Current Indicator For 723

by P Needham this is either Rs, Rs //Rs, or Rd/Rs. A difficulty arises if it is intended to provide an overcur- rent indication for the shutdown circuit with the aid of an exter- nal transistor fitted in parallel onto pins 2 and 3, since the external and internal transistor are highly unlikely to have pre- cisely the same characteristics. When the internal transistor has the low B-E voltage of the two, the indication will not work, while in the other case the external transistor takes away the base current for the internal transistor, so that the current limiter is rendered ineffective. In this design of a power supply, a current overload indi- cation was realized by fitting the external transistor with a high value base resistor, R7, to ensure that the current limiter in the 723 is not disabled. A further transistor, T3, has been added to keep the base current for T2 as low as possible. Since the base-emitter junction then has a diode characteristic, the associated voltage drop i...