12V - 7.29A ac/dc converter project Need Help

Discussion in 'The Projects Forum' started by Butterworth, Nov 6, 2011.

  1. Butterworth

    Thread Starter Active Member

    May 6, 2009
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    Hell Everyone, I am working on a new lighting project that involves an array of 81x IR leds in a 9x9 grid. The leds I plan to use have a Vf of 1.2V @ 100mA. I wanted to run them at 90mA to conserve on power. I went with a series/parallel design where I have 9 leds in series with a current limiting resistor on the end of each strand of 9 leds, totalling 81 leds when its all done.

    If I did the math right, I came up with 7.29A @ 12V supply. My request is if anyone is willing to point me in the right direction to construct, or even find, an appropriate power supply for the job. I don't think I can find one in the ratings required, so all help is welcome.

    Thank you for everyone's time.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Hello Butterworth,
    If you operate LED's in series, then each LED in the string gets the exact same current.

    So, if you operated 9 strings with 90mA current each, you would need 9 strings x 90mA = 810mA current, total. You never want to operate a power supply at 100% of rated capacity, so divide that result by .7; you'll see that you need a supply rated for 1.16A or more.

    With this new information, if you decided that you wanted to run them at 100mA after all, your total current requirement would be 900mA, and you'd need a supply rated for ~1.29A or more.

    One of my favorite suppliers is Marlin P. Jones & Associates. They're in my state, so I receive my orders lightning-quick. Since you are in Ontario, you would have to telephone or fax in your order; you can't use the online ordering - and I have no idea what shipping or duty might add to the price.
    Anyway, they have this 12v 1500mA regulated plug supply, which would be just right for you:
    http://www.mpja.com/12VDC-2A-Regulated-Plug-Supply-Jasper/productinfo/18776+PS/
    $5.95 USD.

    You might find something in Canada that's similar. I've forgotten what the popular suppliers are up there; perhaps AudioGuru will chime in and make a suggestion - he's around Toronto somewhere I think.
     
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Check second hand stores for line operated power adapters, or as some say- wall warts. If you can find inputs around 100-240 V AC, they normally will be switch mode & regulated.
     
  4. Butterworth

    Thread Starter Active Member

    May 6, 2009
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    Thank you for the insight SgtWookie & Bernard, this is great news to hear that my math is both unreliable & the power supply is readily available!

    I can use any 12V supply as long as it is rated for at least 1500mA?

    I ask this because I have a few old ac/dc adapters from older electronics that I no longer use, so if I can use one of those then I am in luck!

    Recycling FTW!
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    You should be able to find one fairly easily, even in Canada, eh? ;)

    There is a caveat; and that is supplies that are not regulated. You will need a multimeter to test the output voltage when there is no load on the supply.

    A typical "wall wart" plug-type supply might have a rating of 12VDC @ 1000mA, 1500mA or 2000mA - or some other rating; different voltages, different currents. If the supply is not regulated, it will measure significantly higher voltage output than specified, when there is no load on the output.

    If you find this to be the case with a supply you've found, you should tell us what the voltage rating @ current of the supply is, and the no-load output voltage. We can then figure out what the impedance of the transformer is, and what the voltage will be when a 810mA or 900mA load is placed on the output; and then adjust the current limiting resistors to suit.

    Absolutely. :) Like I said though, you'll need to measure them with no load on them, which means you'll need a multimeter that can read ~20V DC.
     
  6. Butterworth

    Thread Starter Active Member

    May 6, 2009
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    This is excellent, I will search tonight for what I have laying around, plug it in and measure the voltage on the ~20VDC scale, then let you know the outcome.

    If I have a 12V @ 2A regulated supply, then I can safely assume that it is what it states on the supply?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    If it is a regulated supply, then the output voltage should not change more than a small amount when the load is anywhere from zero to 80% or so of its' rated current.

    If it is a non-regulated supply, the voltage will be significantly higher when there is no load.
     
  8. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
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    Ok, I am about to check some out on my lunch hour, I will see what there is and pick up a regulated supply. If the supply is about 80% of the rated current, then I should pick up the 2A supply rather than a 1500mA supply... this will ensure my buffer room is sustained?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    If you want to operate the LEDs at 100mA, you will need a supply rated for at least 1.3A/1300mA.
    If you want to operate the LEDs at 90mA, you will need a supply rated for at least 1.16A/1160mA.

    More is fine.
     
  10. Butterworth

    Thread Starter Active Member

    May 6, 2009
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    Thank you again SgtWookie< much appreciated!
     
  11. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
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    Ok so I got the supply.... the ratings are the following:

    AC Input = 100-240V-, 50-60Hz, 0.65A Max, 51-80VA,
    DC Output = 12V, 2A

    Mfg: APD #WA-24E12

    It claims to be regulated... so I am in good shape it seems. Let me know if this is good enough for my requirements.

    Thanks!
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    That seems to be just fine. Double check that the output is regulated, though - because if it is not the right voltage, you could burn up your array of IR LEDs!
     
  13. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
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    So measuring the VDC with my multimeter while the supply was plugged in, I measured approx 12.82V DC. This measurement is consistant, so should I recalculate the resistor valve based on this new voltage? Or is there a predicted drop in voltage when everything is hooked up?
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, re-calculate the resistor values.

    Then post what you calculated. ;)
     
  15. Butterworth

    Thread Starter Active Member

    May 6, 2009
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    Ok this is what I get:

    1.2Vf x 10 led = 12V
    12.82V - 12V = 0.82V

    0.82V / 100mA = 8.2Ω resistor.

    8.2 x 100mA = 82mW

    I opted for the 80 led route as the packages of leds I am getting come in 20ct / bag, so I am doing 8 parallel strings of 10 series leds now.

    8 strings of 90mA total current per string should give me approx. 900mA.

    How are the numbers?
     
    Last edited: Nov 11, 2011
  16. SgtWookie

    Expert

    Jul 17, 2007
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    Ahh, wait a minute.

    You have 12.82VDC in, and your Vf is 1.2v @ 100mA - is that the "typical Vf @ current" specification in the datasheet? You need to make sure that's correct; if it's not, you will fry your LEDs in a moment.

    Post a link to your IR LED's datasheet, or attach a .pdf of it.

    Max @ of LEDs = INTEGER( (12.82v-0.75v) / (typical Vf @ current) )
    For example, if your Vf turns out to actually be 1.2v @ 100mA, then:
    12.82-0.75v / 1.2 = 11.82 / 1.2 = 12.07 = 12
    You could run 7 strings of 12 for a total of 84 LEDs instead of 9 strings of 9 if you wanted to.
    That would save you some power over the long run. Your

    If you want to use 7 strings of 12, then you would need:
    (12.82v-12.) / 100mA = 8.2 Ohms, which is a standard value of resistance.
    You can use 1/8 Watt or 1/4 Watt resistors for these.

    If you want to use 9 strings of 9, then you would need:
    (12.82v - 10.8) / 100mA = 20.2 Ohms; 20 Ohms is a standard value and is close enough.
    You would need to use 1/2 Watt resistors for these.
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    OK, I posted after you posted your calculations that I didn't see.
    I'm not at home at the moment; it'll have to wait until I get back there.
     
  18. Butterworth

    Thread Starter Active Member

    May 6, 2009
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    Ok, thank you!

    I found some specs based on a user's findings after current testing them individually...

    "These led's are quite dim at 1.5 volts and run at their normal brightness at about 1.75 volts.
    These led's run at 100ma best.
    I Current tested 200 and they do vary, so you'll want to match them by current draw.
    They run fine at 100ma but if glued to a curcuit board they'll run more.
    1.5v = 30-40ma
    1.6v = 45-57ma
    1.7v = 65-82ma
    1.8v = 90-115ma
    1.9v = 112-140ma max for continual dc use
    2.0v = 136-170ma
    2.1v = 152-190ma
    2.2v = 169-212ma
    2.3v = 188-235ma
    2.4v = 200-260ma Max brightness for pulse operation"

    Now how accurate these findings are is questionable...
     
    Last edited: Nov 11, 2011
  19. SgtWookie

    Expert

    Jul 17, 2007
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    They are what they are rated for.

    You need to provide a link to the datasheet, a manufacturer and part number, or at least ask the vendor what the typical Vf @ current is; what is the Vf and what is the current specifications?
     
  20. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
    3
    I am looking for it now. I cannot find it on the site I found them on. I will get back momentarily.
     
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