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How to Power These LEDs?

Discussion in 'd.i.y.' started by Suffolk Tony, Oct 18, 2021.

  1. Suffolk Tony

    Suffolk Tony Aim low, achieve your goals, avoid disappointment.

    Hi folks, I'm fitting a digital amp into an old Denon case. This has an LED “Strip” arrangement, four LEDs in two strips as per the photo. I'd like to use this to show when the amp's powered up, but I've no idea how to work out the required power. The amp's got a 24v unregulated output which I'd like to use. Any help would be appreciated!

    [​IMG]
     
  2. garyi

    garyi leave blank

    well with the amount of leds I have blown up I am willing to bet they arn't 24v.
     
    chiily and Suffolk Tony like this.
  3. mansr

    mansr Objectionist

    What does the other side of that PCB look like? What model is the Denon?
     
    Suffolk Tony likes this.
  4. davidsrsb

    davidsrsb pfm Member

    We can figure out the circuit from the other side. That pcb already has a series dropper resistor of what looks like 510R.
    You want to run the strip at only 1 or 2 mA unless you want to light your room with it.
    Worst case all leds are in series means a drop of 8x1.5V so the resistor drops 12V.
    You want about 10k for 1.2mA
     
    Suffolk Tony likes this.
  5. Suffolk Tony

    Suffolk Tony Aim low, achieve your goals, avoid disappointment.

    Thanks for the comments & help chaps. The Denon's an old AV half-width UPO-250 amp. The LEDs are connected in series, with no additional components on the backside of the board. Yes, the resistor's 510 ohms. Not sure why there are three connections though.

    [​IMG]
     
  6. a.palfreyman

    a.palfreyman pfm Member

    4 LEDs in series, say about 8V drop. That leaves about 16V which, across 10k is 1.6mA. You could go as high as 33k for 0.5mA, depends how bright you want it.
     
    Suffolk Tony likes this.
  7. Suffolk Tony

    Suffolk Tony Aim low, achieve your goals, avoid disappointment.

    Many thanks, Mr Palfreyman. I'm not after anything bright, just enough to show the amp's powered up. I'll give the 33K a try.
     
  8. a.palfreyman

    a.palfreyman pfm Member

    Yes, I have some blue LEDs and these are very bright even at 0.5mA!
     
  9. mansr

    mansr Objectionist

    I found the datasheet for those LEDS:
    [​IMG]

    [​IMG]
     
    davidsrsb and Suffolk Tony like this.
  10. davidsrsb

    davidsrsb pfm Member

    With old technology LEDs like these, I would not go below 1mA as brightness may be very uneven.
     
  11. earlofsodbury

    earlofsodbury Wastrel.

    If in (complete) doubt, slap 'em on an a variable DC PSU and slowly dial in volts and amps until you get the light level you want, spec the R accordingly (may involve small amounts of maths). Crude but effective.
     
    Suffolk Tony and Mynamemynaim like this.
  12. Mynamemynaim

    Mynamemynaim 38yrs a Naim owner

    ^^^That's what I did with my red ones ..^^^^^^

    Ended up with 100k resistors
     
    earlofsodbury likes this.

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