On the one hand, the transformer provides suitable AC voltage, on the other hand, it plays the role of electrical isolation. Ac and DC * * * use a pole, which means at most that the AC potential at this point is the same as DC, and it has no influence. Of course, if the transformer is omitted, the cost will be reduced, but the circuit behind it is not isolated from the power grid, so the safety is very problematic. I suggest you don't do this, but think of the users. If the cost must be reduced, I suggest that you increase the secondary voltage of the transformer to three times that of the AC power supply, which can be realized by a diode and a capacitor, and because the power supply capacity itself is very small, this does not increase the cost of the transformer.
Oh, the input is AC, and there is no negative pole. The output is DC, with positive and negative electrodes. It can only be said that in your circuit, the negative pole of DC output and the AC terminal of input are shared, which has nothing to do with it.
You don't know electricity? Does a wire (except ground) have voltage? What does it mean to measure "negative voltage"? You have to measure for other lines! The so-called voltage is the potential difference between two points! If the male * * * line is taken from another DC output terminal, it gets a DC voltage; If you connect another AC input line, you will naturally get AC voltage!
It is okay to remove the transformer, but it is not responsible for the user. After all, there is no electrical isolation measures, which brings security risks to users. There will be no one when the forest is big. It is not uncommon for people to rob money in the street.
I really met a soldier, and I don't want to answer anymore. I see you are ridiculous, so I'll be wordy for the last time!
There is no reference point in the above picture, that is, there is no grounding point. The polarity of DC power supply is positive and negative, and its voltage is DC, which is about three times the peak value of AC voltage of transformer secondary pole. If the secondary voltage is 220V, the output DC voltage is 933V (this is the open circuit voltage, which will be lower than this value after receiving the load).
In the picture below, there is a reference point, which is commonly known as the grounding point. Since there is a grounding point, the name can't be called negative output, just output! Triple voltage power supply, the positive pole is grounded, and your DC power supply is negative pole, which is generally called negative pole power supply. The output voltage is DC to ground, otherwise it is not DC power supply! If there is no transformer or the secondary AC voltage of the transformer is 220V, the output voltage to ground is negative 933V (this is the open circuit voltage, which will be lower than this value after receiving the load). I really don't know where your fallacy of "according to the truth, the DC voltage between DC negative electrode and ground is zero V" comes from.
All right, stop here and give it to me in good conscience. You can find any top expert to judge my answer!