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Explain current and voltage to me!!
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dice
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Dec 6, 2007, 06:28 AM
 
Hi all,

Can someone explain current and voltage to me? I'm a little confused but I think I do actually get it.

Also, I lost the power supply for an external hard drive. The bottom of the drive says 12V 3A. Am I right in saying I can't use a 12V 2A powersupply because it won't be able to give enough current.?

Can I use a 12V 4A supply? will the drive get too much power or will it just take what it needs??

thanks!
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Doofy
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Dec 6, 2007, 06:38 AM
 
Originally Posted by dice View Post
Am I right in saying I can't use a 12V 2A powersupply because it won't be able to give enough current.?
Yup. Likely to overheat the power supply and make it shut itself down (that's what happens on my synth supplies if I use the wrong one). Or, if the supply doesn't have thermal overload protection you could end up with a fire.

Originally Posted by dice View Post
Can I use a 12V 4A supply? will the drive get too much power or will it just take what it needs??
It'll take what it needs.
Think about your wall socket - it's supplying a certain current which you can use for a kettle or a cell phone charger with. When using the kettle, it pulls a load of current. When using the charger, it pulls hardly any.
Been inclined to wander... off the beaten track.
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analogika
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Dec 6, 2007, 06:44 AM
 
Amperage is the maximum current flow.

Liken it to water pressure and maximum throughput.
You have a machine that's built to withstand a water pressure of 12 bar, and can accept a maximum throughput of 3l per minute.

Attaching it to a pipe that can only supply 2l per minute will starve it, while a pipe that can supply 4l per minute won't hurt, since the machine will only take up 3l per minute anyway.

Hooking it up to a water supply with too much pressure, however, will cause the gaskets to blow.

IOW 12V 4A is fine, 12V 2A will probably starve the device and cause it to not work properly (or overheat the power supply), and 15V anything will probably fry it.
     
Doofy
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Dec 6, 2007, 08:35 AM
 
Originally Posted by analogika View Post
Liken it to water pressure and maximum throughput.
And that's what's called an analogy.
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MacosNerd
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Dec 6, 2007, 09:16 AM
 
current like water is the movement of electrons through a medium, like wires. The measurement of current is Amphere

Voltage = watts divided by amps(v=w/a), that is the measurement of potential energy.

You can be hit with 15,000 volts and live as long as the amps were tiny.

I was hit by a step-up transformer and that was at 15,000volts but only a few milli-amps. It hurt but it wasn't lfe threatening.
     
ghporter
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Dec 6, 2007, 09:16 AM
 
Great analogy. Voltage is like the height of a waterfall, while current is the width. The water can fall faster from a higher waterfall, but only so much water can get through the width. The combination of how much water and how fast it's falling generates the amount of power produced at the bottom of the falls. With electricity, the power (in Watts) is the product of current and voltage. A 12VDC, 2A power supply is capable of providing a total of 24 Watts of power, while a 12V, 4A power supply can provide 48 Watts.

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Sherman Homan
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Dec 6, 2007, 10:07 AM
 
The water analogies are really good. Consider a 9 volt battery, touch your tongue to the two terminals and you get a mini-zap. But the voltage is low and the current minimal; the height of the waterfall and the amount of water are both very small. Stick your fingers in an outlet and you will get seriously burned and knocked on your butt. Plenty of voltage and current, probably 110 or 220 volts backed up by your local electrical power plant; in other words a fairly tall waterfall with lots of water.
     
Chips G
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Dec 6, 2007, 10:24 AM
 
IOW 12V 4A is fine, 12V 2A will probably starve the device and cause it to not work properly (or overheat the power supply), and 15V anything will probably fry it.
Are you sure about this? Do hard drives have built in limiters to ensure that only 3amps gets delivered to the motor, etc? I thought one should be worried about overheating by supplying too much current.

-Chris
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SirCastor
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Dec 6, 2007, 10:29 AM
 
Current is a measure of the rate at which electrical charge carriers, usually electrons, flow.
A Current of one ampere (1A) represents 6.24 x^18 charge carriers per second past a given point.

The standard unit of Electromotive Force(EMF) is the volt. An EMF of one volt (1V), when it is placed across a component with a resistance of 1 Ω (one ohm), drives a current of 1A through that component. Does the term EMF seem sophisticated or esoteric? If so, think of it as "electrical pressure." Most people call it voltage.

It is possible to have EMF without current. This is Static Electricity
Water metaphors never really worked for me. The above did. It's taken from Electronics demystified
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ghporter
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Dec 6, 2007, 11:07 AM
 
Originally Posted by Chris Gilpin View Post
Are you sure about this? Do hard drives have built in limiters to ensure that only 3amps gets delivered to the motor, etc? I thought one should be worried about overheating by supplying too much current.

-Chris
The motor can only draw a certain amount of current; in this situation the water analogy breaks down because you can't pour more current into a device that only "asks for" a certain amount. As long as the current demand is less than the available current, everything will be fine.

Glenn -----OTR/L, MOT, Tx
     
analogika
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Dec 6, 2007, 01:09 PM
 
Originally Posted by ghporter View Post
The motor can only draw a certain amount of current; in this situation the water analogy breaks down because you can't pour more current into a device that only "asks for" a certain amount. As long as the current demand is less than the available current, everything will be fine.
Mine worked.
( Last edited by analogika; Dec 6, 2007 at 01:17 PM. )
     
dice  (op)
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Dec 6, 2007, 03:21 PM
 
Hey everyone, thanks for all the great replies you really helped to explain it all!
sheesh, that took 8 hours for me to be asked to change my sig...
     
iMOTOR
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Dec 6, 2007, 05:11 PM
 
Ohm's Law is your friend.
     
   
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