How can i tell if a transformer is a dc or ac?

gmoon8 years ago
By themselves, all transformers are AC devices by their very nature (AC in -> AC out.)

If it's a wallwart or similar power source that includes other circuitry, such as a rectifier, filter capacitors, voltage regulator, etc., then it's probably DC output.

The best way to tell for sure is to use a VOM (volt-ohm meter), and test it for both DC and AC voltages.