How to test unknown LED configuration

Discussion in 'General Electronics Chat' started by Tedzz, Mar 5, 2016.

  1. Tedzz

    Thread Starter New Member

    Mar 4, 2016
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    Hi,
    I bought a lamp at an auction. I didn't want the lamp. It's a Yashica camera attached to a angled lamp stand (like an anglepoise lamp) that's been adapted to take an LED plate . It looks quirky and odd. I'm not sure what to do with it (but that's another story).

    It didn't come with a mains adaptor although the end of the cable terminated in a DC jack. So thinking "what the heck", I coupled the light to a 12vdc 1.5amp transformer and it lit up.

    My question is; Is there anyway of calculating the right v/a that this unit actually needs with a meter? My worry is that I'm giving the lamp too much and it won't last long. Should I start off with lower voltage?

    SDC11519_800x600.jpg
     
  2. #12

    Expert

    Nov 30, 2010
    16,257
    6,763
    A wad of LEDs facing me doesn't say anything about the circuit. You have to look on the back-side.
     
  3. hp1729

    Well-Known Member

    Nov 23, 2015
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    Just going by the picture it looks like ...
    21 V, 115 mA, 3 .3 W
    Do you have an ohm meter that you can trace LED connections?
     
  4. Tedzz

    Thread Starter New Member

    Mar 4, 2016
    4
    0
    An ohm meter, yes. But as there's no access to the back of the LEDs - siliconed up to the max...

    But, I think you're right about the type of LED's. What do you reckon I can "safely" supply to them?
     
  5. hp1729

    Well-Known Member

    Nov 23, 2015
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    Variable power supply. Start low and work your way up.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    Can you tell if there could be anything else in the circuit besides the LEDs? If it's just LEDs and not also a constant current driver, the LEDs won't light until you reach Vf for the LEDs and max current will be at maybe 0.5V above that. In other words you may be lucky you haven't blown them up already! But maybe the lamp was designed for 12V and you got away with it. Or maybe it does indeed have an integral controller inside it.
     
  7. Tedzz

    Thread Starter New Member

    Mar 4, 2016
    4
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    I bit the bullet and stripped it down - photo attached. Does it help?
     
  8. WBahn

    Moderator

    Mar 31, 2012
    17,716
    4,788
    Now that you have both sides, you can back out a schematic of the circuit.

    The SS14 devices appear to be 1A Schottky diodes. The R680 part is probably a 0.68 Ω resistor (the R is being used as the decimal point), but that's not guaranteed as it could also be a 68 Ω resistor or a 680 Ω resistor depending on what marking protocol the manufacturer used. The cap is a 4.7 uF, 50 V cap (pretty obvious, that one). The large device at the bottom looks like it is probably an inductor. The IC is probably a driver that senses the current and then pumps the voltage in order to maintain the desired current. I couldn't find a part with those numbers, which may mean that it is a house-labeled part (or it might just mean that I didn't look the right way).
     
  9. Tedzz

    Thread Starter New Member

    Mar 4, 2016
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    0
    Thanks. I'll ask my brother in law to work that out as he's far more proficient than I am. Thanks for your help!
     
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,518
    1,247
    Looks like a constant current output switching power supply. SS14 is a 40 V / 1 A Schottky. The resistor is 0.68 ohms, but without knowing the internal voltage reference in the controller chip, we don't know what current level this sets. For example, Linear Tech sometimes has reference voltages down around 55 mV, which would equate to 80 mA.

    ak
     
  11. hp1729

    Well-Known Member

    Nov 23, 2015
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    Those four diodes might form a bridge rectifier so maybe it was built for AC in. Take an ohm meter and see if you find two cathodes connected together and two anodes together. Electrolytic cap across the bridge connections?
    50 V cap? So voltage is not more than 20 to 30 Volts. More support for the 21 V LED possibility.
    If it didn't light at 12 V do you have a 24 V DC source at about 1.5 amp? Or AC if it those diodes do make a bridge. Try it real quickly. LEDs will take a little abuse.
     
  12. hp1729

    Well-Known Member

    Nov 23, 2015
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    Maybe this?
    3.6 V per LED at 350 mA???
     
    Last edited: Mar 6, 2016
  13. hp1729

    Well-Known Member

    Nov 23, 2015
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    Physical layout might suggest three rows of four LEDs.
     
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