LED:current,voltage,misc questions

Discussion in 'General Electronics Chat' started by Jan Luthe, Jan 10, 2015.

  1. Jan Luthe

    Thread Starter Member

    Jan 10, 2015
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    LED/voltage/current questions:

    I want to modify 2 led consumer products so I am trying to understand stuff:

    1) LED’s of the same color but different physical size: Do they generally require the same voltage?

    2) Can LEDs run on AC?

    3) If an LED requires a certain voltage range(say 1.7-1.9v). How much higher in voltage can one go without seriously decreasing the life of the LED?

    4) What is a general guideline for volts/current for clear LEDs?(My solar light LED has a voltage drop of 1.2 v when measured on the circuit board)

    5) On a circuit: 9v battery, 75 ohm resistor, and 3 red LEDs(2.5v each) in series: Is the last LED limiting the current and therefore the first 2 LEDs at higher voltage won’t burn out because of this ‘limiting’? If a 4th LED was added would it probably not light (because of insufficient voltage) and therefore none of the previous ones won’t light either because there is no current?

    6) When using a resistor in series before a LED: Does the resistor just drop the voltage and the LED just uses the current it needs or does the resistor actually limit the current? Can one put the resistor after the LED (cathode side)?

    7) Standard 120VAC wall dimmer light switch: What does the dimmer do? Can I use it to control voltage or current on a circuit board directly or indirectly thru a power supply?


    I would appreciate answers in very simple laymans terms as I am just starting to learn. If possible I would like a PM notifying me that there is a response so I don’t need to check the forum frequently.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    1) Although LEDs have a nominal forward voltage (Vf) you don't drive them with that voltage. LEDs should be current-driven. You read the datasheet to determine the maximum current they can handle, then choose a running current I a good bit less than that (to maximise LED life). Knowing the running current you can calculate (using Ohm's Law) the resistor value needed to drop the supply voltage Vsup down to Vf. Thus R = (Vsup - Vf)/I. Be aware that Vf differs from one LED to another, even when they are nominally the same. It also varies somewhat with current and temperature.
    2) No. But you can connect two LEDs in inverse-parallel for use with AC. Each LED will pass current on respective alternate half-cycles.
    3) See 1.
    4) See 1.
    5) No. The resistor limits the current. With 4 LEDs none will light.
    6) See 1. In a series arrangement the sequence is unimportant.
    7) A dimmer provides phase control to enable current to flow for a selected fraction of each half-cycle. Don't mess with mains voltage until you have more electronics knowledge. It can KILL you.
     
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  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    we will keep the answers simple
    1) N0
    2) No
    3) The forward voltage rating of an LED is simply the minimum operating voltage required to get an LED to start working.. LEDs are current driven NOT voltage driven. You could have 1000 volt supply and with the appropriate resistor to limit that circuit to the needed current it would work just fine
    4)Most simple "indicator" type LEDs require from 5 to 20mA of current. Higher power LEDs like Cree 3W,etc.. run on like 350mA up to 3A or more.. Typically 700-1500mA
    5) The resistor is limiting the current. LEDs do not and must be used with some form of current limiting like a resistor or constant current power supply.
    6)resistor can be on either side of an LED and it still limits the current flow for the LED
    7)You will not use a standard dimmer for an LED..

    keep learning.. sounds like you are not ready to modify anything yet... no offense. :)
     
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  4. WBahn

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    I'll add my $0.02. As you will see people will give you somewhat different answer with the devil being in the details. Looking at the differences can really help you learn.

    1) Roughly. Two red LED's that are the same wavelength but are very different in size will likely have forward voltage drops that are much closer to each other than a red and a blue LED that are the same size. A given LED will typically see the voltage increase by about 100mV when the current increases by a factor of ten (there is quite a bit of variability between LEDs in this rule of thumb, but they are all in this ballpark).

    2) Yes, within reason. And LED is a diode (that's what the D stands for) that conducts current in one direction and blocks it in the other. So if presented with an AC signal the LED will conduct during half of the cycle and light up during that portion only. As long as the frequency is fast enough (more than 30 Hz or so) you will probably perceive it as being on continuously but at about half the brightness. The big caveat is that LEDs, unlike their rectifier diode cousins, generally cannot tolerate significant reverse voltages and will typically break down with a reverse voltage in the 5V to 7V range while rectifier diodes typically have reverse breakdown voltages that range from dozens to thousands of volts.

    3) Not much at all. The current through an LED will increase by a factor of ten for increases in voltage across it of just a few dozen millivolts. This is why most LED circuits are designed to establish a current through the diode and not a voltage across the diode -- in reality, of course, you are doing both, but the circuit design assumes an approximate voltage across the LED and then establishes a desired current that changes very little as long the LED voltage is anywhere close to that assumed value.

    4) Not sure what you mean by a "clear" LED. The term "clear" usually just means the lens/encapsulation color and has no bearing on the LED's electrical characteristics. This depends primarily on the color of the light emitted by the LED. I'm not aware of any LEDs that operate at a forward voltage of only 1.2V. The lowest that I know of are the infrared LEDs that are typically in the .15V range. As a general (but not rigid) rule, the shorter the wavelength (i.e., going from the red end of the rainbow to the blue end) the higher the voltage. The current depends entirely on how much power the LED is designed to handle, which is directly related to how much light it is intended to give off. The more light, the more current. Currents can range from a few milliamps to tens of amps (maybe more, I'm not sure what the highest current LEDs are at -- they are primarily limited by the ability to get the waste heat off the die).

    5) You need to learn about basic DC circuit theory. The LEDs only know about the voltage difference between their terminals and the current flowing through them. So all 3 LEDs in your example see the same thing and none of them behave differently because they are "before" or "after" the others. In this series circuit, if there is any current flowing, then the three LEDs will collectively drop a total of ~7.5V (3x2.5V) leaving the remaining voltage, 1.5V, to be dropped across the resistor. It doesn't matter where the resistor is located as long as it is in series with the LEDs -- it could be "before", "after", or "in between". The current will be set by the 1.5V across the 75Ω resistor which will make it right about 20mA. None of the LEDs are at any higher a voltage than the others because each of them only sees the 2.5V that appears across their terminals. If you add a fourth LED, then the four LEDs will each be dropping about 2.25V, which is 250mV less than their "on" voltage. Thus we can expect the current to be less than about 0.1mA (making the voltage across the resistor less than 10mV). So there is still current flowing, but it is not giving off any perceivable light.

    6) The resistor imposes a relationship between the voltage across it and the current through it (Ohm's Law). Similarly, the LED imposes a relationship between the voltage across it and the current through it (it happens to be exponential). By putting them in series (order doesn't matter), they have to have the same voltage but the voltage across the combination is the sum of the voltages across either. When we apply a voltage to the combination we force the sum of the voltages to be a particular value and the current takes on whatever value it has to in order to satisfy both relationships simultaneously.

    7) There are different kinds of dimmers, but most allow current to flow only during an adjustable portion of the voltage waveform. In theory you could use one to control the voltage on a circuit board, but the circuitry on the board would have to be carefully designed specifically to do this (as would an intermediate power supply). Do NOT try to do this naively as you will likely destroy your circuit board and may possibly expose yourself to dangerous voltages in the process.
     
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  5. Jan Luthe

    Thread Starter Member

    Jan 10, 2015
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    Alect,Mcgyvr, WBahn:
    Thanks for all of your replies. I have lots to think about and absorb. My questions do not necessarily directly relate to what I want to do. The questions are just to help me start to understand stuff. I am presently working with 'used' components so no 'data' available.
    1)Inverse parallel LEDs connection on AC: If one uses only 1 LED will this cause the 'reverse voltage' which in turn causes the LED to fry? Any time frame on this?
    1b) On a DC circuit: If LED is installed 'backwards' is this the same 'reverse' voltage?
    2) "The current through an LED will increase by a factor of ten for increases in voltage across it of just a few dozen mv": Why is this important to me? Is not the voltage across an LED determined by the design of the LED itself? What am I missing?
    3) LED color: I thought the color of a LED is dependent upon the crystal and not the lens. My 'clear' LED (no lens, just glass) is on a circuit board of a solar post light. The 1.2 volts I measured across the LED. The CB supply(1.25v battery) draws about 50mv when the light is on. Would the CB possibly affect my voltage measurement? (details of CB will be on another thread when I am ready for my modifications).
    4) Relating to 3): Once an LED is lit can the voltage be less than the required starting voltage and still remain lit?
     
  6. Jan Luthe

    Thread Starter Member

    Jan 10, 2015
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    Sheldon:

    Could one replace the reverse diode with an LED and have the same result? What does Greek lamda sign on the led symbol mean? What/how is the 0.6 V determined?
     
  7. sheldons

    Active Member

    Oct 26, 2011
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    Heres the full article I found
     
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  8. Jan Luthe

    Thread Starter Member

    Jan 10, 2015
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    Thanks Sheldon. I have lots to absorb now. I have to slow down before my brain goes into overload and fries!
     
  9. wayneh

    Expert

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    Yes they can, but with the reverse voltage limitations explained by WBahn in #4. The contrary answers in #2 and #3 took that limitation for granted and summarized it to "no". All those strings of Christmas LED lights are examples of LEDs running just fine off AC.
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    Not the "same" result, but largely so. With the diode replaced with an LED you have the "anti-parallel" configuration mentioned by alec_t in Post #2. Each LED will limit the reverse voltage across the other to its own forward voltage (2.5V in your example). With a single LED you will get a pulse of light that lasts for something less than half a cycle while with two LEDs you will get two pulses of light, one from each LED and each lasting for something less than half a cycle.

    It just indicates that that diode is an LED (L="light"="lambda")

    The same way that the 2.5V for the LED in your example was. Each LED has an exponential relationship between voltage and current that is closely approximated by the equation

    <br />
I_d \; = \; I_s e^{\( \frac{V_d}{NV_t} \) \}<br />

    where Id and Vd are the current through and the voltage across the diode, Is is the "scale" or the "saturation" current, Vt is the thermal voltage and is equal to about 26mV at room temperature, and N is a quality factor that for silicon rectifier diodes is about equal to 1 but for LEDs can get be up to 10 or more.

    So let's say that we take a silicon diode with N=1 and put 10 mA through it and discover that the voltage is 550 mV. We can use that to find Is and it would turn out to be about 6.5pA. If we now asked what the voltage across the diode would be at 100 mA of current, we would find that it would be about 610 mV. We would further find that it would be about 670 mV at 1A. The point being that the voltage across the diode doesn't change very much over a broad range of currents and so we often (almost always, in fact) just assume that the voltage across the diode is a constant when it is conducting reasonable and useful levels of current. For silicon diodes that voltage is often taken to be either 0.6V or 0.7V. That same idea is where the stated forward voltage of an LED comes from. It's not exact, but it's close enough for almost all purposes.
     
    Last edited: Jan 11, 2015
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  11. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Yes I went with a "NO" answer to keep it simple/basic. I did not want to explain the details of the limitations to a newbie ;)
     
  12. Wendy

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  13. WBahn

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    "AC" and "mains" are not synonymous.
     
  14. Jan Luthe

    Thread Starter Member

    Jan 10, 2015
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    You almost scared me with the formula but I think I get the picture.
    In your formula calculation did you have a typo and mean 670 mv, not 670 ma?

    Regarding the 'anti-parallel' configuration: Since the leds are on less than half the time could I theoretically double the current? (I think I read someplace that the reason leds have to be current limited because of heat generated and if on half the time the current can be increased/doubled because the average current would be the percentage of the time it is on). Am I remembering right? Also, because each LED is at 60 hz, to the naked eye would both LEDs seem to be on all the time?
     
  15. Jan Luthe

    Thread Starter Member

    Jan 10, 2015
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    I agree with you that if one doesn't know what one is doing 'mains' can be a killer. Presently I am just trying to understand stuff and my mind is going all over the place.
     
  16. WBahn

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    Yep. Good catch. I've corrected it.

    Short answer: Yes.

    There are actually several things place limit on the allowed current, heat being the main one most of the time. But there is also a limit on the maximum instantaneous current that is allowed, however in most cases using pulses of several times the average current will not cause a problem.

    Take note of the warning from Bill Marsden, the moderator. If you are talking about actually running LEDs from the mains voltage, then this would be a violation of the forum rules. But if you are using a transformer to step the mains voltage down to something reasonable, say 9V to 24V or something like that, then that's a different matter.
     
  17. WBahn

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    I don't have a problem with a theoretical discussion of running LEDs from the mains -- there are some good technical discussions that can be had. But the forum rules are overly conservative and don't even allow it to be discussed.

    So let's put everyone at ease and stipulate that we are talking about using an AC voltage source that is 60Hz and, say, 12V RMS for this discussion.
     
  18. Wendy

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    Mar 24, 2008
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    A slippery slope, but allowable. All we insist on is a transformer, and you can get AC wall warts just like DC ones.

    The ironic part is I used to argue with the moderators (before I was one) that we needed to keep this section of the eBook above.

    http://www.allaboutcircuits.com/vol_3/chpt_3/12.html

    Look toward the end of the LED section.
     
  19. sheldons

    Active Member

    Oct 26, 2011
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    So why exactly was that post deleted as in no way did I mention leds be connected to ac at 240v mains.....maybe for the benifit of others the led can be connected as shown to a low voltage ac supply.....there was no mention of capacitive type power supplies and I do understand the rules regarding connection of anything to high voltage ac mains is dangerous if you don't know what you're doing. .....
     
  20. wayneh

    Expert

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    Does a SMPS qualify?

    If not, this is going to become an increasingly difficult issue in the US. The average DIYer will no longer have good old wall wart transformers on hand. There are kids coming along that will have never even seen one. Transformer wall warts can still be found and they're essentially free, but you have to invest some effort. They don't come with retail products anymore, and easy availability won't last much longer.
     
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