Hi-power led led circuit, current limiter needed?

Discussion in 'The Projects Forum' started by Markku, Sep 18, 2014.

  1. Markku

    Thread Starter New Member

    Sep 18, 2014
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    Hello

    I am planning to build a ceiling lamp using two powerful LED modules and DC PSU.
    • LED modules
      • Rated voltage :22-26v
      • Current: 2000mA
      • Module consists of 20 leds
    • PSU AC230v/DC24v switched-mode
      • Voltage 24v
      • Current 5A
    The components are cheap chinese parts, so not much specs for them. I am going to put the 2 LED:s in parallel with the power supply. But I was wondering, will the psu kill the LED:s since it gives 5A of current and the wanted current is 4A total.

    Do I need something to limit the current to 4 amps?
    Would 1 ohm power resistor for each led in series do the trick or how it should be done?
     
    Last edited: Sep 18, 2014
  2. wayneh

    Expert

    Sep 9, 2010
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    Excess capacity of the power supply does not force current into the load. Analogy: Plugging a lamp into the wall - a source of 15A before you blow a circuit breaker - does not force 15A into the lightbulb. The bulb takes what it wants.

    But the rest of your question is a mystery without knowing more about your LED "module". It may have current limiting built-in, but of course you cannot count on that without knowing. Can you supply any information or a photo?
     
  3. ScottWang

    Moderator

    Aug 23, 2012
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    Do the LED modules have the heat sink?
    Could you attach some photos about the LED modules and psu?
     
  4. Markku

    Thread Starter New Member

    Sep 18, 2014
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    Here is the link to the LED module. As you can see the specs are really insufficient.
    And here for the PSU. There is a some sort of voltage tuning potentiomer, I believe.

    I don't think that the modules have the current protection. I wanted to find out, will the modules draw too much current from the psu and fry themselves . I am going to use old active cpu coolers to keep them cool.
     
  5. ScottWang

    Moderator

    Aug 23, 2012
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    One 1Ω/20W resistor for each led module.
    According to the spec that is ok.
    V=2A*1Ω=2V,
    W=2V*2A=4W, choosing a 5 times of 4W or bigger is better for the heat problem.
    24V-2V=22V, the voltage for led module is 22~26V, so if the spec is right then it will be work, otherwise you have to reducing the resistor to 0.5Ω/10W.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    I agree those modules appear to be nothing more than LEDs on a heat sink. You will need current-limiting to protect them.

    The one review at that site mentions getting "only" 700mA when supplied 24V. If that's true, and your PSU supplies 24V, you may not get as much brightness as you expect and an external resistor will not be needed. This will help with the heat issue, but maybe not give the result you hope for.
     
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Yes you need current limiting (LED driver) with those or they will go up in smoke...
    And you need a sufficient heatsink or they will go up in smoke.

    LEDs in parallel is not a good idea either really as you have no guarantee they will share current equally..

    IF I was to build a fixture using those (and I've built plenty of high power LED fixtures) I would use a meanwell LPF-60-30 driver for each LED module and a suitable heatsink like this http://www.mpja.com/Duron_Athlon-Socket-A-CPU-Heatsink-Fan/productinfo/17045 HK/ (will need a regular 12VDC power supply for the fan).. That should get you around 50-60deg C junction temps and should last a long time..

    If you want dimming via a potentiometer I would use the LPF-60D-30 for each LED module and a 100k potentiometer for dimming.

    If you want to save money and don't need full brightness you could use the same LPF driver and run them in parallel.. They will then hopefully share the 2A current equally and run at 1A each.. In the event of thermal runaway you are still not exceeding the max current for the LED module and all should be ok assuming you have a sufficient heatsink to keep them running cool.
     
  8. ScottWang

    Moderator

    Aug 23, 2012
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    If considering the led life and brightness, One 1.25Ω/15W resistor for each led module.
    You just using the 80% of 2A rating current that it is 1.6A.
    R=2V/1.6A=1.25Ω
    W=2V*1.6A=3.2W, choosing a 5 times of 3.2W as 15W or bigger is better for the heat problem.
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Not using a resistor is "better for the heat problem".
    Resistors are fine for "low power" LEDs.. but should NOT be used with "high power" LEDs..
    Not only just because of heat generation/efficiency but because a fixed value resistor cannot accommodate for changes in LED forward voltage.
     
  10. ScottWang

    Moderator

    Aug 23, 2012
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    Unless you changing the psu to constant current, otherwise using the limiting resistor is necessary.
     
  11. bertus

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  12. Markku

    Thread Starter New Member

    Sep 18, 2014
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    Many thanks for everyone's input.
    I really need to keep the cost down for this project. I think this power supply would be indeed the optimal solution, but it costs double the DX PSU. As I mentioned, active CPU coolers will be used for cooling.
    Is there any other cheap solution to restrict the current? Or if I just get a lower current PSU for the LED:s like this one? I don't quite understand why the current wouldn't divide equally to the two LED:s, if the conditions and coolers are the same. I was thinking about a high power resistor in parallel to the LED:s to eat, for exaple ~1,5Amps of the total current. Just an idea, I'm not sure if it would work.
    Is the forward voltage so critical in this case? It doesn't seem really significant, and I assume that the temps are not going to rise really high since the surface area of the module is quite large and it consists of many smaller LED:s and not single high power LED. If it changes for example 10%, from 24v to 21,6v, and still would be powered with 24v. Would that be enough to fry them?

    What would be the optimal solution, or do I just need to get the meanwell led driver?
     
    Last edited: Sep 18, 2014
  13. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    You would want the LPF-90-30 anyways..
    You must ensure the DC output voltage is ABOVE the max forward voltage of the LED.. As 26>24 the LPF-90-24 won't work or shouldn't be used.. You could just have a bright blinking LED as they tend to blink when the output voltage is not in the range of the driver as it can't regulate properly..


    If cost is an issue
    you can pick up a single meanwell LPF-60-30 for $28 USD..
    Run both LED modules in parallel and done. each LED should get 1A and be PLENTY bright..

    LED modules like that aren't that much more efficient (brighter) when driven to the max like that.. And if you aren't willing to spend the money you can't have the fastest car either.. ;)

    In general both modules in parallel "should" share the current equally but the world isn't perfect and neither are the conditions/resistances/cooling with 2 separate LED modules (fit shappens).. That why a "smart" designer would ensure they couldn't get over driven EVER..
     
  14. ronv

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  15. mcgyvr

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  16. Markku

    Thread Starter New Member

    Sep 18, 2014
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    Okay thanks. That one doesn't offer international shipping, but I found a alternative one here. So these kind of PSU:s adjust the voltage to optimal value and offer constant current, or what is the main reasons to use these kind of drivers? If a grab two of those, it shoud give 1,5A for each module. So that would be somewhat good setup to run each module straight from the driver?
     
  17. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Yes in general there are "constant voltage" and "constant current" power supplies..
    Constant voltage adjusts to maintain the same output voltage.
    Constant current adjusts to maintain the same output current.
    And LED driver is just a constant current power supply.

    And LED is not like a regular lightbulb..

    In very general/beginners terms it works like this..
    LEDs MUST be "fed" current or they will try to suck it all up and blow up...
    Resistive loads (lightbulbs, etc..) will only "draw" the current they need.


    and yes 2 of those that you found.. 1 per module will be just fine.. As the output voltage is slightly greater than the max forward voltage of the LED and the output current is just below the "max" for the LED..

    have fun and PLEASE do NOT stare into the LED when its on.. These are powerful enough to cause permanent eye damage in seconds..
     
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