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? on Resistor placement w/ LEDs

Discussion in 'LEDs and Optoelectronics' started by Raul, Oct 16, 2012.

  1. Raul

    Raul

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    Sep 25, 2012
    Why is it that (a) people say it matters not whether the resistor is on the anode end or cathode end of an LED? and (b) Usually one sees the resistor drawn as placed on the negative (cathode) side?

    How does a convention like that arise? Is there a rationale other than monkey see monkey do? Or maybe it's deeper than that: People follow this as a convention because the consistency makes things less complicated later on when someone else is trying to debug the circuit?

    Granted, I'm largely uninformed about electronics, but intuitively it seems to me that putting the resistor in the Anode side makes more sense because one is preventing a spike or surge of excess voltage.

    I sort of, kind of, get it that placing the resistor on the cathode end is OK because if the power is resisted anywhere along the circuit it is resisted everywhere. That is to say that the power can't flow beyond the resistor's ability to limit it.


    I found this paragraph:
    "A semiconductor with extra electrons is called N-type material, since it has extra negatively charged particles. In N-type material, free electrons move from a negatively charged area to a positively charged area."
    Is this why? Because the power is flowing in exactly the opposite direction from what I assumed?
    My assumption ( a life long one) is that the + side of a power supply is the hot side from whence the electrons flow.
    I still think this is correct. The flow of electrons across the depletion zone is merely the flow of electrons inside the diode as it eliminates the depletion zone and not necessarily that power going to or from it - yes ?

    But if the resistor is on the cathode end what about one element in the circuit malfunctioning and admitting some excess of power? Wouldn't a resistor on the Anode end prevent that power from surging?
     
    Last edited: Oct 16, 2012
  2. BobK

    BobK

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    Jan 5, 2010
    (a) is correct, all the rest of your post is redundant.

    When two components are in series the same current flows though both of them. If you have a black box that has the resistor and LED in series with access only to the other lead of each, you cannot do any measurement that will tell you which side the resistor is on.

    Bob
     
  3. CocaCola

    CocaCola

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    Apr 7, 2012
    Because it doesn't...

    What ever's clever some like to stick to a pseudo standard...

    All of the above, I suspect...

    When I worked at Motorola all the hand place through hole boards had all the components polarity top-right '+' (might have been reversed it's been a long time) this aided in assembly as the assemblers didn't have to refer to the silk for each part as there was consistency...

    Draw a series LED-resistor circuit out and explain how the 'spike' or 'surge' is not forced to go through the resistor no matter of it's location... When doing this remember that no electricity flows until the circuit is complete, so there can't be a 'spike' or 'surge' until the circuit is complete...

    Again you need a complete circuit for that power to flow, it just doesn't rush into the parts and wait around to be let out...
     
  4. screwball

    screwball

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    Jan 9, 2012
    This ^,
    In series circuit, current is the same throughout, voltage is not
    In parallel circuit, voltage is the same throughout, current is not

    It makes sense in a way, i've always preferred to put the resistor after output component though to keep everything uni-formal so to speak
     
  5. duke37

    duke37

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    Jan 9, 2011
    If you have several leds, off the board, then it will be neater to connect all the leds to the supply and put all the resistors on the board.
    Just a matter of convenience.
     
  6. Raul

    Raul

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    Sep 25, 2012
    All really good explanations, dispelling my suspicion that I was doing something wrong. Thanks

    About surges and what not:
    So many things that travel through conduit are compressible. Coming to electricity late in life, with a set of preconceptions all firmly rooted, I easily fall into the trap of looking at electrons like I might many other things that flow along conduit.

    For example: "No part of a circuit can get power flowing through it unless the circuit is complete," is not a conclusion to which I'd default. That's the sort of thing that must be drilled into one's head, or mine anyway.
     
  7. screwball

    screwball

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    Jan 9, 2012
    It will soon be default to you lol, when you learn you can swing off a 400,000 volt overhead powerline without getting killed (providing you dont complete the circuit) then now you should not forget :p
     
  8. CocaCola

    CocaCola

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    Apr 7, 2012
    Just look at all the birds chilling on power lines as you drive down the road... Watch the squirrels use power lines as bridges across roads, yeah some of these lines are insulated but others are clearly not...

    Not, that I recommend it (in fact I don't) but I have won a few bets using an extension cord with the ends cut off to show that touching one wire at a time causes no 'shock' as long as you are not grounded or complete the circuit by touching the other wire/ground... The result of a bad habit I developed during my stint in home remodeling while changing lights and sockets when they were live, while mumbling to myself only touch one wire at a time... Again don't do this at home, it's always advisable to turn the power off, especially when it's a potentially lethal power level...
     
  9. screwball

    screwball

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    Jan 9, 2012
    Yeah haha insulated or not i still wouldnt dare try it :rolleyes:
     
  10. Raul

    Raul

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    Sep 25, 2012
    I must have always had some level of ground because whenever I've contacted the hot AC line I get a good buzzing.
     
  11. screwball

    screwball

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    Jan 9, 2012
    To get the buzzing you would have had a low enough resistance to earth for the electricity to flow through you to ground (whether it is through your body to your feet and to ground or from 1 finger touching the hot and another part of your body touching something else)

    If ive got it the right way then the better your connection to earth (lower resistance) then the bigger current that will flow through you, so if you touch hot while your standing on the floor you wont get as much as a wack as touching hot and touching something earthed
     
  12. CocaCola

    CocaCola

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    Apr 7, 2012
    As I said don't try it at home :) but yes you must have been grounded or else Mr. Sparky would not have come out to play...
     
  13. BobK

    BobK

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    Jan 5, 2010
    I prefer to get my buzz a kinder and gentler way.

    Bob
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    There are awards given to people who do this... (The kind of award you can't get if you've already reproduced)

    I'm seriously disturbed when I get a tingle from an unearthed power supply that has some capacitive coupling to the mains.

    Whilst I wouldn't claim to be paranoid, I'm certainly VERY careful about not poking my fingers around even moderate voltages.
     
  15. Raul

    Raul

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    Sep 25, 2012
    Who do what? Deliberately or recklessly handle hot lines?
    I don't believe that is cognizable from what I said.
    If you do enough renovations and reconstruction of existing buildings where you alone are all the trades and all the labor, you will (I guarantee you) end up contacting hot lines more often than you'd prefer. I'm on my Fifth ancient building. My first had knob and tube wiring.
     
    Last edited: Oct 18, 2012
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,175
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    True.

    I guess you're lucky to be living in a country where the mains voltage is not quite so lethal as ours.
     
  17. donkey

    donkey

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    Feb 26, 2011
    ok this is new info, I thought the resistor had to be on anode end... this makes my common annode tricolour LEDs a lot more useful.
     
  18. screwball

    screwball

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    Jan 9, 2012
    I am in the UK so quite strict regulations against things like this,
    I wouldn't consider our domestic mains to be particularly safe either (230v 50hz)
    I worked for an electrician for a long while and they disconnkected and reterminated cable while power was still on, it wasn't all the time but you only need to f up once for it to kill you

    simply not worth the risk, like Steve i am very aware of it and for that reason take much care
     
  19. screwball

    screwball

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    Jan 9, 2012
    well no not quite so straight forward, you can't just put it anywhere with tri colour diode as there's not just 2 connections like a single diode
     
  20. Raul

    Raul

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    Sep 25, 2012
    Yah we have 110/120 mostly I have 220 for the machinery in my shop.
    Still, you can get your heart to stop with bloody few amps. I have read that one milliamp can be sufficient. Our 120 lines carry 15 or 20 amps depending on the fusing and line size.

    I can't imagine living under a regime where I was disallowed from doing my own work on my own property. We do have building inspectors who have to approve or disapprove based on codes, but mostly they are idiots.
    I was rebuilding and expanding my kitchen a few years ago and the electrical inspector showed up to the rough-in examination. He got all excited about these little loops I put in the line where ever I passed into and through a stud. I put them in in case later on I ever need a few more inches in the box there is something left to pull. Plus, I just like the way they soften the transition.
    But he was excited about these loops. Mind you they are Not required by any code.

    He actually said that he'd not have passed my work if the loops were not there. Which would have got him in trouble if he had tried.

    I asked him why. I asked: "Why not? Because the electrons, going so fast, would fly off the wire if I'd turned a tight corner?" I was being tongue in cheek.

    And believe me brother's and sisters the man actually said "YES."
    I had to turn away so he wouldn't see my face.
     
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