Control transformer for do it yourself repairs of funace and air conditioner equipment. If the transformer has failed it is often caused by a shorted wire. If the transformer is replaced without taking care of the issue the new transformer will fail as soon as power is applied.
Common issues we see are rodent damage inside walls and crawl spaces, knicked wire insulation where wire passes through furnace or air conditioner cabinets and electrical enclosures or trimming too close to exposed wiring around air conditioner or heat pump with a weed whacker.
Transformer failures are usually related to issues with the low voltage (24 V) side of the equipment.