step-down principle safety capacitance step-down principle of work is not complicated.
Is the use of capacitance in the ac signal frequency must be produced to limit the maximum working current capacitive reactance.
For example, under the condition of power frequency of 50 hz, a 1 uf capacitance of capacitive reactance is about 3180 ohms.
When the ac voltage of 220 v in the ends of the capacitor, is through the capacitance of the maximum current is about 70 ma.
Although through the capacitance current of 70 ma, but on the capacitor does not produce power consumption, because if the capacitor is an ideal capacitor, the imaginary part of current through the capacitor current, it did work for reactive power.
According to this characteristic, if we again on a 1 uf capacitor in series a resistance element, the impedance components at both ends of the voltage and the power produced by it all depends on the characteristics of the impedance components.
We will, for example, a 110 v / 8 w bulb with a 1 uf capacitor in series, in 220 v / 50 hz ac voltage, the light bulb lit up, sends out the brightness of the normal without being burned.
Because 110 v / 8 w bulb needed current for 8 w / 110 v = 72 ma, it produced with 1 uf capacitance current limiting characteristics are identical with each other.
In the same way, we can also use 5 w / 65 v bulbs and 1 uf capacitance in series with the received 220 v / 50 hz alternating current (ac), the light bulb will also lit up, but not destroyed.
Because 5 w / 65 v light bulb working current is 70 ma.
Therefore, safety capacitance step-down flow is actually using the capacitive reactance.
In fact have a limited current and capacitor and dynamically allocated capacitors and load voltage on both ends.
Using step-down should pay attention to the following: 1: according to the size of the current load and the work frequency of the alternating current (ac) to select the appropriate capacity, rather than on the basis of the load voltage and power.
2: current limiting capacitance must adopt non-polar capacitor, electrolytic capacitor will not be used absolutely.
And capacitance pressure must be above 400 v.
The most ideal capacitance for iron shell oil immersion.
Three: safety capacitance step-down cannot be used for high power condition, because not safe.
Four: safety capacitance step-down is not suitable for dynamic load conditions.
6: when need dc work, try to use half-wave rectifier.
Bridge rectifier is not recommended.
And want to meet the constant load conditions.
Safety refers to the main application circuit for capacitance safety capacitance on such occasions, the capacitor failure after, will not lead to electric shock, do not endanger the life safety.
Safety capacitor used the main occasions many, mainly include: resistance capacity buck circuit, EMI filter two kinds.
Resistance capacitance step-down circuit refers to the use of capacitors in a ac signal frequency of capacitive reactance to limit the maximum working current.
As shown in figure as follows: the resistance capacity step-down power supply EMI filter circuit diagram of the main role is of high frequency pulse filter outside the grid to the interference of power supply, but also reduce electromagnetic interference of switching power supply itself.
Generally two groups of EMI filter capacitance, the crossover between the power cord of differential mode inhibition between X capacitor and after in the power cord and Y capacitor in one of the common mode rejection.
For X capacitance its rated voltage and grid voltage, its capacity can choose a few bigger, typical value of 0.
05年µfµf ~ 1。
For Y capacitance values allow cases, the bigger the better, but if Y capacitor failure can lead to shock, so have limit on the maximum leakage current.
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