Actually I have mine set up so the fan blows continuously when the enlarger is powered up, only the lamp is on the timer switch. That reduces the load on the timer circuit and cools the bulbs (there are two 200 watt bulbs) a bit better by keeping the air cooling after the timer cycle is done. Same reasons as you would for a slide projector. You should also have a power box that is a separate unit, made to ride on the enlarger chassis. I think it is just a transformer and not a regulator but I'm not expert on this point.