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What are the reference voltage points of ATX power supplies?

Last Update Time: 2019-04-15 14:08:24

When reading information about ATX power supplies, often encounter the words "hot ground" and "cold ground". What is the connection and difference between "hot ground", "cold ground" and "land"? In fact, the terms "hot ground" and "cold ground" are more popular and convenient, but they are not scientific.

 

The ground in "hot ground" and "cold ground" is actually the concept of the reference point rather than the real earth. This directly gives us an important hint: is there a need to select two reference points in the ATX power supply? It is. In ATX power supply, it is generally considered that the upper level of transformer belongs to high voltage (220V AC), that is, hot zone, and the lower stage of transformer belongs to low voltage zone, that is, cold zone. Thus, the "hot ground" is the reference point of the high voltage zone in the ATX power supply, and the "cold ground" is the reference point of the low voltage zone in the ATX power supply.

 

Remove the enclosure of any "ATX power supply" and look at the ground wire in the socket, and you will see that the ground wire is interconnected with the shell of the ATX power supply through screws. A progressive look at the four screw holes on the ATX power board reveals that the wiring gathered at the screw hole is also interlinked with the housing through the fixed screws. In summary, for the ATX power supply, the four screw holes and the outer finish are on the ground. More step by step, we can also determine that the ground wire in the ATX power output terminal is the ground by measuring the connectivity between the ground wire and the housing in the ATX power output terminal, and we can conclude that the ground wire in the ATX power output terminal is also the ground.

 

To sum up, each DC output in the ATX power output terminal and the circuit in the low voltage zone on the circuit board are ground-based reference points.

The following question: since there are two reference points for voltage measurement in the ATX power supply, according to the author, one of them is the earth of the low-voltage zone (the shell of the ATX power supply, four screw holes), and the other is naturally in the high-voltage zone. Where is the reference point in the high voltage zone?

In the ATX power supply with a "two-tube and half-bridge" topology, the 310V DC voltage rectified by the full-bridge will be added to the ends of two large-volume electrolytic capacitors in series (which I call the "main capacitance"). As shown in figure 1.

 

Point A in the figure (that is, the negative pole in the full-bridge rectifier) is the reference point in the high-voltage area of the ATX power supply. This reference point is defined as "full bridge negative" or "main capacitance negative".

 

In the running line and in the actual maintenance process, the ATX power board has usually been removed from the housing, for convenience, you can use the wire to draw the two reference points to the convenient measurement. Figure 2 shows a special tool made by the author for the convenience of maintenance measurement.

        

Figure 1. Main capacitance                           Figure 2 a dedicated tool

 When it is used, weld the two cables to the corresponding location of the power source. A protruding copper sheet for the clamp (ground) of the oscilloscope probe

This article is from Allicdata Electronics Limited. Reprinted need to indicate the source.