An output short circuit protection circuit using two transistors VT1, VT2 and a relay K is composed. The normally open contact K1 of K is connected to the output terminal of XL4016 (the positive output terminal of the voltage regulator).

Figure 1: Output short circuit protection circuit

When the power is just turned on, the voltage across the capacitor C1 is zero, so VT1 can automatically conduct, the collector electrode connects to the relay K to work, and the contact K1 closes, and the voltage regulator has a voltage output. Then, due to the conduction of VT2, VT1 remains conducting.

When the positive and negative output terminals of the voltage regulator are shorted, the upper end of the resistor R4 is grounded, at this time, VT2 is cut off without bias voltage, causing VT1 to also cut off, K loses conduction, and K1 opens, thereby protecting the voltage regulator. When the output short circuit is released and the machine is restarted, the voltage regulator can resume normal operation.

The capacitor C1 in this circuit should use an electrolytic capacitor with small leakage current, and the specific capacity is determined according to the experimental requirements. Since there is only a 12V relay available, a voltage dropping resistor R1 is added here. The specific size of the resistor is related to the input voltage and the operating current of K. Here, a 330Ω / 3W metal film resistor is used.