A motherboard assembly includes a motherboard and an expansion card. The motherboard includes an expansion slot with a first idle pin connected to a standby power through a resistor. The expansion card includes an edge connector having a second idle pin and first to fifth electronic switches. When the first electronic switch receives a high level signal through the first and second idle pins, the first and fourth electronic switches are turned on. The second, third, and fifth electronic switches are turned off. The second system power outputs a standby voltage through the standby voltage output terminal. When the first electronic switch receives a low level signal, the first and fourth electronic switches are turned off. The second, third, and fifth electronic switches are turned on. The standby power outputs a standby voltage through the standby voltage output terminal.