Science Fair project

Adjuster

Joined Dec 26, 2010
2,148
A much, much, muuuuccchhhh better analogous word is pressure, since it is possible to have voltage with zero current. Please stop saying speed.

I think I speak for all of us when I say your idea is not entirley clear, if you could please post a schematic or block diagram it would be most helpful.
In any case, you can't increase power through a buck converter, you can only do that through adding a power source.
Converters cannot increase power, but they may optimise the power source loading, allowing the maximum output to be realised.

I wonder if the OP's ideas about "speed" came from an analogy to an automotive gearbox? (NB moderators, an analogy only.)
In this situation, selection of the appropriate gear ratio allows the engine to run at an appropriate speed so that it can deliver full power. You can't get more horsepower than the engine has available, but you can certainly get less available power with the wrong gearing.

I really do not like the voltage = speed analogy either, but it does work to a degree. It goes like this: an engine has a certain torque/speed characteristic, just as the solar panel has a certain current/voltage characteristic. The solar panel maximum voltage is a bit like the maximum rpm limit, and the maximum current is a bit like the maximum torque.

That said, the the analogy breaks down at the extremes as the panel will deliver full current into a short-circuit, whereas an internal combustion engine will stall below a minimum rpm. The panel voltage limit is inherent, with no prospect of connecting rods coming out of the side, so the engine would have to be imagined as having an effective rpm limiter.

It is important in the end to remember that this is just an analogy: electric potential is not really the same as speed. The analogy of voltage with pressure and current with fluid flow is more commonly used, and in my opinion easier for a general audience to understand.
 

Thread Starter

russpatterson

Joined Feb 1, 2010
353
Thanks all for the replies; and the automotive analogy. So exactly what is voltage potential then, forgetting about the 13-year-old getting it requirement? The electrons are moving faster? There's more unbalanced atoms wanting to balance out with other atoms that have the part they're missing?

The issue I have with the water pressure analogy is that not a lot of 13-year-old's have really discussed or put much thought into the idea water pressure before. They certainly understand it from a practical perspective from experience with garden hoses and super soakers and things like that but I find you have to start explaining what water pressure means and then coming back to the analogy with electricity. Everyone understands how fast the water's moving and word's like speed, fast, quickly are inherently more interesting and positive sounding than "pressure". That said I don't disagree that pressure is probably more accurate. The analogies are just that; and all have their flaws. It comes down to a subjective decision where you make a choice based on the flaws you think you're willing to live with, (similar to a presidential election).

For the Science Fair project I think I will recommend leaving the battery out of it and just using graduated resistors and LED's.
 

Wendy

Joined Mar 24, 2008
22,368
When I was new to electricity I used the water hose analogy. Current is how fast the water is flowing through the hose. Voltage is water pressure, especially when there is no flow. A resistor is a kink in the hose that restricts the water flow.
 

Adjuster

Joined Dec 26, 2010
2,148
If we take speed as an analogy for voltage, then we have to use force (or torque if thinking of rotation) for current. Unfortunately, I think many folk would say force is a better analogy for voltage, so speed would best represent current...
IMHO, this mechanical analogy is not so good. I would agree with Bill Marsden and Magnet18 that the fluid flow analogy is much better, but in the end any analogy is not the full story. It may be better not to get youngsters to spend too much time with these simplified explanations, even if we may find these concepts difficult ourselves.

At a certain age, it may be well to quit certain metaphors, and call a volt a volt, an amp an amp, bringing us naturally to current.
Current represents the flow of electric charge, which is a basic property of matter. Subatomic particles called electrons each have a negative charge, and are responsible for most ordinary conduction e.g. in metals. Other particles carry charge, such as a nuclear particle called a proton (positively charged), and atoms or molecules with a deficiency or surplus of electrons (ions).
I don't have the faintest idea of what charge really is, but accept that it exists: ask a physicist for a better picture.
Anyhow, every electron has a charge of about 1.6×10−19 coulombs , and a flow rate of one coulomb per second is a current of one ampere http://en.wikipedia.org/wiki/Ampere . Now we see why, if an analogy is needed, fluid flow works for current.

It may not be especially helpful to think about the speed of the electrons themselves, as this varies a lot according to the circumstances. Good metallic conductors are so stuffed with free electrons and atoms for them to interact with that the average "speed" of the individual electrons at ordinary currents works out to be pretty low, although an electrical signal in a cable can propagate at a fair fraction of the speed of light. Physicists explain this by the way electric fields work, but you can sort of get the idea by thinking about those wretched hosepipes, if you must. If a pipe is already full of water, then if we turn on a tap feeding it some water may come out of the far end long before the water that was at the tap end gets there.
Electrons do move pretty fast in vacuum devices like X-ray tubes, old-fashioned TV CRTs and even grandfather's old radio valves (US: electron tubes). In a high vacuum with negligible collisions, it is true that the speed reached by an electron is related to the potential difference that it has crossed.

So what is potential difference? http://en.wikipedia.org/wiki/Electric_potential_difference It is the thing we commonly call voltage, although purists may dislike that. Voltage is or was sometimes referred to as tension, or pressure, as in that old hydraulic analogy. http://en.wikipedia.org/wiki/Hydraulic_analogy In any case, it is measured in volts . Basically it describes how much energy has to be expended to move a charge from one potential to another, or conversely how much energy would would be released if the charge moved in the opposite direction. For this reason volts can be defined as joules per coulomb.

The potential difference tends to make charge accelerate (an electric field produces a physical force on a charge, but let's not follow that digression). In any case, the charged particles move, constituting current, and they can release energy, in many cases as heat in the jostling atoms of a metal wire or a resistor, or in a variety of ways at the end of their flight as they smash into the anode of an X-ray tube. Since potential difference (in volts) is equivalent to joules per coulomb, and current (in amperes) is coulombs per second, we find that (for DC quantities at least: AC can get complex*) multiplying the voltage in volts by the current in amps tells us the number of joules per second, that is, the power in watts. http://en.wikipedia.org/wiki/Watt

I seem to have used too many words, as usual, but hope this helps. I will have made mistakes, perhaps serious ones, but maybe someone else will be along to tell the story better. Finally, note that I have not even tried to check out all the several Wikipedia references here. These may be helpful, but if you find errors don't blame me: I didn't write them!

*http://en.wikipedia.org/wiki/AC_power
 
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Wendy

Joined Mar 24, 2008
22,368
Fluid flow is closer than you think. The specific definition for current is a certain number of electrons past a set point per second.

http://en.wikipedia.org/wiki/Ampere

The analogy of water pressure is also accurate for volts.

http://en.wikipedia.org/wiki/Volt

After that is gets more iffy. Like all analogy's it breaks down quickly if taken too far.

There might be something in there for a science fair if you can figure out an angle. How is electricity like electricity? Look at the works of Ben Franklin concerning electricity, back then they really thought it was a fluid.

When it was my kids time for a science fair I wasn't so good at helping him. I respect your helping your kid, it isn't easy.

I'm going to point out a fairly easy experiment that isn't too expensive or complicated, but it may be too advanced for what you're wanting to do. It is a primitive LED/resistor voltmeter. Not accurate in the slightest. It is simple over all. I can help with parts if need be, PM me if you're interested.

http://forum.allaboutcircuits.com/blog.php?bt=676

Look at Figure 3.2.
 

wayneh

Joined Sep 9, 2010
17,152
So what's the over/under on the voltage differential. Does bucking a 22V panel charging a 12V battery to 14.4V make sense in your opinion?
I doubt it, but I'm just guessing what the delta-V needs to be before bucking becomes attractive. If open circuit in full sun is 22V, then the optimum load might be whatever current brings it down to 18V. There's not much spread between that and 14.4v. And the gap narrows even more when there is less illumination.
 

wayneh

Joined Sep 9, 2010
17,152
At a certain age, it may be well to quit certain metaphors, and call a volt a volt, an amp an amp, bringing us naturally to current.
FWIW, when I had circuits in college, the analogy to water flow was discussed briefly on day one and then dropped. The reasoning was that, while an analogy can be useful very early on, it breaks down later and can actually make other concepts more difficult while your mind is busy trying to maintain an analogy that just doesn't work. Water flow isn't very useful in the AC world, for instance.
 

THE_RB

Joined Feb 11, 2008
5,438
...
Question: Can changing the voltage of electricity from a solar panel maximize it’s power output?
Hypothesis: If I alter the current being used by a solar panel, can I measure and plot the power curve in order find an optimum spot for maximizing the solar panel’s power output?
...
This is a good science fair experiment but has been written up rather badly.

Putting a resistance in series with the panel and battery can indeed increase the panel's power output, which I think is the purpose of the experiment to chart the panel's volts/amps curve, and to allow the student to realise that if the panel is operated with less current and a higher voltage the panel will produce higher power output than it will directly connected to the battery.

The questions should clarify (or emphasise) the key point, which is that placing some impedance between the panel and battery causes the PANEL to produce more power (as the MPP will occur at around 17.5v for a 12v panel).
 

Thread Starter

russpatterson

Joined Feb 1, 2010
353
Thanks for all the replies. Interesting stuff.

I doubt it, but I'm just guessing what the delta-V needs to be before bucking becomes attractive. If open circuit in full sun is 22V, then the optimum load might be whatever current brings it down to 18V. There's not much spread between that and 14.4v. And the gap narrows even more when there is less illumination.
I recently read paper on MPPT (attached) that a friend sent and in the "Hardware Design" section they briefly discuss buck, buck/boost, and boost topologies. They choose boost because it's simpler. However what didn't make sense to me is why you "boost" the panel voltage, which is already higher than the battery voltage? If a panel operates at 22V, and say in full Sun it's MPPT voltage is 18V then how do you track to that optimum voltage by boosting? Seems like buck is the obvious choice but I am clearly missing something.
 

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wayneh

Joined Sep 9, 2010
17,152
...but I am clearly missing something.
I think what's missing is realization that this is a pretty complicated topic for this age group. Rather than solving the issue with state-of-the-art buck or boost technology, simplify the problem to the core issue; matching a load to its power source. Like changing gears on a bicycle.

I think it'd be fairly easy to demonstrate that more useful power can be extracted with a matched load (several lights in series?) than a single, low voltage load (same lights in parallel?). This demonstration is plenty for this age group and the principle - load matching - may actually be more memorable when demonstrated as simply as possible.
 

Thread Starter

russpatterson

Joined Feb 1, 2010
353
@Wayne, agreed. In previous posts I said the SF project would be just plotting the I-V curve by measuring a panel with different resistor values. Then identifying the MPPT from that.

I'll start a new thread for the MPPT & magnetic topology stuff.
 
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