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?
In article <8fa5a24f-9d8d-465a-9d73-605f64a48...@c65g2000hsa.googlegroups.com>, panfilero <panfil...@gmail.com> 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?
You had it when you said drawing less than 2 Amps. Turn on surge current can cause problems. its a major issue driving 12 lamps. Sometimes the supply will shut down.
> 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?
> > 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?
> > > 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....
> > > > 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.
>> > > 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.
>> > > 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"!
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.
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:
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.
