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Electronics Help guys!! LED meltdown!!

Discussion in 'Modding' started by El_JimBob, 20 Feb 2004.

  1. El_JimBob

    El_JimBob What's a Dremel?

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    plz refer back to my previous thread if u need more info....
    http://forums.bit-tech.net/showthread.php?t=48438

    finally got my circuit finished and added the fans and LEDs:
    [​IMG]

    problem is, once I bring use the pots to bring voltage up to max (just over 10v), the LEDs go thermonuclear and start fizzing and melting.
    The LEDs are rated at:
    FWD Volts: 3.3v
    REV Volts: 5v
    DC FWD Current: 20mA

    I have been using 470 ohm resistors as the bit-tech calculator suggests.

    At the moment, the circuit is being powered off my dad's 12v test supply.
    At around 5v the LEDs light up, around 7/8v they start to change colour - by 12v they are dead.

    Can anyone suggest what i may be doing wrong? My dad says the fan may have a certain amount of reverse current on it, but he's not sure.
    Any ideas?
     
    Last edited: 20 Feb 2004
  2. cpemma

    cpemma Ecky thump

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    Most likely is a shorted or wrong value resistor. Check your 470R resistor value - colours should be yellow-purple-brown--gold, if they are stick a multimeter on it cos it's not 470R now.

    Case fan ain't to blame, brushless ones don't let any back-emf out.
     
  3. El_JimBob

    El_JimBob What's a Dremel?

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    The resistor is the right value - 470 ohm.
    Checked for shorts but there are none.
    My dad dug out a 1,000 ohm resistor and replaced the 470 ohm with it - instant success - 12v on the test supply gave just under 3.3v to the resistor with a current of just under 10mA.
    Just out of interest cpemma, my dad lowered the volts on the test supply until the fan stalled. He span the blades both ways with his hand and observed an amount of reverse current on his multimeter - could this be the fan or more likely the circuit i have made?
    Im beginning to wonder if i have out-of-spec LEDs....
    ????
     
    Last edited: 21 Feb 2004
  4. cpemma

    cpemma Ecky thump

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    Is this an ordinary case fan? I've never seen any "dynamo" effect with brushless PC fans, the motor coils are switched by a transistor which won't work without the supply. Odd...

    If you've got a 1k resistor fitted, LED current is probably no more than 7mA if the max voltage is around 10.5v; with 470R it shouldn't be more than 15mA so those leds seem a tad fragile. :worried:
     
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  5. El_JimBob

    El_JimBob What's a Dremel?

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    ...exactly.....i bought them in bulk - didnt wanna take Maplin up on their £2.99 for 1 offer :lol:

    It is an ordinary case fan - an Evercool 12cm case fan rated at around 3w and 0.28A
    The current we measured at the LED (with both the 470ohm and 1,000ohm and voltage to LED at 3.3v) always hovered around the 10mA mark.
    My dad said it was more likely that the LEDs were out of spec, but as he had observed the fan dynamo effect (as you called it) he wasnt so sure.

    As it goes, the LEDs seem quite happy with their 1,000ohm resistors in place now - seeing as your the 'dimming LEDs' king round here, here's one for ya while yr on a roll....

    The circuit voltage goes down to about 5v - The LEDs dont appear to dim much from 10v down to this point. Is the only way to get a good brightness variation (to signify fan speed) to lower the brightness in the first place (ie: the volts going to the LED)???? I noticed variation is a lot better when theres less volts going into the circuit - could you approve/disprove this theory and suggest what i might do?

    You are the absolute boff king round here. Many thanks for your help tonight.
    :thumb:
     
  6. cpemma

    cpemma Ecky thump

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    Best way I've found to get a decent-looking dim is with a few diodes to increase the forward voltage likee so, which gives a much bigger change in current as you turn the voltage down.
    Try keeping the 1k resistor and adding 4-7 cheap diodes (1N4001 or 1N4148) till you get the effect you want. :cooldude:
     
  7. El_JimBob

    El_JimBob What's a Dremel?

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    Thanks again - u are a star
    As soon as i can get hold of some diodes, ill let u know how it goes :thumb:
     
  8. El_JimBob

    El_JimBob What's a Dremel?

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    Works like a charm matey :)
    In an amazing example of pure coincidence, my dad happened to have a bag containing about 100 said diodes - Im currently weighing up whether to use 5 or 6 per channel - 6 dims the LED to almost an off state, but im not sure if thats what I want yet.
    I did notice that the heatsinks on my 7805s are a little bit hotter since i added the diodes. I spose this is down to the extra current the diodes are pulling?
    My fanbus is perfect now - gotta do the modding part but ill post some circuits and pics here once ive finished it.
    Once again, thanx for your help.
     
  9. ConKbot of Doom

    ConKbot of Doom What's a Dremel?

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    I've had that work for me too, a 40mm led fan lights up when I blow on it (rather hard) I think I spin it backwards, and the leds light up.

    I also took a stack of 3 80mm fans turned on the bottom two, and was getting ~2v from the one that wasn't powered.

    (Isn't the hall sensor on the low side of the coil and not on the hot?)
     
  10. cpemma

    cpemma Ecky thump

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    Yup, the A276 below is used in a lot of fans. The open-collector outputs will sink 300mA so can switch most fan motor coils directly. I'd have thought with the transistors off there was no path for juice to flow but theory goes out the window when facts show different. :confused:

    [​IMG]

    :idea: Perhaps the fan generates juice that works the electronics? A bit chicken & egg... :blah:
     

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