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What voltage are these clear color LEDs?

Discussion in 'General Electronics Chat' started by gary350, Aug 5, 2017.

  1. gary350

    gary350 Well-Known Member

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    I bought this sign $2 at a yard sale it has no power supply. Light bulbs are plastic with an LED inside. 8 LEDs wired in parallel. There is a place to plug in a power supply/transformer but voltage is a mystery. All the lights are wired in parallel, there are no other parts, no resistor.

    It will not light up with 1 AA battery. It lights up very dim with 2 AA batteries in series. It lights up brighter with 3 AA batteries in series. It is still not very bright with 3 batteries at 4.5v.

    I can not find a store with replacement light bulbs I was hoping to find a voltage on the light bulb package.

    I have a 12V power supply with the correct plug. 12v is probably too much it might burn out all the LEDs.

    What voltage are these 3/16" diameter LEDs?

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    Last edited: Aug 5, 2017
  2. be80be

    be80be Well-Known Member

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    There supplied with a
    [​IMG]
    Constant current supply too high of voltage will burn them out. Got be a current supply that just gives whats needed
     
  3. JonSea

    JonSea Well-Known Member

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    Good idea Burt, but if the LEDs are in parallel, a constant current supply isn't going to work.
     
  4. dave

    Dave New Member

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  5. be80be

    be80be Well-Known Member

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    Maybe but I've installed a bunch of those signs and the all used that kind of power supply it outputs only whats needed. I have some I don't think there just currant control because they control voltage to that sign use about 6 volts ones I use are 6 volt too 12 but they only output whats needed.
    I'm a Electrician 34 years I've seen these none used a wall wart.
    I would start at 6 volts and go up
     
    Last edited: Aug 5, 2017
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    LED flashlights (torches in the UK) have 24 LEDs in parallel. The forward voltage of all the LEDs are matched then a single current source powers them perfectly.
    This sign has parallel LEDs so their forward voltages are also matched. If the forward voltages are not matched then the LED with the lowest voltage will be the brightest because it hogs the current and it will probably burn out soon.
     
  7. be80be

    be80be Well-Known Member

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    I've never installed any that didn't use a Constant current supply. Now led lighting most them strips they use a resistor on each led and a plain 12 volt supply. Then there the color ones and they get fancy.
     
  8. Beau Schwabe

    Beau Schwabe Member

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    If you can isolate one of the LED's then get a digital voltage meter and put it on DIODE check.... look at the forward voltage reading.... typically this is somewhere near 1.7 Volts, but depending on the LED this can vary ..... 12V would most likely be just fine for 8 LED's in series.


    EDIT: Sorry, I somehow thought that the LED's were in series
     
    Last edited: Aug 9, 2017
    • Disagree Disagree x 1
  9. be80be

    be80be Well-Known Member

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    12 volts and its bye bye leds it's 8 LEDs wired in parallel.
    and it uses a Constant current supply if one led fails they still light up and you replace the bulb I no i do this stuff for a living 34 years.
    They come in little bulbs like these
    [​IMG]
     
  10. JonSea

    JonSea Well-Known Member

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    Walgreen's is a pharmacy chain here, with a sign of red script letters. I think most of these signs have been converted to LED illumination....apparently very cheap LED illumination. The stores usually have signs on two exterior walls. It is extremely rare to see these signs without a burned out section.

    If you have a constant current power supply, you could apply around 10mA to a bulb and measure the LED's forward voltage.

    Could there be resistors in the base of each bulb?

    Burt may be correct in that the sign uses a constant current supply with parallel LEDs. As AudioGuru says, if the bulbs aren't well matched, the one with the lower forward voltage will hog all the current. I have usually seen constant current supplies used wirh series bulbs. The down side of a series connection is that a blown bulb will extinguish the whole string.
     

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  11. alec_t

    alec_t Well-Known Member Most Helpful Member

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    I'm guessing there's a series resistor within the bulb, since bulbs with an MES base seem to be spec'ed for a specific voltage, e.g. 6V or 12V as in this example. If that's the case then a fixed voltage supply is what you need.
     
  12. gary350

    gary350 Well-Known Member

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    I worked for a sign company once many years ago, I never saw a sign that did not use some type of constant current transformers or circuit breaker the #1 reason is, being outside in all types of weather it will not over load and start a fire if there is a short circuit. That is why neon signs have shunt transformers in a wide range of volts and ma. We designed and built signs according the amp load needed to run all the lights. Signs were also designed in sections if 1 section shorted out and turned OFF the remaining part of the sign stayed ON.

    When I worked in the neon sign department large signs sometimes had 8 to 24 transformers. The length of the tube determines the voltage. The number of tubes in parallel determined the current. The type of gas also determined current and voltage.

    This sign I have is 1 section 8 LEDs in parallel. I have no clue what current for each LED is or if there is a hidden resistor inside, if each LED is 1ma x 8 = 8ma. If a standard size PS is 10ma that is probably what this sign needs. Voltage can not get too high or all 8 LEDs will burn out.

    I have a box full of those plug in powers supplies with a whole range of voltages, 5, 8, 9, 12, 15, 18, 24 volts. I took a risky chance and tried the 5v and all 8 LEDs light up but not very bright. I can not just keep trying different voltage 1 by 1 soon it will be the wrong voltage and all the bulbs will burn out. I need a target voltage to shoot for unless I can find replacement bulbs with voltage information on the package?

    I can see down inside each light bulb there is a female plug with the LED plugged into it. I used my analog meter to test the bulbs because the meter has 2 AA batteries inside i get no reading on ohms even after swapping the red and black wires.

    I need to take my good meter apart to see what is inside, don't know how many AA batteries it has, might even be a 9v battery in there?
     
    Last edited: Aug 6, 2017
  13. alec_t

    alec_t Well-Known Member Most Helpful Member

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    There are plenty of MES bulbs with known ratings advertised.
     
  14. gary350

    gary350 Well-Known Member

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    What is MES?

    I am going to try 6v PS if I can find one. Might test it with 4 AA batteries first.

    Seems like I saw a chart once that tells the voltage of each color and size LED?
     
  15. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Miniature Edison Screw.
     
  16. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    5 mm Red LEDs are like a 2V zener with an ESR of about 10 Ohms. White are 3.1 V and 12 Ohms ESR

    What size are these around the lens? 5?8?10?(mm)

    8 LEDs of this type in parallel is still like the same Zener voltage but now ESR/8 Ohms or between 1 & 2 Ohms ESR. (Zener specs use Zzt to mean ESR which is about 1/Pd the max power rating of the low voltage diode.)

    Did you actually measure the battery voltage Voc, Vload and use Ohm's Law to compute the result. This will tell you the exact divider ratio of the battery ESR and Load ESR. Since AAA batteries have an ESR of about 1 Ohm and AA about 2/3 of that give or take depending on Voc / pulse short circuit current=ESR.

    Ohm's Law will tell me exactly what Voltage and source impedance to use, or current rating.

    You never use a CC source because if one LED fails, the rest increase their share of current.

    I might try a 1 Ohm series R from a high current low voltage supply like 3.3 V or a couple Ohms with 5V ATX PSU. Then the voltage drop in the known R value tells you the exact current being shared.

    Come on guys, this is basic Ohm's Law. The only trick is understand the ESR or 1/load regulation delta v/delta I of the source and the load. The ideal match is to match the load ESR to the source ESR such that the source voltage drop matches the load voltage rise . e.g. RED LED rises from 1.8 to 2.0 over 20mA approx or 0.2V/0.02A= 10 Ohms. while White may rise from 2.85 to 3.1 V or 250mV/20mA = 12 Ohms. Normally a 25% tolerance and good ones will be lower ESR, and poor ones higher. Also higher power LEDs will be lower again where ESR=1/Pd power rating of LED.

    If in series, then add the Vf and ESR's to match the equivalent load.

    I've sold about a million of these types (previous 10 yrs during my retirement) of 5mm LEDs in RYGBwhite types. with very little variation. I still have 20k excess if anyone wants some , send postage or buy 200 for any reasonable offer, and I will mail it to you. The LEDs in each bag are the brightest in the world and all ESD protected.. e.g. 16000 mcd 30 deg ANSI neutral white or 20000 mcd or 10,000 mcd RED or 12,000 mcd yellow 30 deg. The angle affects peak brightness with lens ~2x ((1.8))for each reduction in beam-width /2.

    Each batch is so perfectly matched they can all be run in parallel without risk of thermal runaway ( <<1% matched Vf)

    People use CC regulators in large power LEDs so they can substitute a wide range of suppliers with >25% tolerance and deliver a constant shared current in series or use a resistor drop of at least 3 V in strip LEDs so it can be operated from a car from 11 to 14 .2V at varying brightness.

    These LEDs in your sign will be fairly well matched if from the same batch, so a CV with supply ESR to match the load is ideal. If each LED is 8mm is 100mW and they are RED then you need a 400mA at 2.2V maybe , guessing size and quality of your sign LEDs, more if poor quality, but if one battery was weak, or the LEDs are poor quality, you may need more. If they were clever, they would have hidden a series R equal to the tolerance of the ESR for those LEDs in each bulb. (to equalize or reduce the variation of If on each chip, the shunt current. Or perhaps they added 10 Ohms in series raising the Net ESR . again Ohms Law rules above threshold voltage, Vth where (Vf-Vth)/If~ESR using about 10% of rated current for Vth
     
    Last edited: Aug 7, 2017
  17. gary350

    gary350 Well-Known Member

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    I tested each LED light bulb with my Victor VC9805A meter. I get no reading on 200 ohms, 2k, 20k, 200k 2m ohms, swapped red & black wires still no reading. No reading on diode test either. All the lights still light up with 3v, 4.5v and 5v. If i tough the meter leads together i get 0 ohms. The bulb measures about 3/4" diameter. The LED looks like 3/16" diameter.
     
    Last edited: Aug 7, 2017
  18. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    What did I say to mislead you into thinking you can measure ESR with an Ohm Meter. You use a power supply with a 1 ohm series R or or < 100mV drop R to ΔV/I. If you power supply drops, thats a problem, the the LEDs are still dim but the voltage stayed constant, then you dont have enough voltage.
     
  19. JonSea

    JonSea Well-Known Member

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    There is some conflicting information here from different posters, including that you can make an LEDs light using the diode test or resistance measurement of a DVM. Your reply seems a bit hostile to the OP just trying to follow the various suggestions.
     
  20. gary350

    gary350 Well-Known Member

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    Tag on PS body says, 5v 1.2a

    PS voltage is 5.15v with no resistor with all 8 LEDs on.

    I have a 1 ohm 3% resistor soldered in the circuit in series with 1 positive wire.

    PS voltage is now 5.16v with the resistor.

    Voltage cross the resistor is .14v

    [​IMG]
     
    Last edited: Aug 8, 2017
  21. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    3/16" = ~5mm rated for 20mA nom. thus with 8 LEDs = 160mA or 0.16V drop on 1 Ohm. You are getting 0.14 drop so these must have series R's built-in and it seems to be matched to a 5V supply. Is it bright enough? This is all you can expect from these bulbs. If you want much brighter, I can send you something 100x brighter but ~< 1/3 the beamwidth. 30deg instead of 100deg. You can then put 3 in parallel for each bulb and draw 0.5A and it will be blinding at arms length but clearly visible at 100ft. with added Rs of 1.9V/If for a 5V supply. or use a 3.3V supply with a smaller Pd. and smaller R value. Your choice.

    The photo shows white. is that correct?

    I prefer 4500'K rather than warm or cool. So in White, this is what I use.
     
    Last edited: Aug 8, 2017

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