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Offlinesplifner180
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Registered: 03/24/06
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Re: Ghettofied [Re: splifner180]
    #5589594 - 05/04/06 11:02 AM (17 years, 8 months ago)

skeletor: Having more amps doesn't do you any good. All amps are is potential for juice.

Pretend that your garden hose is your power source. Think of the voltage as the diameter of the hose and the amperage as the knob you use to turn on the water.

Now suppose you're hooking that hose up to a sprinkler system on your lawn. Obviously the hoses need to match size (voltage). And obviously, the spigot knob (amperage) can be turned up but only so far. For the sake of argument, pretend the sprinkler system can control the spigot knob.

Once you've got the correct hose size (voltage) then you can connect the two systems. But if you try to connect more sprinklers than the hose can provide pressure (amps) for, the system won't work properly. The water won't go far enough and maybe the pressure won't even be enough to make that clack-clack-clack ratcheting action that causes the head to turn.

Hopefully that makes sense.

Ultimately, all you have to remember is make the voltage match exactly and make the amperage equal to or greater than the amp rating on the device(s) you have attached. You can go lower on voltage BUT it may work poorly or not at all.

Never go higher on voltage unless you're prepared to possibly destroy whatever you're attaching the power to. A good example is our goal here, spinning a fan. Who knows, maybe 13.5VDC wouldn't destroy the fan and give you extra spin.

Are you willing to possibly destroy a fan to find out? If so, fire that sucker up! =)

splif


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First Grow: Ecuador -> LC -> HPoo/Straw -> Monotub
Build a Do-It-Yourself Magnetic Stirrer in thirty minutes with no money.


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Invisiblecreamcorn
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Re: Ghettofied [Re: splifner180]
    #5589784 - 05/04/06 11:55 AM (17 years, 8 months ago)

well this will confuse everyone i'm sure, your analogy gets the point across but is pretty well wrong.  its semantics i know but as an electrical guy i can't keep letting this slide ;-)

using your terminology:

voltage = potential = how far open you turn on the tap (V)
hose size (including things attached to the hose) = resistance (R)
current = resulting flow (I)

ohms law: V = I*R or I = V / R or R = V / I

power (watts) = V*I  (in case you come across a wattage rating instead of voltage/amp ratings, you can figure it out just as well.)

your fan is the "R" value.  it is what is resisting the flow of current from the voltage source.  it's like the hose.  if it says for example 12VDC/0.35A its providing roughly 34ohms of resistance.  it draws 4.2 watts of power.  these are useful numbers if you want to use  a potentiometer (a dial) to control the speed of your fan.  a typical pot might provide variable resistance from 0-35ohms up to 5 watts and would work great here.  wired in series with your fan you can then adjust your circuit R value from its initial 34ohms up to 69ohms, basically allowing you to go from half speed to full speed.  hence, turning up the resistance allows less current to flow.  motor speed is related to current flow.  it spins faster with more voltage, but if you look at the ohms law formula that should explain why.

output current rating (as written on a power supply) = maximum it can safely provide.  its not a like hitting a brick wall.  it WILL provide more, given a lower resistance.  it will also overheat and cook itself if you get carried away.

10% tolerances are very common in electric devices like these.  meaning you can generally safely use a voltage +/- 10% different than whats called for.  so with your example 13.5V to a 12V fan is pushing it ever so slightly, it would probably work just fine though.  when we're loading a magnet on that fan then a jar of liquid above it, we're adding mechanical resistance to the fan, which in turn increases its electrical resistance.  so its spinning slower.  its not drawing as much current as it says on the sticker.  using a slightly higher voltage again will help over come that, but at the possibility of stressing the fan to the point of failure.

dont mean to get technical on people but THAT above is the correct explanation. :smile:


Edited by creamcorn (05/04/06 12:03 PM)


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Offlinesplifner180
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Re: Ghettofied [Re: creamcorn]
    #5590540 - 05/04/06 03:48 PM (17 years, 8 months ago)

Oh. Hey. I learned something here. Thanks creamcorn! I've never had a good grasp of how adding resistance (by way of a pot, like you mention) fits into the equation.

When you say something like "0-35ohms," you mean K, right? As in 35Kohm?

splif


--------------------
First Grow: Ecuador -> LC -> HPoo/Straw -> Monotub
Build a Do-It-Yourself Magnetic Stirrer in thirty minutes with no money.


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Invisiblecreamcorn
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Re: Ghettofied [Re: splifner180]
    #5590647 - 05/04/06 04:30 PM (17 years, 8 months ago)

well in trying to way oversimplify the dc resistance of a motor there i think i goofed on some of the numbers.  ill take a look in a bit and will point out an appropriate pot to use.  thats a super simple addition though to the DIY stirrer that costs a buck or two so you can get control over your RPMs.

and yeah, adding resistance means less current can flow.  in this case, because a motor is magnetic, and magnetic flux is determined by current only and not voltage, we can control the motor speed.  with a pot we can use pretty arbitrary choose values actually (as long as we dont go over the power rating) - they'll just determine to what extent we can make those adjustments.  one too small (probably like the one i initially suggested) will only let you turn down the speed part way... one too big means most of the movement of your dial will probably leave the fan off say 3/4ths the way around the dial, with the last little bit of movements giving you rather coarse control... so we want to strike a balance where the dial gives you nice even control from near 0 to max.  and because of ohms law, we know less voltage gives you the same end result, but the motor isn't spinning slower because the voltage is low, its spinning because the lower voltage at the same resistance means less current flow.

same concept too if you look over the LED teks, they wire em with resistors - thats because an LED alone needs a certain voltage threshold to turn on, has near infinite resistance when off but has near zero resistance once on - so without adding an amount of resistance, the moment it turns on it will allow as much current as possible through (heading towards the infinite as close as your real-world system can supply)... resulting in a very bright flash and one dead ass LED. :smile:


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