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Voltage Regulator

  1. Nov 2, 2007 #1
    I was going to use a +5V Fixed-Voltage Regulator 7805, for a project until I noticed that I need 2 amps and the 7805 can supply at max 1 amp. what can I do?
     
  2. jcsd
  3. Nov 2, 2007 #2

    NoTime

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  4. Nov 2, 2007 #3

    ranger

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  5. Nov 5, 2007 #4
    go to digikey and seach linear regulator. put in a filter for 5v and 3amp
     
  6. Nov 13, 2007 #5
    after doing some more research I found out I need about 900mA to 1 amp. will the 7805 guarantee putting out that much? and should I put a heatsink on it to help (and how)?
     
  7. Nov 13, 2007 #6

    ranger

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  8. Nov 13, 2007 #7

    berkeman

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    Staff: Mentor

    I agree, datasheets are our friends. Also, google can be useful if you know a couple keywords.... I just googled heatsink tutorial, and got lots of good hits. This is one of the first ones on the hit list:

    http://sound.westhost.com/heatsinks.htm

    It describes the concepts of thermal resistance and heatsink sizing, plus gives some useful tips for how to use heatsinks.
     
  9. Nov 13, 2007 #8
    yes I know, but you did not read my question. my question was how does heat impact its performance, I know the hotter the chip is the lower the current output will be. do I need to worry about a heatsink on it at all? according to this at 25 degrees Celsius, it will output 1 amp ( http://www.national.com/mpf/LM/LM78M05.html )
     
  10. Nov 13, 2007 #9
    thanks
     
  11. Nov 13, 2007 #10

    berkeman

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    The current of a voltage regulator does not go down with higher chip temperature, at least not until any internal thermal current limiting circuit kicks in.

    Check out the heatsink link I provided -- that should help you figure out your answer.


    EDIT -- we're posting simultaneously! LOL.
     
  12. Nov 13, 2007 #11
    I hate datasheets...:grumpy:
     
  13. Nov 15, 2007 #12
    I have a couple more questions. is it absolutely necessary to have the two capacitors in the circuit

    "*Required if regulator input is more than 4 inches from input filter capacitor
    (or if no input filter capacitor is used). <- ON INPUT
    **Optional for improved transient response." <-ON OUTPUT

    and is there a recommended input current? I know the higher the current the more heat made.
     
  14. Nov 15, 2007 #13
    input current depends on how much you draw.

    heat depends on Vin and current draw. if your 5V regulator can take Vin up to 24V, applying Vin of 24V would generate more heat dissipation than applying Vin of 12V keeping all else constant. Remaining unused voltage gets dissipated as heat i believe.
     
  15. Nov 15, 2007 #14
    I guess for the current question i should have rephrased it. I need the regulator to output one amp, would I be safe putting more than one amp to the input of the regulator?
     
  16. Nov 15, 2007 #15
    if your regulator can output more than 1 amp, then yes. it will draw what it needs
    .
    lets say your regulator can output upto 2A, and you require 1.5A,then yes it will draw only that much
    if it can output upto 500mA, and you require 1.5A, your regulator will still output 500mA and cap at that much. You cannot put in 1.5A into the regulator because it can only take in 500mA.
     
  17. Nov 15, 2007 #16

    berkeman

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    A linear regulator like you are asking about has current going through it from input to output, and it drops a controlled voltage to maintain the output voltage in regulation, even if the input voltage varies. You do not put current into a linear regulator. You supply it with an input voltge, the regulator drops whatever voltage is necessary to make the regulated output voltage, and the output current is determined by that output voltage and the load impedance. The input and output currents for the voltage regulator are equal. (Well, except for the small bias current Iq which flows down through the regulator circuit to ground, which means that Iin = Iout + Iq.)
     
  18. Nov 15, 2007 #17
    (maybe this is why I'm not an EE major) but if you are supplying the 7805 with power from a DC power supply, how can the 7805 only have voltage as an input when the dc power supply has both a voltage and current output?
     
  19. Nov 15, 2007 #18

    berkeman

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    Yes, you are getting a bit confused about the details and the semantics. I'll try again, the output voltage across the load impedance determines the output current of the regulator. This output current is basically identical to the input current, since a linear regulator is basically a pass element. So the input current that the regulator draws from the source power supply is determined by the load impedance, not by any characteristic of the voltage regulator. The input current equals the output current (the current all just flows through the linear regulator), and the input voltage is determined by the voltage setting on the source power supply.
     
  20. Nov 15, 2007 #19
    that clears everything up. thanks
     
  21. Feb 25, 2008 #20
    Yes.

    They keep the regulator stable.

    If you leave them out, it may well oscillate.

    Series regulators are very good at oscillating.

    Usually at about 40 to 50MHz.

    Which tends to make whatever circuitry you are supplying do very interesting and unusual things.
     
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