A Y, Y capacitance is a kind of capacity, safety capacitance is used for such occasions: namely capacitor
expires, will not cause electric shock will not endanger personal safety.
Is that safety capacitance is different from other ordinary capacitance has a place, the ordinary capacitance for a long time after power off will also retain some residual voltage, electric shock will occur once the hand touch, but never safety capacitance.
Y capacitor mostly blue, but there are yellow, because it is safety capacitor, so Y capacitors are generally marked above the relevant certification, such as CQC, VDE, UL certification.
Y capacitance usually meet in the diagram below the zero line and earth or between the wire and ground, the following figure is a class I EMI filter circuit, Y1 and Y2 are Y capacitance, usually two tandem together, according to the resistance to high pressure into Y1, Y2, Y4 three levels, including: Y1> 8 kv capacitive resistance to high pressure value;
5 kv < Y2 high-pressure capacitance value < < 8 kv;
5 kv < Y4 high-pressure capacitance value < less than 5 kv, leakage current for switching power supply, Y capacitors are usually pick up on the primary side (
With the secondary side,
Between, as the chart, Y capacitor can be coupled to a side interference current of secondary side provide return path so why can't Y capacitance is too big?
We know Y capacitor for EMC has a lot of help, because it is a common mode capacitor, so you need to ground, so there will be a leakage current, above or below the EMI filter circuit as an example, the Y capacitor leakage current (I) = 2 PI fCU including: f is the mains frequency is 50 hz, C to Y capacitor capacity (
4700 pf + 4700 pf = 9400 pf)
Here, U of terminal voltage, take 110 v.
Can be calculated leakage current is zero.
32 a, and found that the leakage current is proportional to the capacity of the relationship, in other words, the greater the capacity of the leakage current is also for 220 v / 50 hz ac power supply electric appliances, many countries stipulate leakage current shall not be greater than 1 ma, for handheld and mobile devices, and other electric products also have different requirements to leakage current, so if the power supply products need to meet Y capacitance, capacity is generally not very big, sometimes even remove Y capacitor, therefore, we often see Y capacitance are basically not greater than zero.
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