current and current consuming devices

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?

Reply to
panfilero
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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.

greg

Reply to
GregS

Yes

Reply to
Bob Eld

dc

Which question are you nasering?

Reply to
Richard Henry

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....

Reply to
panfilero

...

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Amp dc

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.

Reply to
Richard Henry

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

Reply to
Paul E. Schoen

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

Reply to
Mike Wahler

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

Reply to
Rich Grise

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

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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
Reply to
Frank Buss

Which question are you nasering?

All of them.

Reply to
Bob Eld

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.

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 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.)

Not if the supplied voltage is correct.

--
Peter Bennett, VE7CEI  
peterbb4 (at) interchange.ubc.ca  
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Reply to
Peter Bennett

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.

Reply to
Charlie Siegrist

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.

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
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Reply to
Tim Wescott

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 =3D 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).

Reply to
whit3rd

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