Three lead Solar Cell?

[Rory Starkweather]

I have a set of solar powered walkway lights. The rechargeable batteries in most of them went bad, so I decided to do a solar cell experiment.

When I opened them up, though, I found that they have three leads instead of two.

1. Red wire to far side of cell.
2, Black to near side of cell.
3. Green to near side of cell.

These all run to a small circuit board which has two leads coming out to charge the battery.

I have a question about the green lead because it may or may not be connected to the black lead at the solar cell. The two leads come out of the solar cell together, but it looks like the manufacturer has gone to a lot of trouble keeping them part. They actually come out through an unidentifiable white piece that has two widely separated holes for them.

The point where those two wires come out of the solar cell has 'CDS' molded into the structure nearby, but I don't think that tells more than the type of cell.

Has anyone seen something like this before? I wonder if it could be a direction connection to the LED that produces the light for the walkway. If so, why does it go through the circuit board?

 

[JWHassler]

One of the wires goes to a photo-electric cell ("solar cell") that generates voltage.
Another goes to a photo-conductive cell that detects the presence/absence of sunlight. Photo-conductive cells are made of cadmium selenide/sulphide.

The remaining wire is a common.

 

[Rory Starkweather]

Thanks. That extra lead to show the presence of sunlight gives me something to think about. Do you think it's on/off or might it be a variable voltage or current that might be used for steering the cell towards the sun?

 

[JWHassler]

They vary from 10's of Kohms in sunlight to 10's of megohms in darkness

 

[(*steve*)]

It is perhaps more accurate to say that they vary in resistance by about 3 orders of magnitude between "daylight" and "darkness". Some have a much lower resistance in bright light, but have resistances of only a meg or so in darkness.

For reference I have included a pdf of a datasheet. This indicates the ranges of resistances. Note that the "on" resistance can vary significantly.

I have also seen the solar panel used in place of a separate LDR. Whilst it is possible you might have some sort of tapping off the solar panel, I think the integrated LDR is far more likely.

Whatever the method, the function is to ensure the light is turned off until it becomes dark.

 

[Electro132]

Cool, I have a bunch of solar cells as well that i got from old walk way solar lamps but when i opened them up they only had 2 wires - black and red. However, when i connected them together to a capacitor which lights a LED it didn't work. But there was voltage going through it (about 1.5 v). So i'm guessing that green wire is for the solar cell.

 

[Rory Starkweather]

Cool, I have a bunch of solar cells as well that i got from old walk way solar lamps but when i opened them up they only had 2 wires - black and red. However, when i connected them together to a capacitor which lights a LED it didn't work. But there was voltage going through it (about 1.5 v). So i'm guessing that green wire is for the solar cell.

You lost me there. Is it possible that one of your cells is open?

Two wire black/red suggests to me that a voltage would be produced with red at a higher voltage than black. So you put in a cap to store the voltage produced and also connect an LED to it, with proper current limiting resistor. (The question arises, "How do I pick the resistor?" I guess it would have to be based on the max value the cap charges to.

If this voltage is large enough to overcome circuit resistance in the LED part of the circuit, and still have enough left to light the LED, I would expect it to light.

It is hard for me to see how the third lead would affect this.

A quick Google suggests that 1.5 volts is often the minimum voltage required to fire the LED. Some take as much as three volts.

If you have the Solar cells connected to the capacitor in parallel, you may very well not have enough voltage to fire it.

Considering circuit resistance I would suggest that you try a series-parallel connection of the solar cells so that the capacitor charges to > 4.5VDC and then try to fire the LED.

The thing I'm concerned about is whether the green lead keeps the LED from lighting, or some more basic issue. I don't have any two lead solar cells. I had decided to just use red and black and see what green does and if there is a use for it.

It may be as simple as power for the LED built into the light, shutting off charging and powering the LED when the solar cell is no longer providing a usable input voltage.

 

[Electro132]

Ok so the green wire is obviously not needed to get a solar cell to work unless you wanted it to charge up a lithium ion batt or something during the day then turn the LED on at night. However, if i try 2 solar cells connected in parallel with each other (have one POS (+) from one solar cell and connect to the GND of the other) and then connect the POS (+) of one of the solar cells to the capacitor that should give me a total of 3v going into the cap / li - ion. That should be enough to charge the LED won't it?

 

[Rory Starkweather]

@Electro132, 3 volts is at the low edge of what I would try. To be sure about triggering the LED even with lead resistance, I would add another solar in cell to get 4.5 volts, approximately. But you need to make sure that each solar cell is actually producing current.

Suppose that you test them as individual cells, with a DMM, and see what voltage and current they are producing.

This is kind of interesting. I chopped up one of my solar cells and connected the DMM between red and black. I got about 0.8 VDC at 0.6 mA. Please keep in mind that I am using a 13 watt economizer bulb for a light source. I don't think even three in series would light an LED with this light source.

By the way, I also checked current between green and black. 0.0. That reduces the possibilities.

 

[Electro132]

Ah i see, i actually chopped up an old solar calculator and found that it generated about 2 volts with both the solar cell and battery combined. The only thing different is that there were 3 wires - red, black and black. Red and black was connected to the battery whilst the other black one was connected to the GND, which is weird because i initially thought the black wire would be connected to the NEG (-) side of the rechargeable battery as well as anything that is negative throughout the circuit.

With the solar lamp, i tested my 3 solar walkway lamps individually and they all generated current. The only thing is i can't seem to get the circuit correct because they weren't powering up the capacitor (You know like building the current up rather than inputting a certain voltage) and so the LED wouldn't light up either. With your one i think the circuit is straight forward - black and red are connected to the pos (+) and neg (-) whilst the green wire is used for the detection of night time.

I also had this idea of placing each pos (+) of the 3 solar lamps directly into the capacitor pos (+) rather than connecting them NEG - POS - NEG - POS - NEG - POS to be connected to the capacitor.

 

[Rory Starkweather]

A capacitor may not be your best bet here.The capacitor may charge, but it will immediately try to discharge through the path of least resistance. It may not hold a charge long enough for you to measure, or do anything useful with. It's a problem.

On my front, I left the DMM hooked to red and black with alligator clips. I noticed today that, while ruffling through the junk on my desk, the black lead had become separated from the solar cell. I'm going to find a soldering iron and re-attach the black lead to the small blob of solder it used to be attached to. These things are definitely not designed for re-cycling/re-use.

 

[Electro132]

Ah yes, i would have to agree. Cheap solar lamps come with cheap components. By the way, how did you go with the experiment?

 

[Rory Starkweather]

TY for asking. It's a mess right now. Nothing does what I expect it to. ALL voltages are unexplained.

I'm going to have to take several steps back. I could have built a thyratron including vacuum, by hand, in the time I have spent on this.

I am currently waiting for the backplane for a Brooklyn style rebuild. I have trashed all the components I started with. I'm just tired of burning things up and getting 12 volts out of a 9 volt regulator.