1,000,000 USD Question: How is it that LEDs in SERIES can take up high voltages? Isn't the limit 3v?

Discussion in 'General Electronics Chat' started by Fitzgerald11, Nov 24, 2018.

  1. Fitzgerald11

    Thread Starter New Member

    Nov 24, 2018
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    Hi,

    Long-time noob here.

    The high-stake question is: I've checked many places, and they seem to say that 10 LEDs can actually be supplied a HIGHER voltage than the 3-5v limit IF they are in series.
    I can't grasp HOW.
    Would that mean that the FIRST LED would he hit by the high voltage (say 12v wall adapter), and then the second LED would be hit by a voltage minus the first LED's resistance value or voltage drop, and so on and so forth until the voltage drops to near 0 when it reaches the last of the 10 in-series LEDs?

    I can understand why when placed in parallel the CURRENT drop would be equally shared by all.
    But that should NOT be the case with VOLTAGES when there are components placed in series, by way of analogy with how RESISTORS work in series.

    Where did the laws of physics failed my pretty logical mind?

    1,000,000 still up for grabs.
     
    Last edited: Nov 24, 2018
  2. wayneh

    Expert

    Sep 9, 2010
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    Yes, LEDs can be in series and then accept a working voltage across the string that is the sum of all their individual forward voltages.

    No current flows through any of them unless the voltage across each individual reaches the minimum forward voltage. Once met, the current can vary quite a lot without much change in voltage drop.
     
  3. dl324

    AAC Fanatic!

    Mar 30, 2015
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    In series the voltage is divided among the individual LEDs; not evenly, but divided nonetheless.
     
  4. oz93666

    Senior Member

    Sep 7, 2010
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    Imagine you have a string of a 1,000 leds in series... each one has a push back voltage of about 3 V ... so no current will flow until about 3KV is put across the whole chain ....

    The individual leds do not experience high voltage ... all they know is the current . While the voltage across the the whole chain is 3KV , that across each led is 3V.

    It's the same with solar panels , we are told not to wire too many in series , not to go over 500V ... But there should be no problem in wiring a million in series to get 20MV @4A .. in one long line , well off the Earth ... each panel will not see high voltage , only know that it's pushing 4A.
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    The key point you are missing is that the voltage limit for each LED is the voltage ACROSS the LED -- the voltage difference between one terminal and the other. This is the only voltage that the LED interacts with.

    You are under the common misconception that the voltage AT a point has any intrinsic meaning. It doesn't. Voltages are ALWAYS voltage differentials. When we talk about a voltage at a point, such as an overhead power line being at 13 kV or whatever, we are implicitly stating it's voltage relative to come common reference point, such as Earth ground. When we talk about a car's power being 12 V, we are talking about the (nominal) voltage of the positive side of the battery relative to the negative side (which is connected to the frame on nearly all vehicles these days).

    Another common misconception is that LEDs placed in parallel will share the current equally. They won't -- and the slight differences can lead to a phenomenon known as thermal runaway where the LED with the slightly higher current warms up more than the others and thus it hogs more current and gets warmer and hogs more current until eventually it fails. At that point another one repeats the process. So if you have a lot of LEDs in parallel, you need to take steps to FORCE them to share the current reasonably.
     
  6. Fitzgerald11

    Thread Starter New Member

    Nov 24, 2018
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    So, in that case, the usual analogy that is made to WATER when explaining how voltage and current work in electronics does NOT seem to apply here. Imagine having 10 smaller-diameter pipes placed in series, analogous to resistors or LEDS for that matter, the FIRST in line would take most of the BLUNT force of the water pressure coming down the bigger pipe. But, in electronics, it seems that this is NOT the case, and somehow the LAST of the smaller pipes takes on the exact level of pressure and current as the first.
    Hence, it's urgent and imperative that memos should be sent out to all electrical engineering departments so that 'teachers' and 'professors' who introduce ppl to electronics won't commit the mistake of making analogies between electricity and water when it comes to voltage and current flow. Because it's confusing, and electricity have laws that are a little bit out of this world, and frankly don't make much sense, and which you have to take at face value.
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The pressure at one end of the first pipe would indeed be the maximum, but the pressure at the other end of the first pipe would be only slightly less, because of the flow resistance provided by the other pipes. So the pressure difference across the first pipe (analagous to voltage across a LED) would be small. The water/electricity analogy still holds.
    Analogy.PNG
     
    Last edited: Nov 25, 2018
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  8. MrChips

    Moderator

    Oct 2, 2009
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    That danger in using the water analogy is well known. Knowledgeable educators are aware of this. The use of the water analogy is frowned on and discouraged.
     
  9. OBW0549

    Distinguished Member

    Mar 2, 2015
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    The only reason it seems that way to you is that you've made the all-to-common mistake of inferring too much from the analogy, apparently to the point of believing that everything we observe in the behavior of electrical circuits somehow "should" correspond EXACTLY to what we observe in the flow of water through pipes. It does not; in fact, the water analogy is extremely crude, limited and loose, and IMO does far more to confuse than to clarify.

    The laws of electricity are not out of this world; they make perfect sense; and massive empirical evidence gleaned over more than a century and a half by thousands of people gives strong support for taking them at face value.

    My advice: LOSE THE WATER ANALOGY. Wipe it out of your mind so it doesn't cause you further confusion. Forget about it. Focus instead on learning circuit theory without that crutch, because it will only mislead you.
     
  10. DNA Robotics

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    Jun 13, 2014
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    Regarding the difference between electricity and plumbing.

    Hydraulic Gold Mining
    A water cannon powerful enough to blast away a mountain only used gravity to create its pressure.
    The water was routed from a source such as a nearby river or stream 200 feet above into ditches and flumes, gradually reducing in size, where it was run through a canvas hose to a high pressure nozzle.
    So did resistance create more amps?
    They start talking about the nozzle at 4:15 and the plumbing at about 6 minutes.



    Hydraulic mining used water under high pressure to wash away gravels and high bench deposits, making a slurry that could be run through a sluice box or other equipment to recover the gold. The water was routed from a source such as a nearby river or stream into ditches and flumes, where it was run through a canvas hose to a high pressure nozzle called a monitor. The monitors were huge cannons, some up to 18 feet long, capable of discharging massive amounts of water under extremely high pressure. These jets of water could literally wash away mountains, removing all the material down to bedrock. They were the perfect tool for reaching the ancient river gravels buried underneath mountains

    Monitors - Water Cannons of Hydraulic Mining
    https://www.sierracollege.edu/ejournals/jsnhb/v2n1/monitors.html

    Hydraulic mining was a variation on ground sluicing where the water delivered to the site would be shot through a nozzle at high pressure onto the face of the cliff, thereby washing away tons of boulders, gravel, dirt, and ounces of gold.

    The first use of this method is credited to Edward Mattison in 1853 who supplied the water through a rawhide hose to a nozzle he carved out of wood. Later miners upgraded their hoses to metal or the more desirable canvas, and the nozzle soon became iron. Technological advances made the hose and nozzle connections more flexible and allowed greater movement. Lavish attention was paid to the design and specifications of the nozzle and companies began producing their competing appliances. The product names were various — Hoskin’s Dictator and Hoskin’s Little Giant are examples. But the name that stuck was the product name of the Craig Company — the Monitor.
    Powerful Force of Water
    The Monitors were powerful to say the least. When the water reached the Monitor it was compressed into a nozzle that was anywhere from one inch to eight inches in diameter. The stream of water that could wash down whole hillsides was impressive to behold. In his multi-volume 1874 - 1890 classic History of California, the historian Hubert Howe Bancroft stated that an eight-inch Monitor could throw 185,000 cubic feet of water in an hour with a velocity of 150 feet per second. Other accounts of the force are less technical but just as startling. One description points out that a strong man could not swing a crowbar through a six-inch Monitor stream, yet another commented on the striking phenomenon of a fifty-pound boulder riding the crest of a jet with the power of a cannonball. Documented evidence recalls that men were killed by the force of the water from 200 feet away.
     
  11. wayneh

    Expert

    Sep 9, 2010
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    Correct, it doesn't apply well because an LED passes no current until it sees a minimum voltage (pressure) drop. Once that minimum voltage is reached, the current can climb to a high level with almost no increase in ∆V across the LED. You might be able to imagine a plumbing device that would behave similarly with water - maybe a big flap that requires a minimum pressure drop to open but could support a large flow once open. Mental gymnastics to rescue the analogy just aren't worth it. Once the threshold is passed and current is flowing, the analogy isn't too bad as @Alec_t 's diagram shows.
    Well that's not really how water or electricity works. Each step in a series arrangement sees the same flow (current) and the same dynamic pressure (voltage) drop.
    Most teachers of electronics agree and the analogy is avoided. It's a little bit useful for the early concepts of Ohms law, but it quickly starts falling apart when the student encounters inductors, EM fields and such. The student is better off in the end if the analogy is never introduced.
     
  12. dl324

    AAC Fanatic!

    Mar 30, 2015
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    Do whatever you need to do to learn how conventional current flows, except, don't use the water analogy. Keep asking questions until you can understand current flow.

    But, be aware that some still use electron current that flows backwards.
     
  13. ebp

    Well-Known Member

    Feb 8, 2018
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    Something more to think about:

    The 12 volt supply mentioned could be a single 12 V supply or three 4 V supplies with one or more LEDs between each, or twelve 1 V supplies distributed in any arrangement throughout the circuit, and the behavior would be exactly the same as long as everything were in series and the polarities of the supplies were the same.

    ==
    "I can understand why when placed in parallel the CURRENT drop would be equally shared by all."

    The language is of "voltage drop" (meaning, as WBahn said, the difference in voltage between two terminals of a device or "nodes" of a circuit) and current "through" a device or a pathway in a circuit. "Current drop" really isn't meaningful except to describe a dynamic event, as in "I did ... and it caused a current drop."

    Kirchhoff's voltage and current laws are very important fundamental descriptions of circuits.
     
  14. KeepItSimpleStupid

    AAC Fanatic!

    Mar 4, 2014
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    The differential voltage that a LED must see is both process dependent (statistical variations) and the color of the LED.
    So there is a minimum and maximum for every color.

    There is also some reverse voltage that won;t damage an LED. Let's assume Vf = 2.1-2.5 V and 3V

    Proper designs needs to be checked against those maximums. If your putting LEDs in series, it wise to match the Vf's so that the intensities track.

    For AC voltages, you might also see two LED's back to back. The other LED makes sure the reverse voltage doesn't damage the other LED.

    LEDs are a current driven device, so a resistor generally drops some of the power supply voltage limiting the current. The current it "regulates" is dependent of the sum of all the Vf's and the supply voltage. I=(Vf+Vf2...+Vfn)/Rseries

    We can;t stress it enough that single LEDS should not be put in parallel because one LED could hog most of the current. they won;t share equally because Vf is different.
     
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