How is it redundant? Here is the scenario... Battery gets charged all day. Night comes and you decide to flip the switch and light up the cockpit. Well, after a few drinks and some great conversation you get tired and head into the cabin to sleep... completely forgetting to flip the light switch off. Thankfully, when the sun rises the next morning the PNP transitor has cuttoff the LEDs and the solar panel continues to charge the battery.The base resistor must go to ground (negative)
And you were told that the transistor is redundant.
If the PNP base is receiving the proper amount of power from the solar panel, how will the collector make a connection to the emitter?The way you have it configured the LEDs will light whether the battery is charging or not.
The PNP will be on because it is connected to the battery.
Ahhh... I see your point. There’s no negative connection with the switch set to the off position. Perhaps a light sensor would make more sense...Ok, so maybe I'm misunderstanding something here, so where does the PNP get its negative from to turn on?
Wouldn’t there be a negative bias on the opposite side of the LED’s if the switch was removed?Actually, I was referring to the fact that the transistor has no negative bias to turn it on.
When it’s sunny, the output of the solar cell is high at the transistors base, which opens the transistor and switches off the LEDs.Yes, but a PNP needs a negative bias at the base to conduct from emitter to collector.
When it’s sunny, the output of the solar cell is high at the transistors base, which opens the transistor and switches off the LEDs.
When it gets dark, the solar cells voltage drops to zero, the current flows out the transistors base and through the solar cell to ground, this closes the transistor letting the current flow through the LEDs.
On a real sunny day I’ve seen the voltage go up to 6V. Manufacturer states that it is a 5V panel.Your total LED current is 150mA. When a transistor is used as a switch, the rule of thumb is to assume a current gain of 10, to ensure the transistor switches on hard. Therefore the required base current will be 15mA, hence the base resistor will be (3.7-.7)V/15mA = 200Ω. Use 220Ω (as ES suggested in post #22).
What is the maximum voltage output of the solar cell? There is a risk that it could cause reverse breakdown of the transistor base junction unless the voltage is clamped (with, say, a zener diode from base to ground).
Alec_t, thanks for all the great info. Just out of curiosity, at what voltage would you begin to consider using a zener diode?If 6V is the real maximum then a zener is unnecessary.
You can have zener's work down to close to 1V, but you need to also be aware that zeners tend to require at least 10mA in order to function. Also, are you preventing too much current from flowing into your LEDs? a 1-ohm resistor won't protect them.Alec_t, thanks for all the great info. Just out of curiosity, at what voltage would you begin to consider using a zener diode?
You don't want the base-emitter junction to breakdown. For the transistor you're using, and just about every one I've ever checked, that's 5V.at what voltage would you begin to consider using a zener diode?
by Aaron Carman
by Aaron Carman
by Aaron Carman
by Aaron Carman