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What are some good resources to learn about basic electricity?
August 11, 2005 6:21 PM   Subscribe

What are some good resources to learn about basic electricity?

I have a device here that says "9VAC / 1300mA" under the power input. I just moved and the adapter that was packed with it says it outputs "DC 7.5V 1A". It's not exactly the same, so I'm afraid to try it out in case I destroy my beautiful equipment. This is just the latest electrical idiot moment for me. I seem to know zero about things like DC, AC, VAC, volts, watts, amps, milliamps, etc. I've looked up the individual terms, but it tends to lead to equations and science history. Is there a resource that will help me gain a basic working knowledge rather than treating electricity like scary witchcraft?
posted by frenetic to Science & Nature (13 answers total) 2 users marked this as a favorite
 
Don't plug an AC->DC adaptor into an AC appliance.

This is a pretty good little slide show on basic electrical concepts.
posted by nicwolff at 6:41 PM on August 11, 2005


google is always a good resource for this kind of stuff. there are a million pages out there with basic electronics tutorials. this one looks to be good, though i haven't read it really, but lots of pictures and not so much math.

this stuff is difficult for beginners to learn because it's all invisible. however there are some everyday analogues that come in handy.

*deep breath*

a short but simple description of these circuit concepts goes like this: electronics is all about the way charge moves around in wires. charge comes in discrete units, called electrons - however an electron is very small, and so there are very, very many of them moving around in your typical household circuit.

voltage describes the invisible force that makes the electrons move. another, geekier name for voltage is electro-motive force, which pretty accurately describes what it does. a Volt (V) is the unit of voltage. voltage is two-directional - it can be an attractive force, or a repulsive force.

so what happens when you get some electrons near a voltage? well, if they're inside a conductor like a copper wire, they start to move, either away from the voltage or towards the voltage, depending on its direction. this is where the notion of "positive" and "negative" voltage come in - a positive voltage attracts an electron, a negative voltage repels it.

you can talk about how many electrons that move through your wire when you apply a voltage - this is called current, and is measured in units of Amperes (A), or Amps for short. a milliamp is 1/1000 of an amp. one Amp is an awful lot of electrons - around 62,000,000,000,000,000,000 of them.

so what determines how much current you get? for most simple circuits, two things: the voltage you apply, and the material properties of the circuit. these material properties are neatly wrapped up in a quantity called resistance, which is measured in units called Ohms. now, i know we don't like equations in this discussion, but bear with me. voltage, current, and resistance are related in an equation called Ohm's law: V = I * R, or if you do a little algebra, I = V / R.

so, basically, if you apply a voltage to some thing with some resistance, you'll get some electrons to move around in the thing. if the resistance is increased, less current will flow; if the resistance is decreased, more current will flow. if the voltage is increased, more current will flow; if the voltage is descreased, less will.

an easy-to-grasp analogue of this is water in hoses. voltage can be thought of something similar to the water pressure, current can be thought of something similar to the amount of water that runs through the hose, and resistance is kind of like the opposite of the hose diameter. turn up the pressure, you get more flow; step on the hose and reduce the diameter, you get less.

i've been talking about DC circuits, which mean that the voltage is constant. however many circuits, for various reasons, use alternating current or AC. in an AC circuit, the voltage is constantly swinging back and forth between positive and negative. this means that the electrons in the circuit first go one way, then slow down, stop, go back the other way, slow down, etc. current is still flowing; it's just that it happens to be the "same" electrons that flow back and forth through a particular segment of your circuit.

as for your devices: "9VAC / 1300 mA" means that the device you have requires 9 volts of AC (alternating current) and has an effective resistance (which you could easily do the math on and figure out) that makes 1300 mA flow. what this means is that it requires 1300 mA worth of current to work properly, and the little power supply thingy you plug in has to be capable of providing at least that much, and at the right voltage.

the one you've got supplies 7.5V (not enough) and it's DC, and it can only put out 1 amp (also not enough). i don't think there would be any harm in plugging this in, since it's undervoltage, but your device is almost certain to not work.

best bet is to shop for a "universal" AC adapter; the best ones are the kind that have a little switch that allow you to select the voltage they put out. make sure you get one that puts out AC if that's what your device needs.

a BIG gotcha here is that there are like a million different sizes of plugs (the end that goes into your thing, not the wall end) and there's no apparent rhyme or reason to how they're selected. so if you get a universal one, get one of the kind that allow you to change the tips and that comes with a big variety.

anyway, best of luck.

man, this turned out to be really long. sweet jeezus. sorry.
posted by sergeant sandwich at 7:05 PM on August 11, 2005


oh, i should add - in general it's okay to plug a DC source into a circuit that's expecting AC (it won't damage the circuit, unless the voltage is higher than expected), but the opposite is not true. an AC source can easily damage a circuit that's expecting DC.
posted by sergeant sandwich at 7:20 PM on August 11, 2005


[Heh, good stuff Sgt. I was typing up a summary as well, which is not a detailed as yours but might help so here goes...]

Volts measure the difference in potential between the sides of a circuit, and you can think of this as if the electrons were water flowing from one tank to another; raise one tank on high, and there's more pressure in the hose. Ohms measure the resistance in the circuit to the flow of electricity - how wide is that hose? Amps measure the flow of current, which is the voltage divided by the resistance - if you want more water out of a hose, you can raise one tank higher or get a bigger hose.

Watts measure the power at some point in the circuit - how much work could the flowing water do? - and that's the product of the voltage and the amperage. If you wanted to turn a waterwheel faster, you could either increase the pressure coming out of the hose, or the speed of the flow.

Now if you just hook up a battery to some wires, all the electrons are coming at you all the time, from the negative terminal around the circuit to the positive terminal. (Remember, the charge of an electron is negative.) That's called direct current or DC, and there's nothing wrong with it - Edison wanted to distribute power that way - but it turns out that if you instead push the electrons back and forth in the circuit - "alternating" the current's direction - you can manage the voltage of the power with transformers, raising it for long-distance transmission and then lowering it for delivery into homes.

So that's the situation; your home is getting 120 volts AC (in the US - maybe 220 or 240 abroad) from the power company, and the breakers in the box in the basement share out that power at different amperages to 120V outlets in different rooms. Most of the simple electrical devices in your house take 120V AC directly - lamps, the refrigerator &c. - but some small devices are designed to take less voltage, so they have a step-down transformer - in your case, the device wants 9 volts at 1.3 amps of AC power.

On the other hand, complex electronic devices run on DC internally so they have either external adaptors or internal power supplies that convert the AC to DC. You've got one of the external adaptors there, which takes 120V AC and outputs 7.5V DC at 1 amp.

So, it's both the wrong kind of power and not enough power to operate that device.
posted by nicwolff at 7:24 PM on August 11, 2005


I suggest that you turn to an actual book for learning the basics. Much of what you find on the web will be confusing to you if you are a beginner. I suggest this book.

As far as power adapters go, here is all it boils down to in a nutshell:

- match AC to AC and DC to DC, don't try to mix.
- the voltage rating of both should be the same. [*]
- the current rating of the power supply should be equal to or greater than the current rating of the device. [**]
- make sure the polity of the plug is correct. Most devices will have a little picture showing whether ring or tip is positive.

That's it.

[*] There are some exceptions to this, however. In some cases you can easily use a DC power supply with a higher voltage than what is called for in the equipment if the equipment expects unregulated DC (and thus it will be stepping it down and regulating it anyway, so the exact input voltage is irrelevent.) Some equipment uses a DC-DC converter (most laptops) and here again the actual voltage of the input isn't really critical to be exactly right.

[**] It's fine for the power supply to have a much larger current rating than the device, as it will only supply as much current as the device will draw. The reverse is not true though.
posted by Rhomboid at 7:49 PM on August 11, 2005


s/polity/polarity/
posted by Rhomboid at 7:50 PM on August 11, 2005


The rule my electrician and electrical engineer friends told me to boil it down to the simplest terms:

It takes one amp to move one volt across one ohm.
posted by lambchop1 at 8:44 PM on August 11, 2005


lambchop1: shouldn't that be, it takes one volt to move one amp across one ohm?
posted by delmoi at 9:17 PM on August 11, 2005


Delmoi is right.
posted by fatllama at 12:05 AM on August 12, 2005


and good show, sergeant sandwich, nicwolff.
posted by fatllama at 12:06 AM on August 12, 2005


In addition to what's been said, an AC voltage can be specified in several ways:

1) "Peak voltage" is the the highest voltage that there ever is. Once the voltage reaches the peak, it starts decreasing again.
2) "Peak-to-peak voltage" is the difference between the strongest positive and the strongest negative voltage, which is twice the peak voltage.
3) "RMS voltage" stands for Root-Mean-Square. If you take the AC voltage and reverse all the negative peaks, you get a voltage that varies between the peak voltage and zero. Then you smooth this out to DC. This is the RMS voltage, which is 0.7 times the peak voltage.

For grid power and home appliances, the voltage specified is always RMS. In other words, an 9VAC adapter feeds out the equivalent of 9VDC but has a peak voltage of 12.7 volts and a peak-to-peak voltage around 25 volts.
posted by springload at 4:20 AM on August 12, 2005


Just saw sergeant sandwich's comment linked on the sidebar on the front page. Great post overall, but I do have one correction:

you can talk about how many electrons that move through your wire when you apply a voltage - this is called current, and is measured in units of Amperes (A), or Amps for short. a milliamp is 1/1000 of an amp. one Amp is an awful lot of electrons - around 62,000,000,000,000,000,000 of them.

Current is not just how many electrons move through a wire--it's how many electrons, and how quickly. The unit of measure which is related to the number of electrons alone is the coulomb. One coulomb is equivalent to about 6,240,000,000,000,000,000 electrons.

If you take 1 coulomb and push it through a wire, you could do it quickly, or you could do it slowly. If you push 1 coulomb through in 1 second, your current is 1 ampere (a.k.a. amp). If you take 10 seconds to push that coulomb through, your current is only 1/10 amp.

Thus, one amp is not any fixed number of electrons--you can't determine a specific number of electrons without knowing how long the current is applied for.
posted by DevilsAdvocate at 1:51 PM on August 17, 2005


DevilsAdvocate is right - missed that one in the proofread. should have been electrons-per-second, and the right number of zeroes.
posted by sergeant sandwich at 4:09 PM on August 18, 2005


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