Electricity / Electronics for DummiesApril 27, 2016 6:26 AM   Subscribe

Please help me understand electricity and electronics. I have read the water analogy numerous times but I still can't quite grok it. Like "Ummm, Where does the water go and does it evaporate?" I have started tinkering with a Raspberry Pi and would love to build more complicated circuits with capacitors and transistors.

Last weekend I decided to hack my sons Hot Wheels track. I was tired of using so many batteries on the 4 different boosters so I decided to hard wire them.

I work with a few electrical engineers and they told me how to do it. This involved buying a power inverter, a high voltage resistor and a potentiometer. The end result was awesome. But the results left me confused. Each booster is a different model. One of them totally stopped working. I am assuming it is because they each have different circuitry and one got burned out. I thought that was what the resistor was for? I did crank up the inverter to 12V to see what happened and the boosters got awesomely fast! It was also curious to me that when I would turn one of the boosters off, the wheels on the other three would spin faster. My guess is it has something to do with voltage and charge but I don't think I fundamentally understand what is happening.

I have silly questions like: "Once electricity hits a load, what happens downstream of that. Is there less of it?". I feel like I am close the getting it, but then I just start getting really confused.

Can you help explain all of this or point me to some articles and books that have really good explanations with examples. I want to build cool shit but my simple mind is holding me back.
posted by jasondigitized to Science & Nature (7 answers total) 18 users marked this as a favorite

this book right here.
posted by sexyrobot at 6:53 AM on April 27, 2016 [1 favorite]

The water analogy has more to do with thinking about the pressure and flow of the water in the pipes and not the H2O nature of the water itself. Water does not compress like air does. Don't worry about the evaporation and temperature stuff. Pretend every system is closed off to the air.

So for example when a pump (aka a battery) builds pressure behind, say, a valve (aka a switch), the water does not continue to flow and build up...it just stops.
posted by JoeZydeco at 7:07 AM on April 27, 2016 [1 favorite]

Neither article nor book, but I found The Great Courses Understanding Modern Electronics series valuable.

The sticker price is daunting, but I was able to get it on DVD from my local library through an Inter Library Loan. The Great Courses also apparently has a streaming service now (finally) which has a free trial and is \$20/month after that (you could easily get through the series in a month). The stuff you want to know (right now) is covered in the first 5 lectures.

Khan Academy might be another resource to check out.
posted by sparklemotion at 7:38 AM on April 27, 2016 [2 favorites]

The water analogy is a good one, but remember that it's just that: an analogy. Don't spend too much time trying to pick it apart because it will, eventually, fail. Having a basic understanding of Voltage, Resistance, and Current (and Ohm's law, which relates them) will get you a long way.

I think there are probably some good books out there, but if I were you, I would bribe one of the electrical engineers you work with to sit down with you for an hour and help you wire a simple circuit. Pick the engineer who seems to be the best at explaining things - the concepts are really straightforward, but you want to try to find someone who doesn't overcomplicate things. I think you'll learn a lot more practical things about circuits by actually sitting down and building a simple parallel and series circuit.

If you don't think that any of the engineers you work with are up to this task, I was going to recommend the same Kahn Academy lectures that, on preview, I see sparklemotion recommended.
posted by Betelgeuse at 7:43 AM on April 27, 2016

Seconding the Mims book recommended by sexyrobot. It's crystal clear and gives you the foundation for future study.
posted by Johnny Wallflower at 8:13 AM on April 27, 2016

I have silly questions like: "Once electricity hits a load, what happens downstream of that. Is there less of it?". I feel like I am close the getting it, but then I just start getting really confused.

In short: yes, there's less of it. Where the water analogy applies is in the idea of a total amount of flow from which your various loads can take some to do their work. Where the water analogy falls down is that in an electrical circuit there's not a set quantity of power that has to be consumed; it's referred to in terms of potential, and potential that isn't used is never subtracted from the system.

Imagine a household lamp circuit (so, in the US, that would be 120V, 15A). That's connected to a circuit breaker rated for the voltage and current. You connect a single lamp with an incandescent bulb: 120V, 60W. That will draw ½ ampere (P/V = i ∴ 60/120 = ½). Also if you know voltage and current you can calculate resistance of your light bulb (V = iR ∴ 120 = ½R ∴ R=240Ω). In theory you can connect 29 more lamps, each at 120V, 60W, and not overload your circuit (in reality you would never load a single circuit that much).

But in an electrical system there are two different ways you can connect multiple loads to the same circuit, series or parallel. Different things happen when you do. In series, voltage drops and current remains constant; in parallel, current drops while voltage remains constant. Your household circuit should be wired in parallel, so each lamp would still get 120V potential, but with less current available. So, if we go back to the idea of connecting 30 lamps, each lamp would see 120V potential but only ½A, but that's enough (ignoring previous warning about the reality of wiring).

If you connected them in series, though, since the current must remain constant the voltage would drop. 30 lamps in series would each see only 4V each. You can also calculate the current draw, although lightbulbs aren't perfect resistors so measured values probably wouldn't match your calculations. Confused? See this photo for an example.

Note also that you can combine loads in series and parallel. I'm not sure how you wired your circuit, but what you want to do is make sure that each booster is fed with the right voltage at the right current. Note that the "right" voltage will vary by design and construction, and some boosters may be more tolerant of over-voltage than others (as you've discovered, by burning one out). Two D-cell batteries in series would provide 3V. Four 3V loads connected in series would drain 12V. If you turned one off, the remaining three would drain 9V if their design limited their power consumption – but your testing seems to indicate that they don't have a limiter, and they're able to run fast and hot, for a while.

Four 3V loads connected in parallel, with no limiter inline, would be likely to burn up at 12V. Four 3V loads in parallel, in series with a properly sized resistor, should hum right along though.
posted by fedward at 9:21 AM on April 27, 2016 [1 favorite]

The water analogy is useful but limited.

It is helpful for explaining stuff like voltage [=pressure], current [=flow] and resistance [=pipe diameter].

One obvious place the analogy breaks down is that water does not require a closed circuit in order to flow, electricity definitely does.

However if you do have a closed water circuit, with a pump in the place of a battery in an electrical circuit, the analogy is pretty good for basic stuff.

I have silly questions like: "Once electricity hits a load, what happens downstream of that. Is there less of it?"

"Is there less of it?" is not quite the right question - it depends what you mean by "it". When thinking about electrical circuits, we don't think about the quantity of electricity per se, we think of the magnitude of voltage, and the magnitude of current.

So, imagine you have your pump connected to 2 large diameter pipes [1 each on the input & output]. The pipes are full of water & the ends are capped. Your pump is producing pressure X, but obviously no water is flowing.

if you connect a teeny pipe between the 2 large pipes, you can then measure flow [or current] Y is passing through the circuit[1].

Replace the teeny pipe with one that has twice the cross-sectional area, and you will then see a flow of 2Y through the circuit.

Switch back to the teeny pipe, and crank up the pump so the pressure goes up to 2X, and you'll also see 2Y flowing.

What do you expect to happen if you now replace the teeny pipe with the larger one?
Or if you ran another teeny pipe in parallel to the 1st one[2]?

The above covers the basics of the relationship between voltage, current & resistance.

[1] if you're measuring your flow as 'volume per unit time' it should be obvious that you'll see the same rate of flow no matter where you measure in the circuit.

[2] In this example, the total current flowing will be equally shared between the 2 small diameter pipes, so measuring the current in one of them will show half the current you measure in the large pipes.
posted by HiroProtagonist at 8:44 PM on April 27, 2016 [1 favorite]

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