a semiconductor diode is connected in series with a 16 ohm heater element to a 240 volt a.c supply.Can the supply voltage be used to work out heater current?

 

Since its AC the diode transient response also needs to be taken into account. The 240 V AC is RMS value of voltage. So whatever current you calculate is also RMS value of the AC current

 

If the heater element is purely resistive the current will correspond to a half-wave rectified circuit configuration.

The diode will drop about 0.7V in forward conduction.

The peak load voltage will then be 240√2-0.7=339.4-0.7=338.7V

The peak load current will then be Im= 338.7/16=21.17A

The RMS load current will be Im/2=10.585A

The average load current will be Im/∏=6.74A