App note from Infineon Technologies about properly choosing resistor value in series to Microelectronic Relay. Link here (PDF)
The International Rectifier Photovoltaic Relay (PVR) devices are current-controlled microelectronic relays with a specified current which must be supplied for turn-on. Therefore, a current limit resistor is necessary when operating from a voltage source. This application note gives the procedure for determining the proper resistor to program the microelectronic relays to operate from any control voltage.