Please help me understand this PNP transistor

Hi,

Here’s a great circuit from Evil Mad Scientist - I’ve built it and it works: it’s to create a garden solar lamp.

When there’s sunlight it charges the battery and the LED is off. When there’s no sunlight the LED is powered by the batteries. Brilliant. But I can simply not understand how the PNP transistor works.

Here’s my reasoning - stop me when I’m wrong.

When there’s sun V solar panel > V batteries so the current goes thru the batteries but also thru the emitter (top), back in the base, thru the 5K and back again for another round.

When V solar panel < V batteries, current leaves the batteries, and reaches the emitter. What’s different at this point then when the “solar” current reached the emitter? What’s the “trick” that makes at this point the current take the path to the collector and onto the LED?

Bonus question: is the 5K resistor the result of a calculation, or simply a “high enough resistor”?

In advance - thank you!

Looking for the million-th time to this diagram something new stroke me. When solar-powered (ie - left battery at 4.5V), current reaches 3 points: the battery, the emitter AND the base. That's of course the difference then when it's only battery powered. But... why is having current both on the emitter and base not moving any of it to the collector and LED?

confused :slight_smile:

The trick is to remember that when there is a difference between the emitter and base voltage greater than 0.7V the transistor turns on. When there is no light the solar cell is in effect not there and the base is at 0V and the emitter is at the battery voltage so it turns on the transistor and the LED lights.

Doesn't this depend on the solar cells having some leakage current to ground when dark, to provide a path for base current from emitter though base to ground??

A solar cell has leakage to ground since its a forward biased diode. In open-circuit conditions the entire photocurrent leaks through the forward biased diode leading to the maximum forward voltage... Its only when you load the cell that some (hopefully most) of the photocurrent is diverted into the surrounding circuit (lowering the forward voltage). With no light present consider the array of solar cells as a string of diodes in series.

Grumpy_Mike:
The trick is to remember that when there is a difference between the emitter and base voltage greater than 0.7V the transistor turns on.

Grumpy - thank you! Yes - the word DIFFERENCE is indeed the trick. Thanks so much for the answer!