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Lamp with LEDs in parallel and very under-rated resistors?

Discussion in 'Product & Service Reviews' started by Flyback, Aug 30, 2013.

  1. Flyback

    Flyback Well-Known Member

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    Hello,

    I just purchased a “72 LED multi-purpose work light” from ASDA supermarket.
    It says it was produced for “Status International (UK) Ltd” (£7)

    This runs on four-in-series AA batteries (Alkaline batteries have on_load terminal voltage of 1.1 to 1.3V)

    72 LED lamp schematic: (also below in PDF)
    http://i41.tinypic.com/17zo6r.jpg



    The LEDs are all basically in parallel as you can see. It’s three lots of 24 LEDs in parallel. These 24 paralleled LEDs have no added resistances to equalise the currents in them.
    Does this mean that the LEDs must have resistance built in to their silicon?

    I supplied one of these Work Lights with 4.4V, and found that the total current drawn was 447mA.
    The voltages across each of the 10R resistors was 1.453V, 1.471V & 1.465V. As you can see, this means about 210mW being dissipated in each of these 0805 resistors. –That is above the absolute maximum allowed for an 0805 resistor. A 2010 resistor would have cost little more than 0805. Why did they not just use 2010 resistors instead of 0805s?
    (remember that AA cells can sometimes be 1.3V,and that would have meant even more disspation in these 10R resistors….in fact , in a separate test, i supplied the product with 5V, and found the dissipation in each of these 10R’s was 412mW)

    The LED forward voltage was 2.93V

    With equal sharing, each LED will only have 6.2mA in it. Presumably these LEDs are 30mA rated LEDs (they look like it). So is this why they are OK with parallel operation?…i.e., because they are being operated at the lower end of the I/V curve where the dnamic resistance is much higher?

    I note that regarding the three 10R resistors, one has the value text pointing up….but the other two 10R’s are “upside down”, and the value lettering cannot be seen. I suspect that this is a kind of “factory coding” to indicate that these units have been through production soak testing.?

    I suspect that 0805 resistors have been used, because if any of the products have widely variant LED forward voltages, then there is more chance of the 0805 resistors burning out during soak test….and the staff can see that a resistor is burnt out , and simply bin the product, rather than ship it?

    How are they getting away with so many LEDs in parallel and so badly under-rated resistors?
     

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  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Try reading manufacturers datasheets and application notes, it's perfectly OK to parallel LED's of the same type (and preferably from the same batch) as long as you have more than a certain minimum number, which I seem to recall is about four?.
     
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  3. Flyback

    Flyback Well-Known Member

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    thanks, though I have never yet found advice like that on a led datasheet.
    These are single leds in parallel. (in three groups of 24)
    Do you mean there should be at least 4 leds in series, forming a string, and then such strings can be paralleled, without series resistors needing to be used?

    what about the 0805's?..is that ok...are they overcooking them so the lamp fails and we have to buy another....thus lining their coffers further?
     
    Last edited: Aug 30, 2013
  4. dave

    Dave New Member

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  5. alec_t

    alec_t Well-Known Member Most Helpful Member

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    The poor young kids who assemble these things probably can't even read :(.
    Testing? At £7 a pop you must be kidding :D.
     
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  6. Flyback

    Flyback Well-Known Member

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    in China wages are nothing..you can have a huge dept of test engineers working 18 hrs a day for nothing (sadly).
    Its a £7 product, and I bet £6 of that went in the pocket of the western entrepreneur who took this work to China.....these western entrepreneurs are coining it in.....they are the go-between for Chinse products.....phase 1 of their work is done....that was dismantling the western electronics industry........so now the west cannot actually compete with the Chinese....and so now these western entrpreneurs will push up the price of their Chinese made goods....safe in the knowledge that there are now no western companies who are able to compete with them.

    I bet that lamp could have been made at a profit for £7 a pop , even in the UK.......Its just that theres nowhere physically open to be able to make this kind of stuff in the west now.
     
    Last edited: Aug 30, 2013
  7. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    No, you can parallel identical LED's as long as there are at least four of them - so the groups of 24 are perfectly fine.
     
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  8. alec_t

    alec_t Well-Known Member Most Helpful Member

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    I'd be interested in the theory behind that. Is it something to do with the Vf statistics?
     
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  9. Flyback

    Flyback Well-Known Member

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    ....myself also.....what total current are we talking?......suppose the LEDs are 1A rated......you mean have four in parallel with 250mA in each of them is ok?....or 4 in parallel with 1A each in them is ok?
     
  10. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    If you check Philips Components, there was an application note by them (as well as various other makes) explaining it all.

    Basically as long as you have enough LED's it averages out.
     
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  11. Flyback

    Flyback Well-Known Member

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    I looked on Philips and I don't find stuff saying you can parallel leds without dropper resistors as long as its more than 4 leds

    eg
    http://www.electro-tech-online.com/custompdfs/2013/08/led_control_rev_d.pdf

    LEDs shouldnt be paralleled....parallel led products require enormous amounts of extended soak testing to weed out the non sharing leds...in China they can do this because they have 1000000's of test engineers all working for a loaf of bread a day....(sadly)...in the west we cant do this.
     
  12. alec_t

    alec_t Well-Known Member Most Helpful Member

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    This was one Phillips' pronouncement I found:-
    "Luminaire designers should also be aware of the risks inherent in connecting LEDs in parallel with each other, or in parallel strings, rather than in series. In a parallel topology, an electrical short of a single LED will cause the forward current to increase through
    some or all of the good LEDs. This increased forward current places additional electrical and thermal stress on the remaining LEDs. This in turn will cause them to fail faster than would have been predicted under normal operating conditions. Additional failures cause the forward current to increase further and further, producing a cascade effect that leads to an accelerating series of failures."
     
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  13. Flyback

    Flyback Well-Known Member

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    LEDs in parallel, I believe , is only done if you have loads of test engineers who work for nothing, and can weed out the non sharing products from a batch....even then , its dodgy.
    Forge Europa sell parallel led lights, with no dropper resistors. I asked then how they guarantee good sharing, and what the worst case sharing might be, but didn't get a full answer.....


    LEDs in parallel with no dropper resistors
    http://www.forge-europa.co.uk/solutions/200mm_round_led_light_engine


    ...so ill come out and say it..the only real reason to put leds in parallel (if you don't have millions of 'peanut-a-day-for-pay' test engineers), is because you and your engineers don't know how to make proper led driver stages.
     
  14. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Have you bought one and reverse engineered it?.

    Completely and utterly wrong, it's done entirely for cost reasons - and NOT by employing 'peanut-for-a-day' engineers (why waste money paying engineers?) - the proof is in the pudding, such lights work perfectly well, if you want to spend a LOT more on a better designed light (in the hope it will be more reliable) then you're free to try and find one :D

    I'm presume the light you bought is working perfectly well?, so why are you making such a fuss? - no wonder you got fired :p
     
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  15. Flyback

    Flyback Well-Known Member

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    yes I have indeed... they sampled me

    weve seen the figures, the 0805s are overpowered, and thus will fail in early time...if rechargeable nimhs are used then the internal res is not high enough to curtail the current...alkalines are better as they have about 200mR of internal resistance.
     
  16. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    That sounds painful :p

    So what do the two terminal blocks connect to?.
     
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  17. Flyback

    Flyback Well-Known Member

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    that's just so you can connect up...theres no way of adding resistors in series with each parallel string. I asked them about current sharing and they confessed sharing wouldn't be always equal, but wouldn't give stat spreads on it.
    they are a cree " official partner" so I guess they can get well vf matched leds served up to them(?)
     
  18. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    'Connect up' to what? - presumably a constant current source?.

    So it's a number of series chains of LED's, with the chains connected in parallel - again there's no problem at all doing that.

    Thinking back to earlier I may have been 'mis-remembering' - the application notes 'may' have referred to series chains in parallel, where as long as you series chains of four LED's or more it was perfectly fine to parallel the chains. This only requires the same make and model of LED, and not even ones from the same batch.

    Grab a load of similar LED's, wire them in chains of four and then parallel them, checking the current in each chain.
     
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  19. Flyback

    Flyback Well-Known Member

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    yes that's right, a constant current source.....theres two connectors, and if you just short the one then thats for single lamp operation.....or you can cascade two (or more) lamps if you wish, that's why theres two connectors

    I still cant find app notes that say that's ok
     
  20. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    There have certainly been some, have you found any that say it's NOT permissible?.

    But like I said, get a few LED's and test it - but the fact a responsible manufacturer does it should tell you something?.
     
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  21. Flyback

    Flyback Well-Known Member

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    I reckon the forge Europa product is for people who want a selv rated (<40v) led lamp but with a lot of leds to give an attractive "light spread" appearance...so they need them in parallel....I reckon the leds they use are from a special process which is quite expensive, but they can afford it because they probably run off a huge quantity for customers who have pre-ordered huge batches of very closely vf matched leds.

    I reckon your leds are cheaper if you just series them and use any old 'cheap as chips' leds that you can find.

    I think youre right, it probably needs to be a minimum of 11 in series leds in each parallel bank.....but as I said, why hassle yourself to have to always make sure that the lamp is made with the exact right batch of leds....if using such lamps you probably have to buy your own pik n place machine and operate it in-house so you can be sure the right leds have been used....or use forge Europa's led engine, and hope that they've got enough in stock for your intended batch
     

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