current and current consuming devices

[panfilero]

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?

 

[Rich Grise]

On Wed, 20 Aug 2008 06:52:35 -0700, 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, if the supply is voltage-regulated. Some cheap wall warts need a
specific load to ensure that the voltage is in spec; that depends on
the internal resistance of the supply.

Cheers!
Rich

 

[panfilero]

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....

 

[Frank Buss]

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, fb@frank-buss.de
http://www.frank-buss.de, http://www.it4-systems.de

 

[Richard Henry]

On Aug 20, 9:48 am, panfilero <panfil...@gmail.com> wrote:

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....

It depends on the device. A linear resistance/impedance will draw
current proportional to the voltage applied. Non-linear devices (such
as many power converters) will draw some power to satisfy their own
needs and more power depending on the requirements of any attached
load.

 

[Peter Bennett]

On Wed, 20 Aug 2008 09:48:42 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

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?

If you supply the correct voltage, the device will draw whatever
current it requires, regardless of the rating of the power source
(unless something is broken, of course). Consider that a car battery
can supply perhaps 300 amps to drive the starter, but a lamp requiring
1 amp will also work just fine from the same battery.

Also, at home, the normal wall outlet is fused at 15 amps, but a
single 100 Watt lamp (drawing .8 amp) will work without burning out.

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?

Sort of, maybe, perhaps....

There are all sorts of exceptions that make it hard to say "yes".

The resistance of an incandescent light bulb increases dramatically as
it heats up, so the cold resistance you measure with an ohmmeter will
bear little relation to the current the lamp actually draws when
illuminated.

With electronics, the resistance you measure with a meter may have
little relationship to the actual current drawn when operating, as
most semiconductor junctions will appear as open circuits at the
voltages used by an ohmmeter, and various circuit paths will open and
close as the voltage is increased from that provided by an ohmmeter to
the desired operating voltage.

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?

I assume you meant "different current ranges" above.

If they all require 9 volts, you should be able to run them from a
single supply that can supply _at least_ the total current required by
all the devices (but there may be ways of designing the individual
devices where this may not be acceptable.)

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

Not if the supplied voltage is correct.


--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

 

[Charlie Siegrist]

On Wed, 20 Aug 2008 15:27:53 -0700, Bob Eld wrote:

"Richard Henry" <pomerado@hotmail.com> wrote in message
news:e279d44b-35d3-4570-99d8-772e49aa7ab5@t1g2000pra.googlegroups.com...
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...

<OP>

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?

All of them.

So.

1) Can a device draw too much current for itself? Yes.
2) Will that device use just what it needs? Yes.

I sense conflict.

Peter Bennett's answer is quite good. The one caution I would add is
that you [OP] make sure the 9V supply you use has a regulated output.
Some cheap wall plug-in power supplies are designed to use the load as
the current regulation, and you'll have trouble in this application if
you use an unregulated supply. Oh, and one more thing that wasn't
completely clear in your [OP] description, make sure the rating of the
supply is equal to or greater than the sum total of all the devices you
plan to use, not simply higher than the highest single load.

 

[Rich Grise]

On Thu, 21 Aug 2008 07:24:45 -0500, Charlie Siegrist wrote:

1) Can a device draw too much current for itself? Yes.
2) Will that device use just what it needs? Yes.

I sense conflict.

A device _CAN_ draw too much current for itself, if the power supply

supplies a significantly higher voltage than the device's rating.

For example, if you plug a 120V toaster into a 240V outlet, it will
draw WAAAAAY too much current, for a little while. Be sure to have the
ABC extinguisher on hand if you do this experiment at home. ;-)

Hope This Helps!
Rich

 

[Tim Wescott]

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?

If the device is designed to be driven from a constant voltage, and if
you give it the right supply voltage, yes.

So you can take the bulb from a one-cell AAA-sized flashlight, power it
off of a D cell, and it'll work fine for a nice long time.

So basically will a 12V 2Amp dc
power adapter work for all devices that require less than 2Amps?

Probably. Those power adapters are a bit tricky in that they give the
advertised voltage (more or less) at the advertised current, but they
often give more voltage at lower currents.

Unless your device is ridiculously sensitive to voltage, your 12V 2 amp
DC adapter will work for all _12V_ DC devices that require less than 2 amps.

But plug in a device designed for 3V and it'll probably die, and may
even send up a smoke signal to mark it's passing. Plugging a 9V device
to a 12V source is generally not considered wise, even.

So the rule is: match the volts, make sure the amps are matched or
exceeded, and have a nice time.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[whit3rd]

On Aug 20, 6:52 am, panfilero <panfil...@gmail.com> wrote:

I just had a couple of questions about current... can I device draw
too much current for itself? ... will a 12V  2Amp dc
power adapter work for all devices that require less than 2Amps?

Maybe. The problem is, that '12V 2 Amp' rating identifies one and
only
one point on the I/V curve (called the "load line") for your power
adapter,
and normal operation may wander over the load line and find
other points. Commonly, these adapters are regulated, i.e. the load
line is nearly constant V = 12 +/- .1V, only dropping to lower V for
currents
in excess of that 2A rating (never exceed 2A, and all is well).
Some regulators require a minimum output current too.

There is one other issue, however, that should be addressed: if you
have
a dozen 0.1A items that need 12V, it isn't always possible to run them
from a single 12V supply. That's because there are no GROUND
connection
issues with an isolated wall-tumor adaptor, until you connect both
a positive-GND and a negative-GND device to it. The first attempt to
connect the grounds of those devices will short out the power supply.
Here, again, the simple '12V 2Amp' rating doesn't tell you if the
device needs to use negative-ground, or center-ground, or positive-
ground
power. Rarely, a wall-tumor may have internal ground connection
(to the third-prong on the wall plug).

 

[Richard Henry]

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?