Hi Gruber, The current-setting pot in this project has a tolerance of 20% and the values of the resistors were chosen so it provides 3A if the pot's resistance is 20% high. If your pot's resistance is 20% low then with it set to max, the project will try to produce 4.2A. Such a high current will overload the transformer and cause its core to saturate. A transformer with a saturated core isn't a transformer anymore, it is a piece of wire across the mains, blowing fuses.
You should add a trimpot in series with the current-setting pot. Adjust the trimpot so the project supplies a max of exactly 3A when the current-setting pot is at max.
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