current and current consuming devices - sci.electronics.design

Message from discussion current and current consuming devices

Frank Buss

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More options Aug 20 2008, 7:49 pm

Newsgroups: sci.electronics.design, sci.electronics.basics

From: Frank Buss <f...@frank-buss.de>

Date: Wed, 20 Aug 2008 20:49:47 +0200

Local: Wed, Aug 20 2008 7:49 pm

Subject: Re: current and current consuming devices

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panfilero wrote:
> 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, a device limits the current to what it needs. See this page for more
information:

http://en.wikipedia.org/wiki/Ohm%27s_law

So if you connect a lamp with 10 Ohm to a 12V power supply, the current is
12V/10 Ohm = 1.2A, even if the power supply can deliver up to 100A, because
the power supply regulates the voltage to 12V all the time (if the current
draw is less than 2A), and Ohm's law does the rest. Note: As other posters
wrote, there are very cheap power supplies, which might not regulate the
voltage for all currents. Use a voltmeter to test it.

For non-linear devices, like ICs and transistors, the resistance may be
more complex, but in the end Ohm's law is true all the time, so the
resistance may change in a non-linear way, but if the device is specified
for 12V and if the power supply can regulate it to 12V all the time, it
doesn't matter how much current the power supply can deliver at most.

If you want to connect multiple devices to one power supply, you can add
the maximum needed current of each device for calculating the needed
maximum current for the power supply. If all devices need the same voltage,
you can connect them in parallel.

--
Frank Buss, f...@frank-buss.de
http://www.frank-buss.de, http://www.it4-systems.de

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