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Paul E. Schoen  
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  More options Aug 20 2008, 7:28 pm
Newsgroups: sci.electronics.design, sci.electronics.basics
From: "Paul E. Schoen" <pst...@smart.net>
Date: Wed, 20 Aug 2008 14:28:50 -0400
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 try to build an analogy to all this stuff in my mind.... I picture a
> board laying flat with some kind of jack on one side that lifts that
> end making the board angled... on one end of the board I picture
> marbles.... when the board is flat that is equivalent to no voltage
> and the marbles dont move, if you raise on end of the board the
> marbles start to flow down the board.... you are increasing the
> voltage... and the marbles flowing is the electrons.... but my problem
> in my analogy is that the electrons (marbles) are only present if
> there is a device that wants them.... if there's no device then they
> wont flow even if you lift the board... I guess you could put a little
> door that slides up on one end of the plank or in the middle of the
> plank and if its completely shut that's infinite resistance and as you
> lift the door open and marbles pass under it you are decreasing your
> resistance.... so the door only accepts the flow of marbles it needs
> no more no less....

Usually, a device will just draw the current it needs when the voltage
supplied to it is within its allowable input range. But you cannot simply
measure the voltage (V) of the supply and the impedance (Z) of the load,
and then connect them together and expect the current to be V/Z. Simple DC
power supplies have filter capacitors and trannies with intentionally poor
regulation, so their open circuit output voltage will be much higher than
it will be under rated load. And most loads other than a simple resistor
will be non-linear, so that the applied voltage will affect the impedance.
For instance, an incandescent 12 V lamp might read 1 ohm on a multimeter,
but only draw 1.2 amp at 12 VDC, because the filament resistance can go up
by a factor of 10 when it is white hot.

Musical equipment contains circuitry that can draw varying amounts of
current, and usually will have a regulator that will allow a reasonable
range of input voltage without problems. But if you use an adapter that is
rated at a much higher current, it might apply a voltage that is too high,
and the device could overheat.

I've always used the water analogy, and I have not heard of the marbles
analogy. I guess it's better in the case of a major circuit malfunction, to
"be all wet". rather than "lose ones marbles"!

Paul


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