# Science Fair project

#### russpatterson

Joined Feb 1, 2010
353
Hello, my 13-year-old daughter asked for some help with ideas for a science fair project. I was going to make a buck circuit with knob (pontentiometer) to change the PWM percentage. I want the girls to wrap the wire around the core of the inductor and connect it to the buck circuit and do the power tests. Do you think we need to just buy the inductor(s)?

Does the idea sound reasonable? Anyone know where a good source for the iron core and wire to make the inductor?

Here's the science fair project details:

Question: Can changing the speed (voltage) of electricity from a solar panel increase its power output?
Hypothesis: If I alter the speed of electricity from a solar panel using an electromagnet, can I measure an increase in the solar panels power output?
Control: A solar panel connected directly to a 12 volt battery. Using two meters, measure the voltage and current between the solar panel and the battery. Voltage (V) multiplied by current (I) gives power (P) (in Watts). P=VI
Variable: Connect an adjustable electromagnet between the solar panel and the battery, vary amount of current allowed to flow through the magnet. Using two meters, measure the voltage and current between the solar panel and the battery.
Other: Make two electromagnets, one to demonstrate picking up nails, the other used to vary speed of electricity from a solar panel

Joined Dec 26, 2010
2,148
I do not know where your project definition came from, but there seem to be some really basic problems with it:

1. We do not normally refer to voltage as the "speed of electricity": this might normally be considered to be the 3*10$$^{8}$$ metres per second speed of propagation of electromagnetic waves, or a somewhat slower speed associated with transmission e.g. in a cable. [Electrons in a conductor on the other hand may be moving quite slowly, but however you think about it voltage ≠ speed.]

1. An inductor might be used in a circuit to increase voltage, but speaking of an electromagnet increasing the speed of electricity is a very strange way to put it, and possibly very confusing to a youngster given that the maximum "speed" of electrical phenomena is generally supposed to be a fundamental limit.
Of course, if this gobbledygook has come to you courtesy of a teacher at the school you may not be able to do much about it, but it seems a pity to get basic concepts garbled in this way.

#### Markd77

Joined Sep 7, 2009
2,806
If you have any dead compact fluorescent lights around and you crack them open, you should find a little toroid like this: (SgtWookie told me).

#### SgtWookie

Joined Jul 17, 2007
22,230
Dead PC power supplies are very good sources for toroids.

Have a look at Ronald Dekkers' "Flyback Converts for Dummies" page:
http://www.dos4ever.com/flyback/flyback.html
It does a really good job of explaining how these things work to a layperson, and there are experiments for you to try.

Radio Shack sells 100uH 2A inductors for a couple of bucks each. They also have assortments of power toroids for ~\$7; or at least they did carry them a few months ago.

I came up with a design using a 555 timer as a high-side PWM regulator for an N-ch MOSFET for driving LEDs; it's kind of interesting, and you might want to play with it.

See the attached.

#### Attachments

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#### wayneh

Joined Sep 9, 2010
17,504
Old TVs or monitors (CRT, not LCD) also have wire-wound coils in them, and people will pay you to take them away. Of course you have to pay also, once you're done gutting it.

#### atferrari

Joined Jan 6, 2004
4,798
Is this a project suitable for a 13 yo student?

Sorry to say but it seems to involve rather abstract concepts (I could be wrong here) that at least for now should be accepted by her in sheer bonna fide.

#### magnet18

Joined Dec 22, 2010
1,227
1 your hypothesis can't be a question, it has to be a statement of prediction.
2. How is connecting an electromagnet in series with the panel/battery going to change the voltage?
4. Where does the PWM come into this?
5. How are you going to make sure the battery is in the same state each time? The further discharged the battery, the more current will be able to flow.
6. Speed of electricity makes NO sense whatsoever, unless you are referencing the speed of propagation of the charge distribution along the line, which I doubt given that this is a middle school science fair project.

As a suggestion, maybe a 13 year old interested in electricity should start simpler, like maybe something involving ohms law or the efficiency of diodes (put different value resistors in series to change the amount of current and measure the voltage drop across the diode, I have a LOT more information on that one that I can post if you're interested, I did it myself in middleschool.)

#### wayneh

Joined Sep 9, 2010
17,504
Does the idea sound reasonable?
Not a bit. Your hypothesis seems to be that adding a variable resistance (an electromagnet) in series with your solar charger might increase power output to the battery. Since it adds resistance and can only dissipate power, not add any, you'd have to violate many well-established laws of physics for your hypothesis to be correct. Your control will show higher power to the battery than the test. This is because the battery will define the voltage - and there's nothing you can do to get more current from the panel at a fixed illumination.

Then you launched into PWM as a way to vary current, and I think confused "speed" with frequency or duty cycle. I also sense that you're thinking of boosting voltage, although you wrote buck. I don't see how any of this fits in with your hypothesis or experimental plan. It just makes things complicated. Ditto for the electromagnet picking up nails. That has nothing to do with optimizing solar panel output. It might be fun, but it's a distraction.

A humble suggestion: Have the kids demonstrate the implication of the I-V curve, the way voltage from a panel falls as the current increases. This leads to an optimum power point at which to operate a panel (at ~80% of the open circuit peak voltage). Too much current, the voltage is too low. Too high a voltage, and there's little current. Find the sweet spot. This exercise will teach about ohm's law, and the relationships of voltage, current and power, and it's highly relevant in the real world. That's plenty for kids that age.

#### russpatterson

Joined Feb 1, 2010
353
Thanks all for the replies. Speed of electricity comes from the water analogy. What items match best:

1) A skinny waterfall, e.g. a small stream in the mountains that falls off the edge of a 1000' cliff (think Yosemite).

2) A wide river that flows along at 3mph (think Colorado River near the border with Mexico)

----

A) The output of an small inverter to run a 4' length of EL wire. 100V at 45mA

B) The output of a 12V bank of batteries to run mountain cabin with all the 12V lights and appliances on, (10 Amps).

If you said 1-A and 2-B I would agree with you.

The word voltage doesn't mean anything to kids, although I agree that learning the vernacular is part of learning about things, the more you de-reference the concepts, the more the kid's eye's glaze over.

To maintain that water analogy; if you do a voltage transformation with a buck converter (the kids wind the inductor) then you are effectively changing the speed of that flow of water, from a skinny-fast stream to a wider-slower stream. Correct?

The MPPT idea is what I was thinking with this experiment.

If a panel runs at 20V open circuit and the battery is at 12V then wouldn't you want a buck circuit to achieve MPPT?

I planned to replace the battery with a large sand resistor so we wouldn't have to worry about the battery's state of charge of voltage level.

Maybe changing the experiment to simply measure the current and voltage and adjust the resistance of the load with a variable resistor would be simpler and more appropriate for this age. I didn't plan to get too deep into the whole PWM and buck circuit details. However, in my opinion, buck and boost circuits are one of the more interesting things you can do with a magnet (electro or otherwise). I bet that most of those middle school kids have no idea that you can do high frequency tricks with magnets.

#### wayneh

Joined Sep 9, 2010
17,504
If a panel runs at 20V open circuit and the battery is at 12V then wouldn't you want a buck circuit to achieve MPPT?
I don't think so. Power into the battery is 12V times the current in. I don't see how adding ANY device between the panel and the battery would increase that current. Eliminating loss across the blocking diode, by controlling a MOSFET as an ideal diode, will gain a few percent, but any power transformation scheme can only cause losses. I'd love to hear that I'm wrong.

#### magnet18

Joined Dec 22, 2010
1,227
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.

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#### russpatterson

Joined Feb 1, 2010
353
Thanks for the input. I simplified it:

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?
Control: A solar panel connected directly loads of varying resistance (LEDs and resistors). Using two meters, measure the voltage and current between the solar panel and the battery. Voltage (V) multiplied by current (I) gives power (P) (in Watts). P=VI
Variable: Alter the amount of current that is allowed to run through the solar panel by changing resistor values and/or number of LEDs. Using two meters, measure the voltage and current between the solar panel and the battery.

#### wayneh

Joined Sep 9, 2010
17,504
Looking much better, but I think the battery still causes a problem by anchoring the voltage. The power going into the battery can only be reduced by anything you put in series with it and the panel.

On the other hand, you could probably light up some LEDs while keeping the same or nearly the same voltage and current into the battery. You won't get more power into the battery but lighting the LEDs would demonstrate that the panel can supply more power when it operates at it's optimum, compared to just wiring it directly to the battery.

#### russpatterson

Joined Feb 1, 2010
353
I could either remove the battery from the whole thing and just use resistors and/or LED's or modify one of my lighting controllers to keep the battery right at 12.5V by PWM'ing the output to a big load, like a 12V halogen lamp or a bunch of LED's (increase/decrease PWM ratio based on battery voltage).

Probably simpler is better as more kids might actually get what's going on with less stuff in the mix.

Joined Dec 26, 2010
2,148
Looking much better, but I think the battery still causes a problem by anchoring the voltage. The power going into the battery can only be reduced by anything you put in series with it and the panel.

On the other hand, you could probably light up some LEDs while keeping the same or nearly the same voltage and current into the battery. You won't get more power into the battery but lighting the LEDs would demonstrate that the panel can supply more power when it operates at it's optimum, compared to just wiring it directly to the battery.
In the context of a chopping buck converter (which has additional elements to the series choke), I cannot agree with highlighted part of this statement. If the output voltage of the panel corresponding to its maximum power output is higher than the battery voltage, then it might in fact possible to deliver more power to the battery by using a switch-mode buck converter, provided that it was sufficiently efficient.

Switching buck converters do not behave like resistors or linear series regulators, which only drop voltage by creating loss, and have at best equal input and output currents: it is possible for a switched-mode system to deliver a net current advantage, provided that there is sufficient input to output voltage drop to more than offset the inevitable losses.

Of course, the maximum possible power output at the optimum conversion ratio is never more than the maximum panel output less losses. It is moot whether the OP would be able to achieve more output than with a direct connection, but at least an optimisation is possible in principle.

Joined Dec 26, 2010
2,148
This paper sets out how a buck converter may be applied to this task. It's a bit beyond the 13 year-old level, but at least it shows that this can be done. The authors claim to have demonstrated a working system. http://www.icrepq.com/ICREPQ'09/502-bernardo.pdf

#### russpatterson

Joined Feb 1, 2010
353

Magnet: "it is possible to have voltage with zero current"

@Magnet, I believe you are probably right but how do you know there is voltage with no current present? How do you measure it?

#### magnet18

Joined Dec 22, 2010
1,227

Magnet: "it is possible to have voltage with zero current"

@Magnet, I believe you are probably right but how do you know there is voltage with no current present? How do you measure it?
If you connect a capacitor to a DC voltage source and let it charge, then disconnect it, it will hold the charge, or voltage. While it is holding it, there is no current flowing, but the voltage is still there. Like blowing up a balloon.

You measure voltage with a voltmeter. One method that allows zero electron flow is to use the voltage to bias the grid in a vacuum tube. Of course, this is life, and nothing is ideal, so there will always be leakage currents, but the principle is sound.

Also, you still need to re-state your hypothesis as a statement, not a question, science fair judges will mark off for that. If you need some pointers with that portion of things check out this site- http://www.sciencebuddies.org/

I'm a senior in highschool, and still reference it whenever I do a science fair project.

#### wayneh

Joined Sep 9, 2010
17,504
In the context of a chopping buck converter (which has additional elements to the series choke), I cannot agree with highlighted part of this statement.
You're right. I was thinking of a situation where the battery is not too far below the optimum voltage. But if it's far enough below, then there's room for a converter to improve the power transfer. Imagine a 20v panel charging a 1.5v battery. Bucking down the voltage and increasing the current would beat the direct connection.