what is the purpose of these components?

[oldguyjohn]

Hi all,
I'm studying other peoples circuits in an attempt to understand more and have a problem with this power supply circuit.  I do not understand the purpose of D10 or R16.  The diode I assume  may be some protection for the transistor, but I cannot get my head around the benefit of R16.  The resistor is specified as a 1K.  Breadboard experiments seem to show that the circuit works pretty much the same either with or without these components.  In fact the output voltage, with respect to varying current, appears more stable without R16, which appears to supply a small proportion of the output by bypassing the main transistor?  Very confusing, but I assume there must be a reason for the inclusion of these components.  Can anyone please explain the designers intention here.

[strawberry]

R10 prevent it from oscilating and D10 blocks negative voltage

[CJay]

Damnit, beat me to it. 

Emitter stabilisation resistor, it helps reduce overshoot and oscillation.

Try simulating it with a rapidly changing load.

[David Hess]

D10 prevents potentially damaging base-emitter breakdown of Q2 and Q4 if the output voltage was somehow greater than the input voltage like if a battery was connected to the output while AC power was removed.  D10 also allows the output to be pulled down and C7 to be discharged via R15 and Q1 when AC power is removed.

R16 counteracts collector-base leakage and removes storage charge from Q4.  This has the effect of allowing Q4 to turn off more quickly and prevents amplification of Q4's leakage current which might be a problem at high temperatures.  Q2 and Q4 are configured as a Darlington pair and Darlington transistors always include base-emitter shunt resistors.  U2 can pull Q2's base to ground so no base-emitter shunt resistor is required although I might have included one anyway.

Be warned that this power supply design is known to have overshoot problems on startup.

[oldguyjohn]

Thanks for the detailed reply David, an explanation of how things work always helps when your just starting out.  This will give me a good starting point for more research.

All replies are appreciated as a beginner, but detailed answers in SIMPLE language help most.

Thanks again for all replies.