Hi,
The explanation to the schematics has a small error - it says:
"Resistor R7 is 100 ohms and develops 4 Volts with maximun load."
It is impossible to get that voltage with all six power transistors
running total of 30 A. One transistor delivers ~5 A, and that gives voltage drop
of ~0.5 V across one of the serial resistors 0.1 Ohm, R1 through R6. With
additional voltage drop on base-emiter PN junction, lets say 0.9 V, total
is 1.4 V, and never can go to 4 V!
If the Rload voltage would go above 12 V (for example, a battery is fully
charged (a battery placed where Rload is shown)), the LM7812 would stop
delivering any current on the OUT, and that would stop current on the IN
side (except quiescent current ~8 mA), and consequently it would stop
any significant Base currents on 6 x TIP2955, limiting (or shutting) current
on Collectors, and finally at the Rload. Some voltage will show up
on an open output (high impedance Rload), as the quiescen current gives some
very tiny current through the Bases of transistors.
So, that circuit behaves as a self regulating high current voltage source at
about 12 V, with a current limitting function. The maximum Rload current is
defined through the maximum OUT current of the LM7812, that is about 1.5 A, giving
about 1.5 A on the IN side, shared by the Bases of six transistors.
1.5 A / 6 =~ 0.25 A per Base. With "beta" =~ 35, it could give maximum of
about 0.25 A * 35 = 8.75 A per transistor. The design specifies about 5 A
per transitor and total 30 A, however, my calculation shows higher total
current, at about 6 x 8.75 A = 52.5 A. If the "beta" is higher, the total
current will go up as well.
Resistor R7 = 100 Ohm plays no role with this example currents.
To get ~4 V across R7, the voltage drop across 0.1 Ohm would be 4 V - 1.0 V = 3.0 V,
meaning 30 A per a single transistor!, and not the total current.
Something is wrong here: the web site explanation, or my understanding of
that schematic.
Thanks,
E-x