drop 6.8 volts. If you take the supply voltage, minus 6.8 volts, then take that number and divide it by .02 amps, you get the appropriate value of the one resistor. R=E/I.

So if you have 14.4 volts, minus 6.8 volts, equals 7.6 volts that needs to be dropped across the resistor. 7.6 volts divided by .02 amps is 380 ohms.

#9

Go with the CL2 to keep the led at full brightness at all voltage levels.

And Battery operations should try to use leds in series as much as possible such as dual head lights or marker lights.

And you can place the CL2 on the positive feed, have a Cab light next, and then parallel the anode of the front and rear leds to the CL2 and select the cathodes for front and rear lighting separately. Cab light will be on as long as either front or rear light is on.

#10

There is a good tutorial on LED circuits on the Ngineering web site.

They also have easy to use calculators for LED and dropping resistor installations.

The calculators not only tell you the value of the resistor needed, but the wattage it must handle.

If using a CL2 LED driver, best to have a diode next to it. I learned the hard way. After a few changes in direction it no longer functioned. A surge probably did it in.

#11

Devon

Another way to control LEDs is through current limiting. Rather then trying to calculate a dropping resistor, I simply use a current limiter set to the current value of the LED. Using this method LEDs maintain their brightness across a wide range of track voltages.

Stan