Trying to understand pass transistors

[dcbrown73]

I've been looking to understand the pass transistor.  The video above is disrupting my understanding transistors.   In this circuit which the video reproduces the circuit below, it's putting out a voltage of 12.5v, but allowing current up to 4amps with a LM317 voltage regulator (max of 1.5amps)  (well, unless he is feeding it a regulated 12.5, but that wouldn't fly in the face if the intent here wouldn't it)



What I'm not understanding is how the LM317 is managing to keep the voltage at 12.5v after you turn on the transistor.  When the transistor turns on, wouldn't that allow not just more current to flow, but also whatever the voltage is behind it through too?   

Or is the LM317 some how throttling the MJE2955 so that it only allows current while the LM317 provides all the voltage?

[magic]

Transistor isn't a binary on/off device. Its conduction is limited by base current (the famous "beta" current gain). For example, if LM317 draws only 1mA of current from its base and beta is less than 100, no more than 100mA will flow through the transistor no matter what voltage difference exists between its emitter and collector. The transistor is not a short.
And LM317 will only pass enough current to pull the output to 12V and no higher.

[ledtester]

What I'm not understanding is how the LM317 is managing to keep the voltage at 12.5v after you turn on the transistor.  When the transistor turns on, wouldn't that allow not just more current to flow, but also whatever the voltage is behind it through too?   

Or is the LM317 some how throttling the MJE2955 so that it only allows current while the LM317 provides all the voltage?

THe LM317 is able to control the transistor by controlling the voltage at the transistor's base.

It controls the voltage at the base by drawing current through the 2R resistor.

The more current it draws through the resistor, the lower the base voltage is relative to the emitter. A larger emitter-base difference results in more current flowing through the emitter-base junction and hence through the emitter-collector junction of the transistor.

It also works the other way... the less current drawn through the resistor the closer the base voltage is to the emitter voltage and less current flows through the emitter-base junction resulting in less current flowing through the transistor.

[David Hess]

They have basically covered it but I will describe it in my own way for thoroughness.  I am very familiar with this circuit and variations of it.

The transistor in this application operates as a current amplifier once the voltage across its base-emitter junction reaches about 0.6 volts.  (1) The current gain is not well controlled in this circuit configuration and will vary between about 10 and 40 but this is not important for proper operation.

As current through the LM317 increases to raise the output voltage, the voltage drop across the 2 ohm emitter-base shunt resistor rises until it reaches the Vbe voltage of about 0.6 volts.  After that, further current is drawn through the base-emitter junction and amplified to appear at the collector as a current.  This adds to the output current of the LM317 increasing the maximum current that it is controlled.

A better variation of this circuit from the National Semiconductor Voltage Regulator Handbook (1980) is shown below.  Its chief advantage is that the current gain is well controlled so the over-current and thermal protection of the regulator can be extended to the transistor.

(1) This is not what is really going on but it is close enough in this application.  Bipolar transistors are transconductance devices which means that their collector current is a function of their base-emitter voltage so voltage in and current out.

[Zero999]

To really understand how the pass transistor works, one needs to look at the internal schematic of the LM317, with the pass transistor added. It forms a  Sziklai pair, with the Darlington pair inside the LM317. The base emitter resistor ensures that it only turns on, when the current exceeds a certain level. At low currents, the external transistor doesn't conduct. At higher currents the pass transistor diverts current around the LM317.
https://en.wikipedia.org/wiki/Sziklai_pair

https://www.eevblog.com/forum/beginners/variable-regulator-pass-transistor/msg1059272/#msg1059272