Electronic – Communication between two AVR microcontrollers over SPI with different power supplies

OK, you have 20VAC from the transformer, which is then full-bridge rectified to give (1.414*20) - (0.7*2)=26.9V (peak volts - diode drops), and passed through D2 (now down to 26.2V) into a Zener diode D3, which is a 9.1V diode! D3 has to pull enough current for T1.1's winding resistance to drop 17 volts. (some of this droop is provided by R118 and L8 as well.)

Toasty!

Power supplies are (usually) voltage sources, approximating ideal voltage sources, so they have a very low impedance looking in. A nonideal voltage source can be approximated as a resistance in series with an ideal voltage source. If we connect two such sources in parallel, we get the following situation, which reduces to a simpler equivalent circuit:

^{simulate this circuit – Schematic created using CircuitLab}

So if \$V_1\$ and \$V_2\$ are not precisely the same, a nonzero difference voltage \$V_1-V_2\$ is applied across \$R_1 + R_2\$. Since \$R_1\$ and \$R_2\$ are very small (ideally zero), even a small \$V_1-V_2\$ can cause a large current to flow.

If \$V_1 \neq V_2\$ and \$R_1 = R_2 = 0\$, then there is a pure short circuit: the circuit cannot be solved for current flow due to division by zero in \$I = V/R\$.