Periodic transformer failure

I have a 120v to 24vac transformer for a lawn sprinkler system that has failed about 3 times since I purchased the house. This time I took the windings apart and found the break in the 120v winding. I live in a lightening prone area. Is this failure most likely from power surges or is there some other possible cause.
Thanks.
Mike
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I do not think the lightning is the cause, I think it was caused by the overheating of the transformer itself, this happened because the transformer is rate too low for your load. Check the voltage before applying the load, and then apply the load to see its voltage drop. If the drop 40% or more then, you'll need to buy a bigger transformer, do not buy the exact model.
Cam
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voltage drop on the load side or the line side? The control box is outside and in sun in the morning when the sprinklers are on, could the sun cause the overheating. Mike
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The Sun could add more heat to it while in operation. You would check the secondary (output side 24Vac)
It should not drop so much, (10% is typical 21volts) 40% = 14.4 Volts or lower is terrible. I've seen these transformers before at Home Depot, they look small to me for continuous operation. I have it too for door bell, door bell is low load, that's why I don't have problem with it. Yours must have a big and continuous load, that's why you're having a problem statically (because in a coil, high voltage is self-generated through energy delay release, this may be why it jumps between the wire after overheating.
Cam
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I narrowed the problem to one zone using your suggestion. The voltage dropped on one zone from around 24v to 15. All of the others had only a 1 volt drop. Next I need to narrow it down to the zone wiring or the box. I am going to switch two zones and see what happens. If that doesn't work I will switch the solenoid. I just hope it isn't the wire to the solenoid.
Mike
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Sorry my news server was down. That's a good idea by narrowing down to a particular zone. If one Zone took that much current then, chances are you got a heavy load someone, plus voltage drop to 15V is not a healthy sign.
-Cam
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