Would 5VDC instead of 4.5VDC be a problem for LEDs?

Discussion in 'General Electronics Chat' started by Johnnyb60, Feb 15, 2015.

  1. Johnnyb60

    Thread Starter Member

    Sep 6, 2013
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    OK so I know this is a dumb question to you guys, but I'm just an amateur.
    Anyway I have a 4.5VDC 1000mA power adapter running an array of LEDs and I want to add more LEDS, but I can only find a 5VDC adapter with the increased load to replace it with and I was wondering if it would still be within range of the LEDs. I don’t know the specks of the LEDs and I don’t know why the original design called for 4.5V.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    LEDs should be current-driven, not voltage-driven. If driven from a fixed voltage there needs to be a resistor in series with the LEDs to limit current to a safe level. Without knowing the spec of your LEDs it would not be wise to try adding more. We don't even know how many LEDs are in your array :rolleyes:.
     
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  3. Johnnyb60

    Thread Starter Member

    Sep 6, 2013
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    Thanks for replying Alex, Sorry about that. I was more concerned about the voltage and didn’t think about the current. So there are actually 4 separate arrays each rated at 300mA at 4.5V DC.

    I think I figured out that the 4.5v rating is because they each can be hooked up to 3 AA batteries although I’m not using batteries and want to have just one power supply for all 4. I’m sick of all the wires and just want to clean it up a bit.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If each LED is being run at its maximum safe current when a 4.5V supply is used, then a 5V supply might fry the LEDs. What limits the current at the moment?
    You could try a 3A diode in series with the 5V to drop it down to ~4.3V.
     
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  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    @Johnnyb60
    If the driver for the LEDs is in fact current driven as ALEC_T suggested, then input will not likely change the constant current source much if any (but does depend on the exact nature of the current source circuit.

    If the driver on your rope is voltage controlled lets assume you have indicator light style LEDs rated at 30 mA each. If white, then, let's assume a forward voltage of 3.3 volts. At this supply voltage you will not have multiple LEDs in series. They are also likely running below their max rating, let's say 25 mA. So, we assume at 4.5 volt supply and 3.3 forward voltage, you need a resistor for each that will drop 1.2 V at 25 mA. That would be 47 ohms.

    Now, let's assume you connect 5 volt supply and you must drop 1.7 volts instead with that same 47 ohm resistor, you will be passing 36 mA.

    Now, LEDs. Are often operated above their rated current, especially when pulsed at short duty cycle. In other words, there is no magical fuse in there that pops once you exceed 30 mA. You may shorten the life by 50% but I think the word "fry" is a bit extreme.
     
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  6. jamesd168

    New Member

    Nov 8, 2014
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    I think this is a better solution than dropping the voltage down with resistor. unless you know exactly how much current you need, the resistor will introduce less or more voltage drop than you want.
     
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  7. Evil Lurker

    Member

    Aug 25, 2011
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    It should be noted that batteries have their own internal resistance. You can hook up an actual power supply and easily cause the LEDs to let out the magic smoke due to over current even though it is the same voltage.
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    The forward drop of a silicon diode is just 0.2V more than you need, but at least it would be safer to be a tiny bit under than half a volt over.

    A 1N4001 per 300mA load should be OK - a 1N5401 would certainly run cool.
     
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  9. Johnnyb60

    Thread Starter Member

    Sep 6, 2013
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    Thanks for all the replies, first I want to mention that I’ve never used the battery option and for years I’ve had a separate 4.5V power adapter for each array. I had 3 sets of lights and now I’m adding one more and they will all be connected in parallel. I don’t know if there is any other resistance connected to the LEDs and I would have to destroy it to find out, but I would think it would have to be inside the power adapter itself if any. Oh I just remembered that some of the LEDs flash if that makes a difference.

    I could just use the 4.5V power supply's that came with them, but then I have 4 to deal with. Three was bad enough and takes two 120V extension cords to plug them all into.
     
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