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# Highly Reliable Voltage Supply

This is an example of a transistor regulated DC power supply circuit which features high reliability through redundancy. The circuit may be considered fail-safe because if any single component fails, or if one of many multiple component failures occur, the regulated output will remain the same with none of the other elements being overstressed electrically as a result of the failure.

Zener diodes, *D*_{3} and *D*_{4}, provide the desired
regulated voltage at the emitters of transistors, *Q*_{1} and
*Q*_{2}. Diodes, *D*_{1} and *D*_{2},
provide isolation of the two sides so that the higher of the two voltages
present at the emitters will appear at the output. Thus, if one side shorts or
opens, the output is unaffected by the failure. Zener diodes *D*_{5}
and *D*_{6} are included as additional protection should
*D*_{3} or *D*_{4} fail. Resistors, *R*_{1}
and *R*_{2}, provide the required drive to *Q*_{1}
and *Q*_{2}. Resistors, *R*_{3} and *R*_{4},
are used to reduce the dissipation at the collectors of *Q*_{1}
and *Q*_{2}.

The value of *R*_{1} and *R*_{2} can be calculated by
considering the drive required by *Q*_{1} and *Q*_{2}
to supply the total current required by the load. Each side should be
capable of supplying this current, and the desired amount of bias current for
*D*_{3} and *D*_{4}.

The value of *R*_{3} and *R*_{4} can be calculated
by considering the total current supplied by each transistor, the maximum
allowable dissipation at the collectors, and the desired
amount of bias current for *D*_{5} and *D*_{6}.

Typical circuit elements and component values for 12V regulator are included in the figure.