Can I get away with 2A LEDs with a 12V supply?

Discussion in 'General Electronics Chat' started by Dleberfinger, Jan 2, 2012.

  1. Dleberfinger

    Thread Starter Member

    Dec 18, 2011
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    Basically I have 4 LEDs, all drawing 2A each at a forward voltage of 4.5. I can't even imagine how to power these. I need them to be triggered by the output from a 555 monostable timer.

    Am I going to be able to do this with 12V? I can't imagine that I would, considering that even if I cut the current to 1A each, I would need a 6 ohm, 36 watt resistor to be safe, and that's not considering the 555 timer or the transistor..

    Even with a 24V supply, I can't see how this would work..

    Any help would be appreciated!
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    U can work this out but I need to know the model of ur LED or a picture may be.
     
  3. Dleberfinger

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    Dec 18, 2011
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  4. Audioguru

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    Dec 20, 2007
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    The LEDs ARE NOT 4.5V. They might be 4.5V which is the typical voltage at 1A.
    But you cannot buy "typical" LEDs, you get whatever they have. Each wafer and production lot is different.
    The datasheet says their voltage is anywhere from 3.92V to 5.36V at 1A for Bin Code J.

    I expanded the Typical Forward Voltage graph and got a "typical" forward voltage of 5.4V at 2A. Then an LED with a high forward voltage might be more than 6V at 2A.

    EDIT: If the LEDs have a low forward voltage then they will burn up if you calculate the current-limiting resistor for "TYPICAL" ledS.
     
  5. Wendy

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    Mar 24, 2008
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    The LEDs in question are rated 1A continuous. LEDs can take short pulse at higher currents. They will still have a long life, but it will be shortened. The datasheets define how much you can get away with (and you are getting away with it).

    The specs also list the Vf of these LEDs as 4.5 DC. Thing is, this is an approximation. It can vary a lot, say ±½V. You can measure this and adjust for it, but you do need to adjust.

    The other way is a constant current source. They sell prepackaged versions called buck pucks on the web, but those are expensive. You can make an analog version of a current regulator, it will get hot and need heatsinking, but it is a cheap solution. If you need more information on this just ask.

    I've never heard of a 4.5V Vf LED, but 1A is fairly new device, so I'm not too surprised. You will likely need a 12VDC 3A power supply or greater. Two LEDs will be in series at a time.

    You can do this with a 555. If you are still trying to do the strobe thing it may not work due to the LEDs not being fast enough, but I would be willing to help you with a design. I like Mouser, they deliver in under 24 hours for me because I am fairly local. I know you mentioned it, but have you considered attaching your approximate location on your profile like I have mine? Helps with getting parts sometimes.

    Adding capacitors with such a short duration pulse means you may be able to get a smaller supply and let the caps provide the current. You will need to light these at 1A to measure the real Vf, but we can talk you through it.

    I may get some of these form myself. Interesting parts.
     
  6. Dleberfinger

    Thread Starter Member

    Dec 18, 2011
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    Yes, I am still working on the flash. I was using .5W SMD LEDs, however, they are so tiny that I struggle to solder them, I ruined two of the three that I had bought to test. Also, the light output was significant enough from these LEDs to delve further into this little project and attempt to acquire better results with better materials.

    I would love if you walked me along with using capacitors, I have an even smaller knowledge of capacitors than I do any of the other simple components.

    Where could I get, or how could I make a 3A 12V supply? Currently I am using 8 AAs in series.
     
  7. Audioguru

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    Dec 20, 2007
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    Old green 5mm ordinary LEDs were 2.2V. Modern green 5mm LEDs are much brighter and use the same material as blue and white ones so their voltage is about 3.5V at 20mA. At 1A the modern high power LED is about 4.5V.
     
  8. Wendy

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    Mar 24, 2008
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    OK, you will need bigger batteries, just for the intermediate tests. A lead acid is capable of powerful short bursts, but you don't want to discharge it too much.

    The AA batteries, coupled with a bank of fast capacitors (not electrolytic) would probably work OK for 2A.

    You would need a simple heat sink to do the preliminary tests too.

    Getting a current limiter to operate in the 2µs range could be a challenge, where as a resistor is no big deal. The reason I keep mentioning the preliminary tests is to find the correct resistance you need.

    A short duration pulse means no heat sinks needed. Longer durations means you will need something to draw the heat away. You could probably use 100ms (0.1 seconds) and not need much, but just in case of a malfunction I would have some sort of a heat sink, something that would protect the LEDs until you can yank the power. The 100ms pulse would be useful for a test too.

    I'm basically free wheeling thought mode here. You want some ideas on how to mount these suckers I've done similar already.

    http://forum.allaboutcircuits.com/showthread.php?t=32364

    Follow the links in the thread.
     
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    There was a period back in the late 80s/early 90's when some red LEDs could be lit up by a single 1.5V "fresh" cell, but not green or yellow ones (all of the choices).

    Then, with blue LEDs (Needing 4.2V in some cases) came the "bright" and "super-bright", where they changed the composition, and all need over 2V, sometimes 3V.

    Now they are pretty standard at around 3V somewhere for super-bright LEDs.
     
  10. Wendy

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    Here is a picture of how I tested a high power LED a couple of years back. It was using a old computer heat sink, 286 style, no fan.

    [​IMG]
     
  11. Dleberfinger

    Thread Starter Member

    Dec 18, 2011
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    Well, I can't find a buck that can handle at least 1A even that isn't $50 or more. If I'm not mistaken, a simple 1.5A buck can be made from an STSC chip. I'd much rather do that, considering they are $3..
     
  12. Wendy

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    Mar 24, 2008
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    Problem is speed. I don't think it will have it, which is why I keep looking at resistors.

    You only need a buck if it is going to be on for a long duration, and even then it is just an option.

    Do you have the LEDs yet, or are you thinking about it?

    If you have them, 1st thing is to mount them on a heat sink.
     
  13. Dleberfinger

    Thread Starter Member

    Dec 18, 2011
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    Oh, that's great. I guess I misunderstood then. I thought I would have to have the buck for these higher powered LEDs.

    I have them ordered, they should be here by early next week some time and I have plenty of heat sinks, so I will put one of the LEDs on the sink.

    So would I be ok simply buying one of those insanely high wattage resistors?
     
  14. Dleberfinger

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    Dec 18, 2011
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  15. Audioguru

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    Dec 20, 2007
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    The output current of the simple low cost current limiter will decrease when the transistor gets warmer. It might be a good idea or it might look like a problem.

    If the temperature of the transistor does not change then it is a good current limiter.
     
  16. Dleberfinger

    Thread Starter Member

    Dec 18, 2011
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    Well there is no way that it really should get warm, is there? For my application the LEDs will only be on for fractions of a second. The longest that I think they will ever be on is around 500 μs, and I doubt that will be very often. I plan to use it mostly at 5 μs. This is just a single pulse also, it's not like an osculation.
     
  17. thatoneguy

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    Feb 19, 2009
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    From the datasheet:
    Forward Voltage (@ IF = 1000mA)VF 4.5 V
    Forward Voltage (@ IF = 1200mA)VF 4.7 V

    Max pulse current: 2A for <10mS and <10% duty cycle, so only 1mS of every 10mS for 2A operation.

    It doesn't state what the VF would be for 2A, but I'd guess around 4.9V as current ramps up exponentially once Vf is met.
     
  18. Audioguru

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    Is your application for a human to see or for a camera to photograph?
    Pulses shorter than 30ms look dimmed to a human. Your extremely short pulses might not be seen by a human.
     
  19. Audioguru

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    These are "typical" spec's but nobody can buy a "typical" LED.
    At 1A the minimum forward voltage is 3.92V and the maximum is 5.36V.
    You get whatever they have. If their forward voltage is high then two in series at 2A won't work from only 12V.
     
  20. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I agree, he will need to test them.

    I mostly posted that info to point out the 10% duty cycle for high currents, and those numbers were on the same page for copy/paste.
     
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