要注意有些二种电源电压的地气(线)。一种是专门高压开关电源地气，负极接地，另一种信号电源低压地线，也是负极接地。二种地气线不能混用。这个负电压是那一点测量的，都有很大关係，电路不正常情况下会出现这种情况，电路正常后可能会得到正常。供你参考。

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　　那说明你所测量的电压就是79系列电源芯片输出端的电压，78系列是正，79系列是输出负，例如7805输出+5V%207912输出-12V。