Series pass voltage regulator circuit anyone?

You already asked this, and the same answer applies, need to see the circuit.

Probably the 107 is used to amplify the error signal and to drive the 50, which is probably the series pass transistor. But this is all anyone can tell you, is guesses.

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Series Pass Voltage Regulator

The BC107 is to provide enough current to drive the base of the larger BFY50 transistor. The problem with regular BJT trasnsistors is that they have a very low input resistance. The output current depends exponentially on the base emmiter voltage (a bit like the current through a diode depends exponentially on the voltage across it), so the base emmiter voltage only needs to be tiny to cause a large current in the collector (the output). But the base current depends directly (by a factor of beta or Hfe) on the colelctor current. Many large power transistors have a beta value that is quite small, so when the collector is taking the full rated current, the base current may still be qutie large! This is a problem for the circuit that needs to drive the transistor. If it is say, a small integrated circuit, it wont be able to provide enough current with out exploding to drive the large power transistor. So we need to use a smaller transistor, with a large current gain (the beta) to take the signal from the drive circuit, boost up the current to a level that can drive the large transistor. The is probably why this circuit has a small transistor driving the larger one.

It is likely a typical series regulator circuit. It operates on negative feedback principal.

The BFY50 is the control element. The transistor absorbs the voltage fluctuation in the input power source and its output is a stable voltage to the load.

The BC107 is the error amplifier. The resistor divider samples the output voltage at the load. The Zener diode provides a stable reference voltage which is used to compare with the sampled load voltage. Deviation of load volatge from the preset value becomes the error signal. The error signal is amplified by BC107 and is used to drive the BFY50 the output of which is so controlled as to offset the error signal. In other words, if the load volatge increases above the preset level, the error signal will cause BFY50 to decrease the output voltage to the load.

In an analogue voltage regulator, the series pass element (BFY50) dissipates quite a large amount of power. The potential difference between the input and output and the load current is dissipated as heat in the transistor.

BC107 increased the current sensitivity of BFY50,or in other words,it increased the beta. Therefore,BFY50 obtained enough current drive at the base to provide max. current able to pass through it.