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Basic PSU circuit questions.

  1. Apr 16, 2013 #1
    I have a 300W PSU that I use to power all kinds of small DC electronics around the house. I recently bought a strip of 12V LED lights. I tested the 4 pin peripheral with a multimeter and got 12.4V which is OK voltage-wise, however, the LED strip says "2A" on it and from what I know the PSU can output 13 amps (someone might want to double check that). So the question is: if I connect the LED strip (positive with positive, negative with ground) will everything work? Will the extra amperage cause the LEDs to blow or will they just draw 2A and let the rest flow to the ground? Also, since my PSU can generate 13 amps, can I link up 6 LED strips in a parallel circuit?

    After this is set up, I will definitely have some more questions regarding dimmers and microcontrollers. Since I never got proper physics education in school I thought I`dd do this project and hopefully learn something. Thanks in advance for any help!
  2. jcsd
  3. Apr 16, 2013 #2


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    The Power rating of a supply only refers to the maximum available, so one or more smaller loads are fine.
    However, you might find it worthwhile to fuse the various outputs (small, in-line fuses right near the PSU and individually at 2A) because the wires to the LEDs would fry if there were a short circuit with 13A available. A fuse in each leg would remove this risk. This is good practice for permanent installations (kitchens etc.) where the lightweight 'bell wire' feeding the LEDs could easily get damaged long after you forgot where they run.
  4. Apr 16, 2013 #3
    Thanks for the quick reply!

    Does this diagram look about right? http://www.bildites.lv/images/dmaqby8255rvro4judcz.jpg
    Wouldn't the PSU blow out the 2A fuse if it is connected there? Where else would I put the fuse?
    Also, DC- = ground. is that OK?
  5. Apr 16, 2013 #4
    The circuit looks ok, but you should also check the resistance between your LED. The resistance will determine how much current will flow. Remember as well if your resistance(LED) is in parallel the total resistance will be lower hence you will have higher current flow.
    V= IR
    Where V is Voltage
    I is Current
    R is Resistance
  6. Apr 16, 2013 #5


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    The fuse only blows if more than its rated current flows. If both LEDs take 2A then that 2A fuse will blow (it will take 4A, in that position). You need to split the supply right at the PSU and put a separate 2A fuse in each (+) leg, so each fuse will only be passing 2A. This could be regarded as overkill but those in-line cartridge fuseholders are only pence each and you'd feel a pratt if the wire burned the insulation and started a fire. The alternative is to use very thick connecting wires - which would cost you more!

    Yes the DC '-' terminal could be connected to Earth or, more likely, you will have 'twin' wires going to the LEDs and bring the negative wires back to the negative on the PSU. It is not necessary to connect, explicitly, to 'Earth'.
  7. Apr 16, 2013 #6
    Here is what I don't understand though: so the PSU can supply 13 amps. The first LED strip would pull 2A. The second LED strip would also pull 2A. Wouldn't that cause 4A to flow through the fuse? That would surely blow the fuse! I only have 2A fuses lying around the house and even though I could get a 4A fuse, I would like to be able to add more LED strips eventually without having to change my fuse every time. Is there any way of connecting a 2A fuse to every strip so that I have multiple fuses? Also, since the LEDs say they draw 2 amps, does installing a 2A fuse wont leave much room for error. Should I go ahead and get 2.5A fuses or do I stick with the ones I have?
  8. Apr 16, 2013 #7
    sophiecentaur answered my question while I was typing... ignore the last reply...
  9. Apr 16, 2013 #8
  10. Apr 16, 2013 #9


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    You need to check that the LEDs are dimmable first. LEDs are usually dimmed by pulsing the supply between mostly on and mostly off, at a high rate (Pulse-width modulation). Some units do not handle this.

    OH yes - your modified fusing is fine.
  11. Apr 16, 2013 #10
    I added a 3rd LED strip (this one is 5A) to the circut: http://www.bildites.lv/images/g2cj9ebs96n5j30xp9.jpg
    Besides having to change the 2A fuse for a 5A fuse, do I have to add resistors or something or will this work fine?

    Also, how would I measure the exact current produced by the PSU? I would assume sticking an ammeter in there would just fry it. Specs say the PSU produces 13A but I`m not so sure since it`s a really cheap one from China..
  12. Apr 16, 2013 #11


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    No worries there. It should work just the same as putting a 50W bulb next to a 150W bulb on the same supply - as long as the supply can supply enough total current.

    The only way to measure how much current that PSU will supply would be by testing it under various loads. You could use a number (5?) of 55W car headlamp bulbs (in parallel, of course) and see if the Volts stay up and the PSU doesn't smell of 'cooking' after a half hour or so. If you have no meter then you can start with one bulb and gradually add bulbs until they go noticeably dimmer when you add yet another bulb. However, if you are only using 2A (=24W) LEDs then you shouldn't need to worry about connecting several!

    If you have even the remotest plans for making electrical circuits up, I would suggest that a cheap DMM (with current measurement capabilities up to 10A) could be a good investment. There are not so many cheap ones that measure more current. If you wanted to measure the total current capacity of this PSU ( 25A) you can measure the current through just one bulb and multiply up by the number of bulbs.
  13. Apr 20, 2013 #12
    Thank you for the advice! I bought a DMM and spent the last week measuring everything I can find around the house with it.

    I do have another question about my PSU setup though: I want to connect an external HDD to the PSU so that I can use its DC adapter to power other stuff.

    Output for the DC adapter: 12V 2A

    So if I was to attach the HDD to the PSU`s 4-pin 12V (http://www.playtool.com/pages/psuconnectors/connectors.html#peripheral ) it would work, right? Would adding a 2A fuse be a good idea? I have a whole box of them and I dont mind fusing anything I can get my hands on.

    Also, the article linked above says that the manufacturer specified max current is 13A although they would recommend not running more than 5A. So my previous LED setup would be over the safe limit.. :( How would I calculate how much stuff I can hook up to the PSU from this label?:
    Would it be 10A per +12V 4-pin peripheral?

    Finally, what is the best way to connect power wires? Would twisting + a drop of solder + heatshrink be OK or is there some other recommended way?
  14. Apr 20, 2013 #13


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    A drop of solder? I am concerned about your soldering technique!
  15. Apr 20, 2013 #14
    Ok so instead of using tin/lead solder I would just wrap the wires around each other (like this: http://w6rec.com/duane/bmw/solder/solder12.jpg ) and then apply a soldering iron? Or would I just go ahead and heatshrink everything without applying heat?

    I would assume that using scotchlocks is not suitable for ~12V, 2A wiring?
  16. Apr 21, 2013 #15
  17. Apr 21, 2013 #16
    OK so the spiraled wire drawing pins attach to the AC power wire, the "switch" icon pins go to + and - DC (10A, 30V).. what about the middle pin? Is that ground? What do I connect that to?
  18. Apr 22, 2013 #17
    I drew a scheme of the relay. Red dots are connector pins. Which two will the AC go through? Which two - DC? Also, my AC wire has a ground wire.. does that go to the middle pin?


    Also, can someone verify if this will work:
    Relay - (10A 250VAC, 10A 30VDC)
    Power source 1 - (220VAC)
    Power source 2 - (12VDC, 10-30A)

    I`m still having trouble determining the output amperage of the PSU. Same one I mentioned earlier in this post. Any help would be much appreciated.
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