1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

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

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

  1. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

    Joined:
    Nov 17, 2003
    Messages:
    39,216
    Likes:
    640
    Location:
    Derbyshire, UK
    ONLINE
    Again, you don't need precisely matched LED's, just the same type - but it's pretty certain that most production LED's will be from the same batch, particularly in the case of SM ones where they come on the same bandoleer. The only 'hiccough' will occur when they change bandoleers, which will only affect one string, and that won't matter because they are all the same LED's.

    So no 'special process' just standard design practice in accordance with the manufacturers recommendations.

    As I said, the manufacturers specified 4 or so.

    Do you have eight (or more) identical LED's? - stick them in two chains with a common feed resistor and measure their current.
     
    • Like Like x 1
  2. Flyback

    Flyback Well-Known Member

    Joined:
    Jan 5, 2007
    Messages:
    1,989
    Likes:
    34
    to be honest i'm not interested in single readings from single experiments, -another batch of leds may not pass this test, as their vf's may be too variant, and thermal runaway will progress with only a small initial variation in vf. I reckon series resistances are needed for each series string that gets paralleled...then you need the hassle of ensuring that all leds are well thermally coupled.

    the manufacturers have not specified this. I have looked everywhere.....no single led manufacturer, anywhere in the world, has any documentation stating that leds can be put in series parallel strings unless series resistors are put in each string.
    I am quite categoric about this....as I have many threads on this, and have researched widely.

    Read the first page of "LED diagnosis in automotive applications" by Osram opto semiconductor.......quoting here....

    osram app note:
    http://www.google.co.uk/url?sa=t&rc...IobZEKXZC-LkpMXdg&sig2=PEVuHdKtcT_SWy8HVSfF9A


    so we cannot count on leds having equal vf's...even in the same batch.
     
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

    Joined:
    Nov 17, 2003
    Messages:
    39,216
    Likes:
    640
    Location:
    Derbyshire, UK
    ONLINE
    So you're not prepared to actually do any tests, just stick to your preconceived ideas, despite all the commercial examples to the contrary.

    Have a look at pages 9, 30, 35 and 36 on this PDF for examples - although it's NOT the document I was trying to find

    www.philipslumileds.com/uploads/208/AB20-3-pdf‎
     
    • Like Like x 1
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. alec_t

    alec_t Well-Known Member Most Helpful Member

    Joined:
    Jul 10, 2011
    Messages:
    9,253
    Likes:
    1,218
    Location:
    Cardiff, Wales

    Ah, that makes more sense. If we assume each LED has a 10% Vf range, then putting 4 in series in a chain would mean that each chain would be likely to have a total Vf range of only ~ 2.5% (if my rusty statistics maths is right :)), which could be acceptable if the chains were in parallel. The more LEDs per chain the lower the likely total Vf range.
     
    • Like Like x 1
  6. Flyback

    Flyback Well-Known Member

    Joined:
    Jan 5, 2007
    Messages:
    1,989
    Likes:
    34
    If effective tests were to be done , then it would need a huge test regime, where by hundreds of leds from different batchs would need to be tested.

    The document you showed said that improved current matching could be had by using series resistors in each series string.
    The current of each led is only 50mA in that application (car indicator)....and I think that parallel operation is less problematic when the individual led power is so low.

    The app note also says that the leds need to be from the same forward voltage group......so that makes them considerably more expensive than cheap leds that don't come in forward voltage groups.
    In summary , I don't see anything in that article that states that its fine to parallel leds without series resistors in each series string (and that's with either constant voltage or constant current supply)


    Page 33 says....
    ....they should elaborate on this and say when the separate resistor per string is needed, and when you can get away with not using it.

    Also, that app note from Philips says its for a 30 led lamp with 50mA in each led. Vin = 12V
    ...That is an easy job for a constant off time boost converter (or sepic or buckboost) and a 30 led series string. Constant off time is the answer to long series led strings as theres no need for slope compensation, and constant off time converters don't suffer from limitations in duty cycle like constant frequency pwm controllers do.

    ...why anyone would want to mess about with particular matched leds and statistical vf variation calculations I do not know...plus the fact that vf matched leds are much more expensive.....plus you've always got to hope that that brand of led is in sufficinet supply when you need it.....just use a single series string and its job done.
     
    Last edited: Sep 1, 2013
  7. ronv

    ronv Well-Known Member Most Helpful Member

    Joined:
    May 30, 2010
    Messages:
    4,725
    Likes:
    488
    Location:
    Tucson, AZ. USA
    I don't think anyone is saying this is a long life design, but it is very cheap. Many LEDs are binned for Vf. It can easily happen during the test process and takes only a few ms. It does not mean any are thrown away they are simply put into different bins for packaging. You might want to test the design as it is in real life to account for contact resistance and the supplied battery performance. A life of 5000 hours, or one battery change instead of 50,000 hours may have been their design guideline. You might also want to measure Vf and the current thru each string vs time on. This would give you an idea if they are in thermal runaway.
     
    Last edited: Sep 5, 2013
    • Like Like x 1
  8. Flyback

    Flyback Well-Known Member

    Joined:
    Jan 5, 2007
    Messages:
    1,989
    Likes:
    34
    I beg to differ...this product is not cheap...its very expensive , and Chinese....this product, with the four 1V1 AA cells gives a piffling 1.3W of LED power (0.447A * 3V).

    It cost £7.

    This is the plan of UK entrepreneurs...initially they got the Chinese slaves to work for nothing so as to dismantle the uk electronics industry, as it wasn't able to compete with slaves....and now the uk electronics industry has gone, these same uk entrepreneurs are upping the price of the Chinese goods, safe in the knowledge that no uk electronics companies exist to compete with them......so they charge £7 for a Chinese lamp which gives just 1.3W of power, and needs 4 AA cells to do so.

    They pocket £6.50 and give the rest to the Chinese slaves.....or rather, to their masters.
     
  9. ronv

    ronv Well-Known Member Most Helpful Member

    Joined:
    May 30, 2010
    Messages:
    4,725
    Likes:
    488
    Location:
    Tucson, AZ. USA
    I see it for L5.35 on line with free shipping. Seems pretty cheap to me for 72 LEDs 4 batteries, clips reflector and a cute case. :D
     
    • Like Like x 1
  10. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

    Joined:
    Apr 17, 2007
    Messages:
    7,253
    Likes:
    961
    Location:
    Loveland, CO USA
    ovspw1bcr4 1W LED Vf=3.0 min, 3.5 typ, 4.0 max.

    TL19W01- 1W LED Vf=2.9/3.3/3.8 Can be sorted in three groups.
    TL19W01-(1) 1W LED Vf=2.9/3.2
    TL19W01-(2) 1W LED Vf=3.2/3.5
    TL19W01-(3) 1W LED Vf=3.5/3.8

    Most of the Chinese LEDs on Ebay; Vf= (min=?/typ=3.0/max=4.0) I don't see Vf(min) rated because these are likely 'fall out' parts.

    I do agree with Alec_t that more LEDs are better. Statistics does not say that half of he 2.9V LEDs will be in string1 and the other half will be in string2. Most of the 2.9V could be in string1 while most of the 3.8V could be in string 2.

    We get the LEDs sorted into 5 to 10 groups. I really don't like paralleling LEDs because some times they don't work. We don't get LEDs 10 at a time form ebay. Last time I looked...1,000,000/month.
     
    • Like Like x 1
  11. Flyback

    Flyback Well-Known Member

    Joined:
    Jan 5, 2007
    Messages:
    1,989
    Likes:
    34
  12. ronv

    ronv Well-Known Member Most Helpful Member

    Joined:
    May 30, 2010
    Messages:
    4,725
    Likes:
    488
    Location:
    Tucson, AZ. USA
    Your right. No batteries. Mines cuter and cheaper. Looks like the dollar store knocked you off for 25% so it's not all the Chinese.

    ;)
     
    • Like Like x 1
  13. ronv

    ronv Well-Known Member Most Helpful Member

    Joined:
    May 30, 2010
    Messages:
    4,725
    Likes:
    488
    Location:
    Tucson, AZ. USA
    • Like Like x 1
  14. ronv

    ronv Well-Known Member Most Helpful Member

    Joined:
    May 30, 2010
    Messages:
    4,725
    Likes:
    488
    Location:
    Tucson, AZ. USA
    Ron, What do you build with all those LEDs? Any idea of the price?
     
    • Like Like x 1
  15. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

    Joined:
    Apr 17, 2007
    Messages:
    7,253
    Likes:
    961
    Location:
    Loveland, CO USA
    RonV, Flashlights and toys. Years ago I did some LED advertising signs. I don't know prices. Last time I checked the store price for the products (with battery) is less than the price of the battery.
     
    • Like Like x 1
  16. MrAl

    MrAl Well-Known Member Most Helpful Member

    Joined:
    Sep 7, 2008
    Messages:
    11,029
    Likes:
    951
    Location:
    NJ
    Hi,

    Lets do a simple experiment...

    We'll take three voltage sources, each exactly 3 volts each, and connect a 1 ohm resistor in series with each ideal source. We'll then connect all three of these sets in parallel. This is our rough starting model of three 3v LEDs in parallel.

    Now with another fourth voltage source of 6v we connect that in series with 50 ohms, and connect the other end of the 50 ohm resistor to the junction of all the 1 ohm resistors. What we have now is similar to three LEDs being powered by a 6v source through a 50 ohm resistor.

    Now we measure the current through each LED, and we find that each LED has very nearly 20ma through it, and a total of nearly 60ma flows through the 50 ohm resistor. This is what we wanted.

    Now we change the 'characteristic' voltage of one of the LEDs, to 2.9 volts, which means we change one 3v source to 2.9v. That's a change of -0.1 volts. Now we measure the currents in the LEDs again and we find that for the two LEDs that we did not change the current went DOWN to around 13ma, and the current in the LED that we did change went UP to about 87ma.

    This is an extreme case however just to illustrate that the lower voltage source draws much more current now. We would not see that much change in a real circuit because the two remaining 3v sources are LEDs and can not contribute to the current in the third LED. So we modifiy the circuit a little. We add 1N4001 diodes in series with each 1 ohm resistor, and change each LED voltage to 2.3 volts. The current in each LED is again nearly 20ma and the total current nearly 60ma.

    Now we reduce one of the LED voltages to 2.2 volts, again a change of -0.1 volts. Measuring the currents in the two untouched LEDs we now see about 7ma through each one, and the current through the changed LED measures about 46ma.

    So we see that the LED with the lowest characteristic voltage draws the most current and that usually also steals current from the other LEDs.

    The effect is a little more complicated than that because the LED heats up more and so the characteristic voltage goes down more (about -3mv to -5mv per degree C for white LEDs) but then that might heat up some of the others too and reduce their voltage also, but we can see this is not a good thing.

    Doing the same experiment with four 'LEDs' in parallel, we see 20ma for each change to about 10ma for the three untouched LEDs and slightly over 52ma for the one that has it's voltage reduced by 0.1 volt. That experiment requires raising the voltage that powers them by 1 volt to 7 volts. If we instead change the series resistor of 50 ohms to 38 ohms (to power four LEDs with 20ma from a 6v source) then we see about the same results so that doesnt change the outcome very much: slightly under 52ma in the changed LED.

    This experiment however is based on only one LED changing which is probably not the best way to look at it statistically, but from my experience the flashlight fails one LED at a time, which would seem to give this view some validity. It could be that the thermal runaway trumps the other effects and so that one LED fails, then the next, then the next, etc.

    It should be noted that this was an extreme case too, as the real LEDs have more series resistance than 1 ohm. But with several in series strings where the strings are in parallel, the LEDs have more chance of survival because there is more series resistance due to the addition of all the series LEDs resistances. More overhead resistance is always better from a lifetime point of view, but of course not from an efficiency point of view :)
     
    Last edited: Sep 17, 2013
  17. rumpfy

    rumpfy Active Member

    Joined:
    May 26, 2012
    Messages:
    384
    Likes:
    61
    Location:
    Melbourne, Australia
    see attached file.
     
  18. Flyback

    Flyback Well-Known Member

    Joined:
    Jan 5, 2007
    Messages:
    1,989
    Likes:
    34
    thanks Rumpfy, though if your analysis is correct,why are these ledstriplights, which power 60 leds from a 12v power supply, clearly using series dropping resistors in each led string?..(you can see the chip resistors in the picture)

    http://www.ledstriplights.co.uk/1m-120-led-single-colour-3528-strip-light.html

    Your analysis of statistics shows what is the most likely outcome, -but still doesnt mean that worser cases cant happen (by "worser cases, I mean one series string having mostly low voltage leds in it)
     
  19. rumpfy

    rumpfy Active Member

    Joined:
    May 26, 2012
    Messages:
    384
    Likes:
    61
    Location:
    Melbourne, Australia
    Hi FB,
    there's a fair bit in your post.
    The stats don't show what is most likely; they show the probability of occurrence of a certain event. With data sheets it is always difficult to know what each manufacturer does with his production. Some processes will produce a far greater spread than the published limits; so it is presumed that the spread of results for a particular parameter will not show the typical bellshaped curve but will show a sharp edge at the parameter limit. Selecting out product in this way will not produce a statistical model which is amenable to analysis.
    In the past, there was an Australian manufacturer of silvered mica capacitors. The 1% types were selected first. Then the 5 % types. Then the 10% types, and lastly, the 20% types. These could be guaranteed to be at least 10% away from the nominal value. We only needed about 50 samples to see this selection was going on.
    Since then manufacturers have gotten somewhat better, but the possibility is that selection does occur which does distort the parameter distribution. My experience tells me that there will always be some customer who will be happy with any spec. "I just want a low price!"
    There is a technique for parameter selection based on exponential smoothing. This judges the inclusion of a particular sample based on what it does to the statistic. If the statistic goes outside the limit, then that particular sample is excluded from production. If the smoothed spec is say 30% of the maximum limit, then the accepred samples will give a production with a 'NORMAL' distribution.
    In the previous attachment, I assumed the population was 'NORMAL'. If the maximum limit for Vf was say 2.5 volt, then this parameter would be exceeded only less than 0.5% of the time. For 4 LED in series, the probability of Vf of the string exceeding 4x 2.25 volt (9 volt) would be less than 0.5 %. The effective spread has therefore been reduced.
    The use of LED's is constrained by the need to guarantee that a device is not used outside its published limits. Using a ballast resistor will achieve this at a power loss cost. What I wanted to show in my previous post, was that a ballast resistor is not always necessary but there are limitations. One of these is the need to use a known and stable power supply voltage.
    In your example of the LED string, you open up another set of requirements.
    From the published data we know:
    in one meter of length, there are:
    120 led's / 9.6 watt / 800 lumen / Led in groups of 3 lamps and a 150 ohm resistor / 12 volt supply.
    Current per 3 led group - 20 mA
    Luminous efficiency 84 Lumen per Watt
    Current per 3 LED group 20 mAmp
    Power dissipation per LED 80 mWatt.
    LED's are on PLCC2 bases (NOT chips)

    data for each LED is a mystery; so in desperation I looked at a data sheet from a real LED maker and found a product ASMT-UWB1-NX302.
    This has:
    Current nom 20 mA / max 30 mA.
    Vf 2.8 min / 3.6 max
    power dissipation 108 mWatt up to 45 deg C
    Approximate dynamic resistance 20 ohm per LED

    If we assume the Vf range 2.8 to 3.6 represents 99% of production, then 3 LED in series reduces this spread (0.8 volt) to around 0.46 volt. Assuming further that Vf mean is 3.18 volt, then the calculated mean current is 12-(3 x 3.18) / 150 = 16.4 mAmp.
    The sensitivity of If to change in supply voltage is then: Delta I = Delta E/ (150 + 3 x 20) = 4.8 mA per volt.

    The circuit arrangement for such a LED string is necessary because it is NOT POSSIBLE for the LED string maker to guarantee the power supply voltage. What does 12 volt actually mean?
    Is it a 12.0 +/- 0.3 volt?
    Is it 'any old 12 v power pack?
    Is it a 12 car battery at say 12.6 volt, or is it a 12 volt battery on charge which is 14.4 volt?
    These questions are real, and go to the safety of the string design. So because the power supply voltage cant be guaranteed, a ballast resistor is mandatory given the potential variation in supply voltage. Given a supply voltage of say 14.4 volt, the LED current could increase to 16.4 +(14.4-12)x 4.8 = 28 mA. This is still less than the published data for the ASMT-UWB1-NX302. Given a string at the low end of VF say 3 x 3.03 = 9.09 Volt, the LED current at 12 volt would be say 19.4 mA. At a supply of 14.4 volt, the LED current could then be 19.4 +11.5 = 31 mA. This would occur less than 0.5% of the time.
    What is not stated in your example is the effect of the series resistance due to the printed conductors. How much current would LED's at the end of the string be carrying. Less than those at the input end!
    hope this helps.
     
  20. Flyback

    Flyback Well-Known Member

    Joined:
    Jan 5, 2007
    Messages:
    1,989
    Likes:
    34
    thanks Rumpfy, tho' as you know we must also appreciate that with leds in parallel, even a tiny difference in vf of the paralleled leds will tend to invite the process of thermal runaway...ie led draws slightly more current, gets slightly hotter, then draws slightly more current due to lowered vf, then draws more current, then gets hotter still, and so on and so on..this is the prime reason why leds should not be paralleled without series dropping resistors.
     
  21. rumpfy

    rumpfy Active Member

    Joined:
    May 26, 2012
    Messages:
    384
    Likes:
    61
    Location:
    Melbourne, Australia
    I agree with you.
    I was looking at another post about LED street lamps. One used 28 LED's in 7 strings of 4 LED in parallel. BUT, the product was a street lamp; designed and built to a specification with a power supply to match. This kind of product is worth REAL money and is definitely not available on ebay! With proper engineering and close consultation with the manufacturer of the LED's, any clever spec could be agreed on with a price to match.
    It's just horses for courses.
    Your (and my) basic point is correct; that car batteries and LED's should not be connected in parallel.
     

Share This Page