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Power supply question

  1. Jul 30, 2013 #1
    Ok this may be a stupid question but what the heck.

    If I have a power supply giving max of 4amps with 35v
    and what I am powering draws 6 amps at 35v,
    then that means the device has 5.7 ohms of resistance.
    If I put a 3.05 ohm resistor (ish) then this will drop the amps down to 4, but will it still power the device... functionally?
  2. jcsd
  3. Jul 30, 2013 #2
    The resistor and your device will form a voltage devider. You'll get 5.7/(5.7+3.05) * 35 = 22.8 V across your device so it is not likely to work.

    The power dissipated in the resistor is I^2 R = 4*4 * 3.05 = 48.8 W wich will be a problem as well.
    Your resistor might require a heatsink and a fan.
  4. Jul 30, 2013 #3


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    if the device needs 6A to operate then its not likely to work with only 4 amps or if it does it will be unstable in operation
    Adding a resistor is only going to make the situation worse

    you need a beefier PSU

  5. Jul 30, 2013 #4
    ok thanks, I forgot about the voltage divider so It seemed possible for a moment, this isn't a project i'm working on, I was just day dreaming.


    Sense I have this thread open, and davenn is here:

    I read that bridge mode in amplifiers will, for example, take a 100 watt amplifier at 8ohms and turn it to a 200 watt amplifier at 4ohms.

    I was going to use 4 tda2050 amps, each running one speaker, but if I use 2 amps in bridge running 2 8ohm speakers in parallel, would this give me double power into both speakers? So as if each speaker had it's own amplifier twice as strong?
  6. Jul 31, 2013 #5


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    As long as you realise that you don't get anything for nothing then you can't go too far wrong. Connected in a bridge, two amplifiers will (if they can) deliver twice the current into a load than they would, individually. They are each 'seeing' a load of half the resistance of the original load. If the amplifier can't deliver enough current then you won't get that power - it will clip.
  7. Jul 31, 2013 #6
    ok thank you! I must say you explained that very well, someone else I asked wasn't being too clear haha.
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