Series circuit heat question

Discussion in 'The Projects Forum' started by john-daley, Sep 9, 2010.

  1. john-daley

    Thread Starter New Member

    Sep 9, 2010
    1
    0
    Hi there,

    I have a lighting circuit which I can't quite work out how it operates. It consists of a 24V 100W power supply which provides 350mA to a 4ft strip of LED lighting(which draws 28w). This strip can also be daisy chained (has both an input and an output flylead on each end of the LED strip) therefore allowing up to 12 ft to be powered off this supply (84w total). Q

    Questions I have: how is it that this circuit operates, as if you are only powering one 4ft strip it works without having to have a "jumper" to complete the circuit.Does this mean it is a parallel circuit?

    The fist strip in the circuit gets a lot hotter than the next 2. Why is this?

    Any help would be appreciated. I have attached a pdf of the circuit to explain, hope it helps!
     
  2. Dyslexicbloke

    Active Member

    Sep 4, 2010
    420
    19
    Asuming the PSU is 24V and not 350mA (Current limmited), which would cause its voltage to = the forward voltage drop of whaever LED's were attached, then each strip must have some form of current limmiting for the LED's it contains.

    I cant be precise because I don know what individual LED's each strip contains or how many.
    However.....
    It is likly that they are arranged in series strings having a forward voltage drop a little under 24V in total and that each string has some sort of current limmiter in series with it. Again no way to tell what but probably just a resistor.

    This is a cheep and simple way that many low end LED products are configured, it is far from ideal, in that it dosnt make good use of the power that is disipated and as you are finding, probably designed right on the edge of what the LED's will cope with in an attempt to maximise the light output.
    (They get hot and as a result won't last that long.)

    It is also extreamly susceptable to to power supply fluctuations and once again in cheaper products the PSU's are rairly that good.

    Sorry if i just trashed you system but if it is a good quality unit then it is also Faulty!

    The hoter first string bit is a little odd, given that each string should handle, and I use the term reservidly, it own current limmiting there must some other explanation.

    Just a guess but if the fly leeds are simply a paralell + and 0 rail that is fed on to each connected strip then you are probably seeing voltage drop along it. .... it would go something like this (Possibly)
    The first string is getting more voltage then the rest because it is close to the supply, seeing a lower impedance.
    As you move down the string along what are likly to be very small the conductors crammed into the strip casing the voltage drop will increese on each rail and, asuming the current limmiting is resistive, so will the current in each individual cluster of LED's.

    Although the voltage drop is cumalative and will be compounded by each strip you add the effect will also be smaller between adjcent strings as you go because the total current in the rails is less.
    IE Strip 1 handles the current for S1 + S2 +S3 +S4
    while Strip 4 only handls S4's requirements. You get the idea I am sure.

    If you check the voltage on S1 and S4 and find that S4 is lower and the strips do in fact carry paralell cobnductors, busses, then you can improve things.
    Connect both ends to the supply and the current will split more equally round the ring formed by each rail.
    This is exactly how ring mains in UK homes are done and for exactly the same reasion.
    The most equal way to jion them would be in paralel all from one point.
    (You must test this scinario by checking the rails first with a good meter. ... DO NOT assume I am correct about thre config)
    If your at all unsure DONT DO IT.

    Hope this helps
    Al
     
  3. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
    24
    Can you supply more info rergarding the LED lights? Are there any markings? Part numbers? Manufacturers?

    PS. Dyslexicbloke, I think when he said his PSU was 350mA that is its maximum rating. Not a 24V constant 350mA PSU. It will only draw as much current as it requires (OHMS Law)
     
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