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Gavilan

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I am just beginning to build simple circuits.

I am building a solar charged - battery operated - LED Array pathway light.

I plan on using a NPN power transistor to act as a switch to close the charging circuit to the battery.

When the cell begins to generate a voltage it will forward bias the gate and allow current to flow to the battery from the solar array. I have that taken care of.

The problem I face is I need a normally closed electronic device that uses little or no parasitic power to turn the LED array off when the solar cell begins to generate a voltage. I do not want to use a photonic device. I want to use a signal voltage from the solar cell to turn OFF the power to the LED array.

Again: When there is no signal voltage the device allows current to flow.
When there is a signal voltage the device quits conducting.

Is there such a device?

Any information you would be willing to share with me would be greatly appreciated.

Thanks

Gavilan
 
Hi Gavilan,
Just a couple of questions.
Usually you can let the solar cell charge anytime it's voltage is higher than the battery voltage so only a diode is needed to keep the battery from flowing backward into the solar cell.
I'm guessing you want to use the absence of solar power to turn on the light?
What is the battery voltage?
 
Thank You for your reply ronv. It is much appreciated.

I will be using a 20 watt 12 volt Solar Charger to charge a small motorcycle or lawn tractor battery. I will drop the voltage to around 3 volts using resistors with my LEDs in parrellel on the 3 volt bus.

Yes; I want to use the absence of solar power to turn the LED array on.

Gavilan
 
Solar switch

Maybe not the best, but straight forward.
 

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Your LEDs are only 3.0V? And your solar panel charges a battery to 12V?
Then they are completely different because you are throwing away 9V. A huge waste of power is wasted.
 
ronv;

Thank you for taking the time to provide the circuit diagram. As I am a beginner; please be patient with me.

I want to see if I understand what is going on.

I do not understand what the labels "panel" and "out" mean. Are they just bus labels?

What does Rser = 6 and 12.6 Rser=.05 mean?

D2 I believe is there to keep the battery (v1) from discharging back through the solar panel when the battery voltage exceeds the solar panel voltage.

The positive side of both the battery and the panel supplies (?) the EMF to the emitter of a PNP transistor. (component Q1=part numberBCW68F)

R2 is used to limit the current supplied to the base as the solar panel voltage increases. This causes a forward bias on the base???? and stopping the flow of current between the emitter and collector????

If this is the case then there is current flowing from battery to the emitter - across the bass - to the collector anytime there is a little or no emf being generated by the solar panel?

In this diagram it appears that as the current leaves the collector to the circuit it splits into two parrellel branches with one branch being a series of R2 and two light emitting diodes. The other series branch being R1 and one light emitting diode.

The drop across the the three component series branch is equal to the drop across the two component series branch.

The resistors of each branch are being used to drop the voltage so that the voltage across each of the leds is the same.

I need to go back down to my shop and see if I can get my radio shack PNP power resistor to conduct across the emitter and collector without a reverse bias voltage.

Do all PNP resistors behave the same way? Or is the 0 bias conducting state of the transistor specific to that transistor part number?

Your assistance in helping me to understand this is geatly appreciate.

Sincerely;

Gavilan
 
ronv said:
I do not understand what the labels "panel" and "out" mean. Are they just bus labels?

What does Rser = 6 and 12.6 Rser=.05 mean?
"panel" and "out" are probe labels for simulation.

He gave the "ideal voltage" sources a voltage (e.g 12.6 volts) and an internal series resistance.

Transistors are somewhat governed by: Ie = Ic + Ib where Ib is small. Ic is Hfe * Ib. Hfe is known as current gain and it's very variable even within the same part number. Some have multiple ranges and are tagged with suffixes to that effect.

NPN and PNP primarily define polarity. Look at the arrows and pretend they are diodes, for the point in the direction of conventional current flow (positive to negative)
 
Audioguru;

Thank you for taking the time to reply to my post.

I am using a 12 volt system because of the availability of components and the energy density of existing inexpensive lawn mower and motorcycle batteries.

I do not know the power consumption of the circuit components other than the LEDs which I have measured and now estimate will consume about 8 watts of power for each 100 feet of walkway that I illuminate.

During the winter months, if the LEDs are illuminated for 14 hours the total energy I will be using each night will be about 8 Watts x 14 hours or about 112 watt hours. A 20 watt collector should give me about 120 watt hours of energy over a period of 6 hours.

Without having researched the actual capacity of a lawn mower or motor cycle battery I believe they will capacitate more power than I need without deep cycle discharging. If not I will move up to a deep cycle marine battery or something like that.

It has always been my understanding that power was a function of current and voltage. The votage (EMF) is only the electro motive force that exists to accelerate the electrons through the circuit. The unit of power in the MKS system (meters-kilogram-seconds) I believe is the watt. I am not "wasting" 9 volts of power but am simply dropping the electro motive force through the use of resistors. I may be wasting some power through the thermal losses of the resistors but my supply voltage does not dictate power usage in and by itself. The supply voltage in combination with the circuit resistance (or impedence in an AC circuit) is what defines circuit power - I think.

I believe a 3 volt supply feeding a .500 amp load circuit (6 ohm) would use much more power than a 12 volt supply feeding a .1 amp load circuit (120 ohm). With the 3 volt circuit consuming 1.5 watts of power and the 12 volt circuit consuming 1.2 watts.

I beleive there might even be some thermal loss advantages with higher voltage circuits but without reviewing my text books I can't remember.

Gavilan
 
Put the LED's in series, not parallel. Make two series strings of them if needed.
Use the voltage drop of the LEDs to your advantage allowing for much less series resistance.
 
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ronv;

I do not understand what the labels "panel" and "out" mean. Are they just bus labels?

Yes, They are just there so you can see which waveform is which.
What does Rser = 6 and 12.6 Rser=.05 mean?
These are the series resistance I assumed for your panel and your battery.

D2 I believe is there to keep the battery (v1) from discharging back through the solar panel when the battery voltage exceeds the solar panel voltage.
Correct

The positive side of both the battery and the panel supplies (?) the EMF to the emitter of a PNP transistor. (component Q1=part numberBCW68F)
Yes

R2 is used to limit the current supplied to the base as the solar panel voltage increases. This causes a forward bias on the base???? and stopping the flow of current between the emitter and collector????
The base is slightly reversed biased when the panel voltage is higher than the battery voltage.
If this is the case then there is current flowing from battery to the emitter - across the bass - to the collector anytime there is a little or no emf being generated by the solar panel?
Back into the solar panel.
In this diagram it appears that as the current leaves the collector to the circuit it splits into two parrellel branches with one branch being a series of R2 and two light emitting diodes. The other series branch being R1 and one light emitting diode.

The drop across the the three component series branch is equal to the drop across the two component series branch.

The resistors of each branch are being used to drop the voltage so that the voltage across each of the leds is the same.
Actually the resistors limit the current thru the diodes. I used the example of 2 in series to show how you could wire your leds to reduce the wasted power in the resistors.

I need to go back down to my shop and see if I can get my radio shack PNP power resistor to conduct across the emitter and collector without a reverse bias voltage.
It conducts when the base emitter are forward biased. This happens when the panel voltage is less than the battery voltage

The thing to remember is that the transistor turns on when the panel voltage is lower than the battery voltage



Gavilan[/QUOTE]
 
What transistor do you have for this? The current is quite high. It will require a smaller base resistor to drive it.
 
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