Normally will have a trap at the air handler (blower and indoor coil unit). Just after the condensate drain connection on the unit. Most installations here have the condensate drain routed to outside the house and dumps on the ground.
If you connect it to a sink drain line, you need to connect above the trap under the sink. Above (upstream) of a trap that is always full with water. The trap at the air handler tends to dry out during heating season.