how to turn off 1 LED in string of 3?

Discussion in 'Digital Circuit Design' started by joewales44, Oct 12, 2017 at 10:50 AM.

  1. joewales44

    Thread Starter New Member

    Oct 8, 2017
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    i'm running 2 strings of 3 LEDs with 1 pwm driver with 12 to 24v. the driver puts out 800mA so each string gets 400mA. i need to have a choice of dimming or turning off 1 LED(3.5Vf) in 1 string. dimming is working fine with 9 ohm resistor across the LED through a switch. how can i turn off the same LED without changing current through the other LEDs? thanks
     
  2. MrChips

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    If you use a constant current source LED driver, you can short across any LED or series of LEDs. The current to the remaining LEDs remain the same.
     
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  3. joewales44

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    Oct 8, 2017
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    i knew that for a single string but does that hold true for running parallel strings?
    thanks
     
  4. MrChips

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    If one string has lower resistance than another it will steal current from the other string, i.e. it is going to run brighter while the other string will go dark.
     
  5. crutschow

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    To keep the current constant you could use a SPDT switch to disconnect the LED and switch in a diode with the same forward drop as the LED.
    This could be two or more standard diodes, depending upon the forward drop of the LED.

    If you are not changing the brightness of the other LEDs with the PWM, then you could replace the LED with a resistor that generates the LED forward drop at the operating current.
     
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  6. ian field

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    No.
     
  7. joewales44

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    Oct 8, 2017
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    i thought if i use a resistor in place of LED with high enough resistance, the current would just go the easiest path through the LED. if i use multiple diodes to burn the voltage, it seems some current would still go through the LED. dimming is easy but i ain't figured out how to turn it off.
     
  8. ian field

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    About 3 standard silicon Vfs in series is somewhere near a red LED Vf. But your 3S2P array won't be very tolerant of even that small difference between the 2 chains.

    How about using more parallel strings of lower rated LEDs and taking out a whole string?
     
  9. crutschow

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    A SPDT switch as I suggested.
     
  10. joewales44

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    Oct 8, 2017
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    i'm using a spdt switch now with a single string and it works right. i'm not sure how to connect circuit when running parallel strings. it seems if i simply bypass the LED, the remaining LEDs would get 20% more current. is that right? thanks
     
  11. ian field

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    A current controlled supply will continue delivering the same current if you remove some of the load - that current will be shared among less LEd strings and they will run brighter.

    Shunting one of the LEDs will give that string a lower sum of the Vf's - that string will clamp the supply voltage and cut off all the other strings.

    LEDs have some parameter spread which includes Vf, you may have got lucky and there's no conspicuous difference in brightnes of each string. Its not a bad idea to give each string a ballast resistor even though a constant current supply is used. You're more likely to get away with replacing an LED with the nearest fit of combined silicon and SB Vf's.
     
  12. joewales44

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    Oct 8, 2017
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    i can't use a ballast resistor because i'm already too low on overhead voltage. i've changed from a schottky to a mosfet to save all the voltage i can. still have barely enough from a 12volt supply. guess i'll stick with a driver for each string since there doesn't seem to be a way to do what i want.
     
  13. crutschow

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    Correct.
    That's why you can put a resistor or a few diodes in the bypass path to drop the same voltage that the LED would.
     
  14. joewales44

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    Oct 8, 2017
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    i understand how to use resistor in series with the LEDs. i'll be glad to look at a circuit with a single switch. i can't use 2 switches and that's the only way i can see how it would work. i need 3 functions, LED off, on, or half power and still not affect current to other LEDs. remember this LED is in 1 string of 2 strings being fed in parallel. thanks
     
  15. ian field

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    Hopefully you'd only be trying to equalise a few tens of millivolts.

    I'm thinking such resistors would be well under 10R.
     
  16. Bernard

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    SW, SP-3T, position 1 - 19 R, pos. 2 - LED, pos. 3 - 19 R in parallel with diode ( .5V ) in series with LED ?
     
  17. joewales44

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    Oct 8, 2017
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    could you provide a sketch of this? as i'm sure it's obvious, i'm not an EE.
     
  18. crutschow

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    I could only do it with a Double-Pole 3-Throw switch.
    R1 is selected to give the same voltage drop as the LED.

    upload_2017-10-16_11-2-27.png
     
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  19. joewales44

    Thread Starter New Member

    Oct 8, 2017
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    you sir, are my HERO.
    i knew this had to be possible, i'm just not smart enough to figure it out. i was trying to break the circuit before the LED instead of after.
    THANK YOU VERY MUCH.
     
  20. crutschow

    Expert

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    Actually it will work either way.
    Just reverse the direction of the LED.
     
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