Relay Driver

The C1, C2 and D1 capacitors make the snubber circuit that is used to limit high voltage transients during discharge. As soon as the transistor turns OFF the resistance across the collector and emitter becomes very high. This causes high collector-emitter voltages. the transistors will fail due to their collector-emitter voltages rising to above maximum ratings. Thus those spikes need to be filtered by capacitors or clipped by diodes. The diode is limiting the voltage because it only allows 0.7 V in forward biasing. The capacitors limit the voltage because they filter high spikes. If the relay inductance is very high then you should choose higher capacitor values to prevent resonance. This is just a general circuit. A small 100 nF capacitor connected in parallel with 100 uF capacitor because 100 uF capacitor has parasitic inductance that prevents it from filtering high-frequency spikes. However, 100 uF capacitor is used to filter low frequencies. You can either eliminate the D1 diode, to reduce the cost. You can also eliminate C1 and C2 capacitors. However, you will need either the D1 diode or C1 and C2 capacitors to limit the Vo voltage during relay inductive component discharge to prevent transistor failure.