Voltage regulator driving a pass transistor...

[Externet]

Hello.
A 3 pin regulator with a maximum input voltage of 35V and 1 Ampere capable like LM7815 can drive a power transistor to raise its current capability. Does the input maximum voltage for the regulator also raises as the pass transistor Vce specifications ?

Or that input voltage increase only works for an adjustable as LM317 ?

 

[AnalogKid]

In the upper schematic it is the voltage between pins 1 and 2 that must be kept below the max spec. With pin 2 at GND, you cannot increase the input voltage above the datasheet spec.

In the lower schematic, it is again the pin1-pin2 voltage that is the issue. But because there is a zener between pin 2 and GND, this allows the input voltage from pin 1 to GND to be greater than Vmax because pin 2 also is above GND.

ak

 

[crutschow]

Note that the bottom circuit has poorer regulation and temperature stability, since the Vbe voltage of the transistor is not is feedback loop.

 

[Externet]

This circuit using a LM317 does not raise its rated Vin beyond 40V by the presence/use of a higher voltage rated pass transistor Q1; right ?

 

[bertus]

Hello,

No, the bypasstransistor will not raise the input voltage.
Here are some pictures that use the bypasstransistor from a german magazine:


The resistor Rs determines when the bypasstransistor will kick-in.

With a pre-regulator you can raise the input voltage a bit:

The pre-ragulator will reduce the voltage accross the final regulator.

Bertus

 

[crutschow]

The pre-regulator could be just an NPN BJT or N-MOSFET configured as a follower with a Zener reference from the base/gate to ground.