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12 volt voltage "clamp?"

Discussion in 'General Electronics Discussion' started by willymc, Jan 26, 2013.

  1. willymc

    willymc

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    Jan 26, 2013
    Hi, I'm new here so bear with me. I have some experience with electronics and circuit design, but it's been awhile. Here is my situation. I have a motorhome and I want to convert my 12 volt incandescent lights to leds. I put in some led boards and they kept overheating and burning out. It appears that my supply voltage varies too much. When my solar panel is charging my batteries, I have 13.5 volts. I beleive that is what is overheating my led boards. Putting in a 7812 12 voltage regulator won't work because they have a drop out voltage of 1.5 - 2 volts. I don't always have the 13.5 volts. I need something that will limit the output to a maximum of 12 volts. If the input voltage falls below 12, the output will follow it down. If the input voltage goes above 12, the device will limit the output to 12 volts. Any suggestions are appreciated. Thanks.
     
    Last edited by a moderator: Jan 26, 2013
  2. BobK

    BobK

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    If you could break open one of the LED modules, it might be no more difficult than changing out a resistor. If you could post pictures we might be able to help.

    Bob
     
  3. willymc

    willymc

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    Jan 26, 2013
    I'll peel off the backing on one of the light boards and try to get a picture of the front and back. May take awhile.
     
  4. willymc

    willymc

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    Jan 26, 2013
    Hello again. Pictures won't help. This is what I found. On the front there are 24 - 5050 SMD brightwhite LEDs. On the back there are 8 - 57 ohm resistors.
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    If the LEDs are burning out at 13.5V (and they have series resistors) then something seriously wrong is happening.

    I would suggest one (or more) of the following:

    1) they're set up for 6V, not 12V
    2) they require heatsinking at the rated current
    3) the ambient temperature is very high
    4) poor design

    Let's do some calculations...

    24 LEDs, and 8 resistors. That means a string of 3 LEDs. Let's assume 3.4V, that means the resistor drops 1.8V. At 12V the current is therefore 31 mA. I think that's possibly on the high side.

    At 13.5V, the resistor drops 3.3 volts, leading to a current of 58mA. That is almost certainly excessive.

    I would recommend the LDO regulator. At the sort of current you're talking about (240mA) the dropout voltage is likely to be small (it tends to increase with current -- check the datasheet of any device you want to use).

    If you're using an LDO regulator, beware that they generally have requirements regarding input and output capacitances to keep them stable.

    From my list of 4 options, the answer is clearly (4). If they had used 2 LEDs in each string, the impact of the change in voltage would have been less. I will note that 13.5 is well below the voltages I might expect to see in a car 12V system in any case.

    Rewiring these in strings of 2 is likely to be far too much effort, although it would allow you to use a regular regulator (something like 9V)
     
  7. BobK

    BobK

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    Jan 5, 2010
    Or, you could try changing the resistors. Measure the current at 12V multiply to get the current and voltage drop the way it is specified, then recalculate for 13.5V.

    Bob
     
  8. willymc

    willymc

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    Jan 26, 2013
    Thanks for the input. I agree, it's #4. Rewiring or changing resistors is out of the question, sandwiched pc board. Here is where my lack of experience kicks in. If I was to put a 7812 voltage regulator (with a 2v dropout voltage) in, what would the output voltage be with say a 12v input?
     
  9. galaxy

    galaxy

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    Nov 3, 2012
    Assuming it is the over voltage blowing the units..
    We are only talking fairly low current and voltage, so why not just use a Zener to clamp it?
     
  10. duke37

    duke37

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    Jan 9, 2011
    You could leave the internal resistors in place and add an external resistor to drop about a volt, say 4 ohm.
     
  11. Timescope

    Timescope

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    Aug 30, 2012
    I am using the characteristics of filament lamps to regulate the leds shown operating off my 12v system . The two orange blobs are the filament lamps connected in series with the leds. Filament lamps have a very low resistance when cold, at low voltage. When the voltage is increased, the resistance rises. At a certain voltage, when visible light is emitted, the lamp turns into a constant current source and the current remains essentially constant as the voltage increases.The attached image shows the lamps on at 14.0 v. When the battery voltage falls, the filament lamps gradually go off leaving only the leds illuminated. Select a low voltage lamp with appropriate current rating, two or more in parallel if necessary. Both lamps are in parallel in this circuit.

    Timescope

    Image538.jpg
     
    Last edited: Jan 27, 2013
  12. willymc

    willymc

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    Jan 26, 2013
    Just got back from Radio Shack. Picked up a 7812 regulator. I put it in the circuit and connected it to a 13.8 volt power supply. I got an 11.8 output voltage and the LEDs lit up. I kept them lit up for about ½ hour and they didn’t overheat. My batteries aren’t always 13.5 volts, so a 2 volt dropout voltage would be too much when my batteries get down to 12 volts. I think I could live with a dropout voltage of .5 - 1 volt. So, looks like LDO regulators will work.
     
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