Joule Thief

[Dave45]

In the pnp setup I dont understand why it even goes through the led, it should run straight to ground.
Unless the coil is actually pulling, sucking.
And if it does pull,,,,well thats interesting.

[MarkE]

In the pnp setup I dont understand why it even goes through the led, it should run straight to ground.
Unless the coil is actually pulling, sucking.
And if it does pull,,,,well thats interesting.

Dave, referring to the drawings:  When the PNP transistor conducts, current builds up in the transformer secondary, lifting the coil voltage above the negative rail.  As the transistor turns off the inductor voltage changes in such a way as to try and maintain the current.  The inductor drives the voltage at the transistor collector down until the LED forward biases enough to carry the current.  The voltage is therefore below the bottom rail by the LED forward drop.  This is just the complement of what happens with the NPN.  There the collector voltage flies back above the top rail by the LED forward voltage.

[tinman]

In the pnp setup I dont understand why it even goes through the led, it should run straight to ground.
Unless the coil is actually pulling, sucking.
And if it does pull,,,,well thats interesting.

All you have to remember is-the current in the inductor always flows in the same direction,but the voltage reverses polarity when the transistor becomes open circuit(switches off)

[Dave45]

I sure hate to conform

[Dave45]

I guess this would be considered a shorted coil, the magnetic field would never collapse.