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Question on Current... — Parallax Forums

Question on Current...

idleupidleup Posts: 46
edited 2005-10-22 13:39 in General Discussion
I thought someone told me a long time ago that a circuit will only use the current it needs and if the source is greater then that is good. Is this true? If I use a wall wart that outputs 500mA at 9v and feed it into an LCD that takes 100mA will I blow the LCD or will the LCD only draw the necessary current?

What got me questioning this is that the spec sheet for the LCD says:
symbol = ldd
condition = Vdd=5v
Min = -
Typ =
Max = 100 mA

And I began to question what I had learned since if it only "drew what it needed" then it wouldnt make sense to have a 'Max' current...

Someone please straighten me out on this elementary concept. Thanks.

- Matt
·

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-10-17 21:42
    Matt,

    ·· Yes, you were right the first time.· Current ratings of devices reflect what the device needs/draws.· Current ratings on supplies tell what the supply can produce.· The 100mA is the maximum·current the LCD will draw/require so your 500mA supply is fine.


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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-10-17 21:43
    Don't get yourself wrapped around an axle -- what you learned was correct (devices only consume what they need). Manufacturers have to list a "Max Current" spec so that designers who intend to use the component can plan accordingly.

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    Jon Williams
    Applications Engineer, Parallax
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2005-10-22 13:39
    There is some 'slop' in wallwarts though. GET ONE THAT IS TIGHTLY REGULATED and make sure it is indeed DC.

    Many unregulated DC devices produce higher voltages when using less current.

    Since voltages generally apply to the ability to insulate a circuit [noparse][[/noparse]called 'breakdown voltage'] often a solid-state device will experience an excessive voltage condition even though it is not drawing much current. Most devices will easily tolterate a 10% over-voltage, but beyond that is risk.

    The real culprit here is the labling on the cheaper wallwarts. If in doubt, check it with your multi-meter to see that it is not producing a no-load voltage in excess of your devices maximum voltage. At the maximum load it comes down to the expected voltage.

    When in doubt, use a battery [noparse][[/noparse]and check that too as some fully charged ones are over the top]

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    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

    ······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
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