A driver comprising: a primary side circuit; a secondary side circuit for driving a load; an optocoupler between the primary side circuit and the secondary side circuit; and a transformer between the primary side circuit and the secondary side circuit for delivering power to the load, wherein: the primary side circuit comprises: a controller for providing a PWM load drive level signal, for setting a load drive level, to the secondary side circuit via the optocoupler; and a sensing circuit for measuring a signal which represents a power consumption of the load; the secondary side circuit comprises: a sensor for sensing a drive level of the load; and a comparator circuit for comparing the load drive level with the sensed drive level and providing an error signal for the controller; and the controller is configured to: adjust the frequency of the PWM load drive level signal; monitor the power consumption of the load in response to the frequency adjustment; and based on the monitoring adapt the duty cycle of the PWM load drive level signal thereby to compensate for errors introduced by the optocoupler, wherein the controller is configured to: adjust the frequency of the PWM load drive level signal between a first, normal operation, frequency and a second frequency; and adapt the duty cycle of the PWM load drive level signal such that the power consumption is the same at the first and second frequencies. a primary side circuit; a secondary side circuit for driving a load; an optocoupler between the primary side circuit and the secondary side circuit; and a transformer between the primary side circuit and the secondary side circuit for delivering power to the load, wherein: the primary side circuit comprises: a controller for providing a PWM load drive level signal, for setting a load drive level, to the secondary side circuit via the optocoupler; and a sensing circuit for measuring a signal which represents a power consumption of the load; the secondary side circuit comprises: a sensor for sensing a drive level of the load; and a comparator circuit for comparing the load drive level with the sensed drive level and providing an error signal for the controller; and the controller is configured to: adjust the frequency of the PWM load drive level signal; monitor the power consumption of the load in response to the frequency adjustment; and based on the monitoring adapt the duty cycle of the PWM load drive level signal thereby to compensate for errors introduced by the optocoupler, wherein the controller is configured to: adjust the frequency of the PWM load drive level signal between a first, normal operation, frequency and a second frequency; and adapt the duty cycle of the PWM load drive level signal such that the power consumption is the same at the first and second frequencies. the primary side circuit comprises: a controller for providing a PWM load drive level signal, for setting a load drive level, to the secondary side circuit via the optocoupler; and a sensing circuit for measuring a signal which represents a power consumption of the load; a controller for providing a PWM load drive level signal, for setting a load drive level, to the secondary side circuit via the optocoupler; and a sensing circuit for measuring a signal which represents a power consumption of the load; the secondary side circuit comprises: a sensor for sensing a drive level of the load; and a comparator circuit for comparing the load drive level with the sensed drive level and providing an error signal for the controller; and a sensor for sensing a drive level of the load; and a comparator circuit for comparing the load drive level with the sensed drive level and providing an error signal for the controller; and the controller is configured to: adjust the frequency of the PWM load drive level signal; monitor the power consumption of the load in response to the frequency adjustment; and based on the monitoring adapt the duty cycle of the PWM load drive level signal thereby to compensate for errors introduced by the optocoupler, adjust the frequency of the PWM load drive level signal; monitor the power consumption of the load in response to the frequency adjustment; and based on the monitoring adapt the duty cycle of the PWM load drive level signal thereby to compensate for errors introduced by the optocoupler, wherein the controller is configured to: adjust the frequency of the PWM load drive level signal between a first, normal operation, frequency and a second frequency; and adapt the duty cycle of the PWM load drive level signal such that the power consumption is the same at the first and second frequencies. adjust the frequency of the PWM load drive level signal between a first, normal operation, frequency and a second frequency; and adapt the duty cycle of the PWM load drive level signal such that the power consumption is the same at the first and second frequencies.