Is there an EE in the house? Solar Power

Factual Questions
1

So thinking I was smart, I went out and got a nice 12 1/2"x5" 12v 2 watt solar panel (sorry, can’t remember the MA output) and a small, battery-powered mini-vacuum that normally runs on 6v (four 1.4 AA cells in a series) thinking that, as my experience hot-wiring toy cars with 9v batteries as a kid demonstrated, the little DC motor in the mini-vac would do one of two things: run like a bat out of hell, or burn out.

Imagine my dismay when I hook up the panel to the little thing, and it does exactly squat (in bright Texas sun, no less). Hmm… Ok, I think, maybe it’s too much juice for the motor, but not enough to make smoke come out… solution–smaller solar panel… so, off I go and get a smaller 3" x 6" (approx.) 6v panel… and, just as much squat… dang!

The panels are working fine (tested them with a voltimeter), and the minivac runs quite well on its intended 4 AA cells, but apparently I need to fill in some holes in my admittedly limited knowledge of practical electronics. Bugger. So, what am I doing wrong?

2

Wattage = Current X Voltage (P = IE)
so 2 watts = 12 volts X 0.167 amps.

167 mA is probably not enough to run a vacuum cleaner. Even a typical flashlight will take 300 mA or more.

3

Not having the specs on PV cells right on front of me at the moment, but I would say that your vac is suffering from a serious case of impedance mismatch. Using your numbers, power law states that your PV cell’s max output is 167mA. Some DC motors can run at such a low current, but a vacuum motor which is (I assume) pulling a belt attached to a rotary brush must be one helluva load for a PV cell. I am sure the PV cell’s internal resistance is getting in the way. Please refer to this thread for a short course in internal resistance of a power source.

Since you know how to use a DVM, you can check for loading problems easily. Connect the meter leads to the cell’s output terminals & watch the voltage level as you connect the motor. I suspect it will drop to almost nothing due to a combination of the current demand from the motor & the cell’s output impedance.

Something else you should look at is the vacuum’s listed power rating. I think it should be much more than 2 watts, but then again I’ve never heard of a vacuum that could operate from 4 AA cells.

4

Just looked up the specs on the panel, and it claims 100 MA (for the big panel), although certainly that’s conservative, and it’s really 167.

However, the micro-vac I meant just that… this is a little bugger that is intended to be used to vacuum crumbs out of computer keyboards, it’s hand held, about the size of a TV remote. There’s no belt or gears, the shaft on this little motor is just connected to little plastic fan (squirrel cage), it’s basically not much more than a toy.

5

That is a light load. I have a small toy model of a wind mill that turns, although slowly & weakly, when sunlight strikes a small (about 3" daimeter circle) solar disc.

I suggect trying the loading experiment with the meter to see if the PV cell’s output voltage takes a dive when connected to teh motor.

The only other thing I can think of is that the vacuum has an idiot-proofing diode in it and maybe you’ve repeatedly connected the PV cell with the wrong polarity. Long shot, I know.

6

You would have to put so many of those panels in parallel that you couldn’t afford to do it. Don’t forget a very large capacitor in parallel with the panels. A capacitor can discharge and absorb it’s power in an instant, so don’t electricute yourself. Include a diode at each panel output to prevent backflow through the panels. I forget what each cell in a solar panel puts out it’s 1.2 to 1.5 volts. You need four or five cells in series to produce 6 volts. At the end of each cell series you need a diode to prevent current from feeding backwards through the cells. Each series bank of cells at six volts needs to be hooked in parallel with a large capacitor. The diodes are between the panels and the capacitor. The capacitor is connected to the motor.

It’s a lot cheaper to run some low powered electronic device than a motor. That’s why you find solor calculators, and not solar vacuums. You can do it, with enough money to spend.

Not as expensive would be to build a solar powered battery charger. They all ready sell them.

7

HD: All that talk of how much I can or can’t afford is well and good, but apparently you didn’t read my first post. The panel puts out 12, twelve, that’s TWELVE volts. You can heem and haw all you want about volts but I’ve got plenty of that, and that has squat to do with it. What matters is the amperage.

Attrayant: thanks for the info. Somehow I got the lowest amperage cells possible. Damn chinese @#$%. I’m going to look for a low inertia motor, I think that might do the trick.

8

Rattler

Sorry to bore you but I had to connect each cell in series when I monkeyed with solar cells. You couldn’t buy them already connected in series, but that was the hey day of Radio Shack, and the first personal computers a little bit later. The diode and capacitor thing still holds true. A capacior between the terminals of a dc motor will greatly help. The connection in series with the diode also still holds true. The only difference is the block of cells you have are already wired in series on one board to produce 12 volts. An individual solar cell does not produce 12 volts, you have a panel of cells in series provided for you.

As for amperage that’s where running the panels in parallel with the diode and capacitor comes in. In parallel each panel will contribute to the total amperage available. Using the diode will keep the current from going the wrong way through a lesser energized panel. The capacitor helps give the extra power to start the motor and stores the back fed energy released during the turning of the motor. It’s a quick charge and release battery. Yes, that dc motor will cause a short back fed current during it’s turning, and magnetic field colapses.

Do you remember the static on the telivision or radio when a motorized appliance was used? (Mom’s Mixmaster or carving knife) A large capacitor and resistor would stop that problem. Have you ever built an actual motor by hand or a capacitor? How about a battery or transformer? Have you built a telsa coil? Have you built a crystal radio? I doubt you could answer yes.

By the way try building a cloud vortex chamber. It’s really neat and shows you a real miniture working tornado.

A physics teacher told our class that by adjusting the magnetic feed back from a motor to just the right phase, you wouldn’t have to pay the electric company to run the motor, the feed back would be correctly phased as to actually negate any use reading on the meter, when it was in use. You use power, but screw up the meter reading the power. This of coarse is stealing. I wish I had gotten that process explained to me in pratical working terms, but power was cheap then. Did you know that you could construct a coil in a building that has high voltage power lines running over it, and get free electricity from the inductance in the coils from the magnetic field in the area surrounding the power lines. It’s an air transformer that uses no metal core to direct the magnetic waves, so it’s much less effective. It is of coarse stealing, because your magnetic air transformer would reduce the magnetic lines of force transversing the power lines by putting them to use powering a load. A large percentage of the current on high voltage transmission lines is carried in the magnetic flux outside of the actual cable.

I started building electronic projects thirty years ago at the miniumum, and don’t really wish too continue this discussion with somebody that doesn’t now shit about electricity.

Have a nice day pin head. Byte me, but don’t Nibble or I’ll have 00000010 put a Hex on you(ASCII code 32).

End of line. <Enter>

9

Clearly I deserved all that–after all, I asked a question.

>An individual solar cell does not produce 12 volts, you have a panel of cells in series provided for you.

For the record, not once did I mistake a solar ‘cell’ for a solar ‘panel’, but then you’d know that if you had read my post in the first place.

>Have you ever built an actual motor by hand or a capacitor? How about a battery or transformer? Have you built a telsa coil? Have you built a crystal radio? I doubt you could answer yes.

>I started building electronic projects thirty years ago at the miniumum, and don’t really wish too continue this discussion with somebody that doesn’t now shit about electricity.

Just so I have the proper visual on this diatribe, should I imagine you beating your chest with both hands in the classic Tarzan manner, or the more sanguine one-handed Gorrila fashion? Is this where I’m supposed to display my hindquarters to you and begin submissive urination?

Congratulations, you figured out I don’t know much about electricity. Not too difficult for most people considering I said so right in my first post, but reading comprehension isn’t your strong suit so I’ll give you points regardless.

As for the rambling stories of your physics teacher, I’m not sure what relevance they had, but thanks for taking me back to my days of visiting the VA hospital as a kid. Should I pop a flare, grandpa? Are there gooks in the wire?

>Byte me, but don’t Nibble or I’ll have 00000010 put a Hex on you(ASCII code 32).

d00d j00 4r3 s0 31337!!!

If making fun of people for asking questions is your bag, this place must be a gold mine for you.

10

Rattler

It was the way you first responded back to me, just like I knew shit. I didn’t answer the question with what you wanted? You could have left it at that. I normaly go out of my way to help people here with technical problems.

Sorry to have dissed you.

I would normaly just have let the comments slip by, and not posted. Those comments were the straw that broke the camels back yesterday. I still haven’t gotten hold of my friend for over 36 hours now, and he is having a medical crisis in the family. My nerves are shot.

11

Here’s a site that somebody who likes to experiment might like.

http://freeweb.pdq.net/headstrong/Default.htm

12

EE here.

First of all, it is important to note that PV cells/arrays are not modeled as voltage sources; over most of their operating range, they are considered current sources. So what does this mean?

a) When there is no load, the current is zero, the power is zero, and the voltage is at a maximum. This voltage is called Voc. This is also what you would measure if you hooked the array to a digital voltmeter.
b) When the array is short-circuited, the current is at a maximum, the power is zero, and the voltage is zero. This current is called Isc.
c) When there is a resistive load on the array, you’ll have non-zero power, voltage, and current. In other words, you’ll be somewhere between the two (extreme) conditions described above.
d) There is a voltage at which power is at a maximum. This voltage is called the Maximum Power Point (MPP).
e) When the voltage is below the MPP, the array behaves like a current source.
f) When the voltage is above the MPP, the current takes a nosedive. As voltage continues to increase the current goes to zero.
g) For a typical PV array, the MPP voltage is around 80% of Voc.

Second, PV array manufacturers are not conservative when quoting specs. When they quote a voltage, it is usually Voc. This is very deceptive because you can’t operate the PV array at Voc. And when they quote a power rating it is usually at the MPP voltage. This is just about as deceptive, because you can only get that power when the load is at one particular value of resistance.

Now to the analysis…

According to your first post, you said the panel is “12V / 2 watt / 100 mA.” This really doesn’t “add up,” if you know what I mean. If we assume 100 mA = Isc, then the MPP voltage would be 20V, and Voc = 25V. Did you measure 25V with your voltmeter? Probably not. I would assume, then, that Voc = 12V (is this what you measured with your voltmeter?), and the MPP would be at approx. 9.6V. Therefore, the current is approx. 210 mA at the MPP and, according to e) above, the current is approx. 210 mA when the voltage is between 0 and 9.6V.

I = 210 mA when V < 9.6 volts.

With me so far? Now what we need to know the resistance of the load . Once we know this, we can determine the operating point.

Let’s assume your motor draws 100 mA at 6V. This means the resistance is 60 ohms. Now hook the motor up to your PV array. Now remember, the PV array is a current source, so it tries to push 210 mA through a 60 ohm load. This requires it to supply 12.6V. And guess what? It can’t do it. Because at 12V (assume Voc = 12V), the current goes to zero. (And in a way, be glad it can’t do it, else your motor would smoke.)

I made a lot of assumptions, but I believe my analysis is correct. If you really want to know the answer, do the following:

  1. In full sunlight, measure the PV array with a digital voltmeter. This value is Voc.
  2. Hook the vacuum cleaner up to a 6V power supply and measure the current (I_motor). Calculate the resistance using R_motor = 6/I_motor, where I_motor is in amps.
  3. Calculate the MPP voltage of your array. The MPP voltage is approx. 0.8*Voc.
  4. Calculate the current of the array: I_array = 2/MPP voltage.
  5. If I_motor > I_array, then you’re automatically out of luck.
  6. If I_motor < I_array, then you’re still not out of the woods. You must now determine where you’re at on the curve. To do this, calculate the voltage the array would generate if you hooked the motor up to the array: V_array = I_array*R_motor. What is this voltage?? If it’s around 6V, then it would work. If it’s much above 6V (like above 9V), you might burn up the motor! But if it is much below 6V (like below 4V) then the motor won’t run.

Hope this helps.

13

I found a site with a nice graph of the voltage Vs. Current for a solar cell, which might help understanding Crafter_Man’s explanation: Solar Cell principles and Applications. Scroll down a bit to figure 4.

Voc is where the intersects the voltage axis. Isc is where the curve intersects the current axis. MPP is somewhere on the upper-right knee of the curve. When the voltage is small (say below 0.4 on the graph), the current is pretty much constant, hence the array behaves like a current source, as Crafter_Man said.

The curves keep going when they cross the voltage or current axes. In particular, if you get above about 0.58 volts, the current can become very large and negative, and burn out the cell. This is the reason for the diode that Harmonious Discord was talking about, to limit the backwards current.

If you had the cell hooked up to a resistor, you could find your operating point by drawing the line where V/I = R. e.g. with a 1/2 ohm resistor, draw a line from (0,0) through (0.5,1) Your operating point is where the curve for how much light you have intersects this line.

Crafter_Man, I question whether the I,V characteristics for a motor really behave like a resistor. I don’t work with motors, but I seem to recall from class that at least some types of motors draw a lot more current when starting than when running. If the motor never starts, you’ll be stuck in the high current region. Still, I suppose it’s the best you can assume with limited info.

Rattler, when I was young, I had a model plane with a small motor to run the propeller (it didn’t fly, it just looked neat). When I turned it on, it wouldn’t start on it’s own, I had to flick it to get it going (and it could go either direction, too!). Have you tried spinning your motor, to see if keeps going once started?

14

You’re correct - a DC motor requires more current at start-up than when running. (I ignored this fact for the sake of brevity.) That’s why a big 'ol cap would be required, which was already mentioned above. In theory, the capacitor would charge to Voc when the motor was off, and (if big enough) would be able to supply the extra current necessary at start-up.

15

One more thing: I know a motor has a reactive component to its impedance, the I-V characteristics of a PV array are non-linear and should be modeled with parallel and series resistance, blaa blaa blaa. But given the application, I think we should keep this analysis as simple and “first order” as possible.