There are two ways we can calculate the maximum input voltage:

1. Power equals:

P = (I^2)*R

I = sqrt(P/R)

= sqrt(10 W/10 ohms)

= 1 Amp

Therefore:

Maximum Voltage = Vr + Vbulb

= Ir*R + 1.5 V

= (1 Amp * 10 ohms) + 1.5 V

= 11.5 V

2. The maximum allowed current for the diodes is 1 Amp. Thus the maximum current across the 10 ohm resistor will be 1 Amp for one of the diodes (they do not conduct simultaneously) plus 0.3 Amp for the light bulb, which equals 1.3 Amp. Therefore:

Maximum Voltage = Vr + Vbulb

= Ir*R + 1.5 V

= (1.3 Amp * 10 ohms) + 1.5 V

= 13 V + 1.5 V

= 14.5 V

We choose the smallest of the two voltages which will be 11.5 V.