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Mike Wahler  
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  More options Aug 20 2008, 7:28 pm
Newsgroups: sci.electronics.design, sci.electronics.basics
From: "Mike Wahler" <mkwah...@mkwahler.net>
Date: Wed, 20 Aug 2008 11:28:55 -0700
Local: Wed, Aug 20 2008 7:28 pm
Subject: Re: current and current consuming devices
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"panfilero" <panfil...@gmail.com> wrote in message

news:f2aae81a-355e-49d0-8cf9-2b5aea6e4851@k7g2000hsd.googlegroups.com...

> On Aug 20, 11:22 am, Richard Henry <pomer...@hotmail.com> wrote:
>> On Aug 20, 7:27 am, "Bob Eld" <nsmontas...@yahoo.com> wrote:

>> > "panfilero" <panfil...@gmail.com> wrote in message

>> >news:8fa5a24f-9d8d-465a-9d73-605f64a48c71@c65g2000hsa.googlegroups.com...

>> > > I just had a couple of questions about current... can I device draw
>> > > too much current for itself?  I mean... if I hook up a device that
>> > > uses 1amp to a power supply without a current limiting control will
>> > > that device just use what it needs?  So basically will a 12V  2Amp dc
>> > > power adapter work for all devices that require less than 2Amps?

>> > Yes

>> Which question are you nasering?

> yeah I'm not sure either.... are you answering "can a device draw too
> much current for itself?" and saying "yes" that it can?

> Tell me, does current just depend on a devices impedance?  so I can
> know how much anything is going to draw by just measuring the things
> resistance and knowing the voltage I'm about to put across it?

> I have keyboards and guitar pedals and such that all require 9V
> adapters at different voltage ranges... so I can just buy a 9V adapter
> which exceeds their current ranges... say the require 500mA and I buy
> a 3AMP power supply.... then I should be good to go right?

> but I guess this is my main question here: "can a device draw too much
> current for itself?"

I'm still an amateur, so watch for anyone correcting me....

AFAIK, yes, current depends entirely upon circuit impedance, which
would include that of a connected load.  But be aware that some
devices (e.g. incandescent lamp) change their impedance when
energized.

A power supply that can deliver current equal or greater
(a bit greater would probably be better) than required by
the load should be OK.

However, a power supply which does not deliver the required
current would cause a drop in the supply voltage (and the
device would probably not function correctly, depending upon
how 'forgiving' are its specifications.)

If concerned about efficiency, you may also want to review
Thevenin's theorem, and the fact that maximum power transfer
occurs when source and load impedances are equal.

-Mike


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