Thursday, September 25, 2014
IC 723 Power Supply Circuit with Current Indicator
- This can be . increased readily by fitting a resistor in parallel with Ra, until the output voltage is5.0 V precisely Switches S1 and Sz are preferably SPDT types with a centre position, but three-way rotary switches should also do if in both cases the centre contact is not used.
- A difficulty arises if it is intended to provide an overcurrent indication for the shutdown circuit with the aid of an external transistor fitted in parallel onto pins 2 and 3, since the external and internal transistor are highly unlikely to have precisely the same characteristics.
- A further transistor, T3, has been added to keep the base current for T2 as low as possible.
- The output voltages of the supply may not be as accurate as required, and this is mainly due to the use of resistors from the E12 series. Close tolerance is especially important in the 5 V range, since the value shown for R3 gives a theoretical output of 4.9 V
- The three output voltages from this supply are probably the , most commonly used for testing asymmetrically fed designs: 5 volt for many TTL and CMOS circuits, 9 volt for battery operated equipment or logic circuits equipped with a 7805 regulator (this requires, an input of at least 8.5 V), and 12 volt for RS232 drivers, and miscellaneous opamp or tran- sistor based circuits.
- Since the base-emitter junction then has a diode characteristic, the associated voltage drop is always lower than that of the transistor internal to the 723.
- The current limiter can be set to l0 mA, 100 mA, or l A for safely power- ing- experimental circuits. Power regulator T1 should be fitted with a heat sink sized at least l0 ><‘10 cm. leds dv (green) and ds (red) are the power on and current overload indicator, respectively.
- 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 indication was realized by fitting the external transistor with a high value base resistor, Rv, to ensure that the current limiter in the 723 is not disabled. IC 723 Power Supply Circuit with Current Indicator
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