....... already been through a couple transformers and a new air handling control board. What is strange is that the 120V side is what is burning out, not the 24V side on the transformer. Both control boards (original and new) resulted in same burned transformer. So, something is causing the 120V side of the transformer to get hot and burn up. I can ohm out the 24V side and it shows 12 ohms... the 120V side is open circuit. Can also smell transformer burn smell.... same on all three transformers (original and 2 purchased transformers).........
For quite a few years now, 120 VAC power transformers have built-in fusible links on the primary side. The fusible links are actually short lengths of smaller diameter wire that are tack-welded onto both ends of the primary winding. The other end of the links attach to the pins of the bobbin.
In an extreme overcurrent condition, whether caused by excessive load on the secondary, or a short in the primary winding, the smaller diameter wire will melt open. A non-repairable event.
My lawn sprinkler timer had a wall-wart power transformer to supply 24 VAC to the controller. The controller was SUPPOSED to be short-circuit protected from any short in sprinkler circuit sector wiring or valve solenoid. They did this by using a Triac in each sector as the on-off device, and a small resistance in series to develop potential over to monitor sector current, and shut down the Triac if sector current was too high.
However, when a valve solenoid suffered a winding short, it blew the Triac, and blew the fusible links in the wall wart. I had some spare sector circuit positions in the controller, so after I replaced the valve solenoid, I moved that sprinkler sector over to an unused one. I replaced the wall wart with a much larger multi-amp heavy-duty one I ordered from James Electronics. And for the real touch, I drilled a hole in the controller's front panel, and put in a 2 Amp push-to-reset circuit breaker I had into the secondary circuit.
The circuit breaker I had scavenged out of the previous controller... which was an electro-mechanical monster that put on a show when a sector ran. It had a big plastic wheel (think Ferris Wheel) that had little time knobs around the edge for each sector. That would connect and time the sectors, and there was a motor to run it. There were other wheels that were the day of week and time of day dials, and they had motors. And there was a bright yellow light that was on when a sector was running. There was a test mode, where you could check your programming, it would turn the big wheel and run each sector for maybe 15 seconds or so, then move on to the next one, a sped-up clock effect. The test mode was fun to watch! What a contraption!
But eventually, a motor was getting iffy, it would stall sometimes, so I replaced it with a smaller all-electronic sprinkler controller with LC Display and one knob, three push buttons, and a slide switch. No more show. But the old monster's 2 Amp circuit breaker lives on!
