Why we need Short Circuit Indicator?
Modern integrated voltage regulators are protected against short circuits but do not give an indication when a short circuit occurs. In the case of regulators with the fixed output (78xx), a short-circuit indicator is easily arranged by connecting an LED and series resistor across the regulator output. The LED will light only during normal operation, that is, input available and output not short-circuited.
If this sort of indicator is used with regulators with a variable output, such as the LN1317, the brightness of the LED will vary with the set output. To make the brightness constant, the current through the LED must be made independent of the output voltage.
The LED current must then not be limited by a series resistor but by a current source. Such a source can be made with only one extra component: T1 in the circuit diagram. The design makes use of the available reference voltage across R1. This voltage is generated by the LM317 and serves in the first instance, in conjunction with R1, R2 and P1 to regulate the output voltage.
Short Circuit Indicator Circuit Diagram:
In the present circuit, the reference voltage is used to keep the drop across R3 constant:
UR3 = 1.25 – Ueb = 1.25 – 0.65 = 0.6 V
The current through D3 is then 0.6/180 = 3.3 mA, which is more than ample for the low-current LED used here.
On the prototype, the LED current remains constant with output voltages from 3 V to 25 V. In other words, over that range of voltages, the LED lights with constant brightness. The base current of T1 is only 15 µA, so that the operation of the regulator is not affected by the branching off of the reference voltage.
Short Circuit Indicator Circuit Description:
In the present circuit, T1 dissipates only 100 mW when the collector-emitter voltage is 25 V. Since this transistor can dissipate up to 500 mW (with heat sink), overheating is highly unlikely. Also, its collector-emitter voltage of 25 V is well below the maximum permissible 45 V. These limits should be borne in mind, however, if the power supply provides higher output voltages.
Capacitor C3 increases the ripple suppression from 65 dB to 80 dB. Diode D2 protects IC1 and T1 against too high a charge current from C3 and too high a base-emitter reverse bias (max. 5 V) when the regulator is short-circuited.
Diode D1 protects the regulator against discharge currents from C4 and any electrolytic capacitors in the circuit being supplied. Without this diode, these capacitors would be discharged through the regulator should the input of this IC be short-circuited or be connected to a lower voltage.
The quiescent current of the indicator circuit is about 10 mA, while the peak current may rise to 1.5 A. At maximum input voltage (35 V) and minimum output voltage (3 V), the circuit can provide a constant current of about 150 mA.