[sdiy] 78xx regulator gives negative output half the time

[JH.]

>The problem is that when you switch it on while the input voltage is in
>the negative half of the cycle, the 79xx is powered first, and through the
>load may pull the positive rail to a negative voltage (the -1.2V you see),
>from which the 78xx is not able to recover when it is powered on a moment
>later.

That makes sense!
I never thought about that, but yes, with half wave rectification, it's obvious once you know it.

JH.