Sometimes having your extraction/inlet fans running constantly is not the best way to regulate your airflow or temperature. A fan speed controller will allow you to set a temperature that you wish to maintain within your grow space for when the lights are both on and off.
Using a fan speed controller can decrease your power consumption and reduce long term noises or humming from the fans. When the temperature drops when the lights go out your fan does not have to be working at full capacity and with a fan speed controller you can set a minimum speed for your fan to run at so it can still exchange the air and still regulate any smells that may have to be cleared.
We have both AC (your traditional extractor fans) and EC (the latest technology) fan controllers. The G.A.S. digital range covers all bases in that aspect and if you like the analogue style then SMS will cover an AC running fan.