Principle and calculation formula of capacitance step-down power analysis in commonly used low voltage power supply, with the capacitor
The actual capacitance current limit)
Compared with the transformer, capacitor step-down power small volume, economy, reliability, high efficiency, the disadvantage is that as the power of the transformer transformer security.
By capacitor introduce ac load, 220 v voltage to ground, people tend to get an electric shock, but if use does not need the body touch the internal circuit of power supply, this weakness can be overcome.
Such as refrigerator electronic temperature controller or remote control the power on/off power supply are all made of capacitor step-down and.
Buck, relative to the resistance for low frequency 50 hz alternating current, small heat loss in the capacitor, so the capacitor step-down more than resistance step-down.
The working principle of capacitance step-down is not complicated.
He works by using capacitors in a ac signal frequency of capacitive reactance to limit the maximum working current.
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 70 ma current through the capacitor, the capacitor does not produce on 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, the 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.
Basic circuit of capacitance step-down type simple power supply is shown in figure 1, C1 as step-down condenser, half-wave rectifier diode D2, D1 in negative half cycle of mains for C1 discharge circuit, D3 is zener diode, R1 to shut off the power supply after the charge discharge resistance C1.
In practical application is often used when the circuit shown in figure 2.
When need to provide to load a larger current, can adopt bridge rectifier circuit is shown in figure 3.
This kind of circuit is usually used in small current power supply of low cost to obtain the isolation.
The output voltage is usually available in a few volts to three a few volts, depending on the use of zener voltage regulator tube.
Can provide the size of the current is proportional to current limiting capacitance.
Using half-wave rectifier, every method of micro capacitance current can be obtained,
International standard units)
44 * 220 * 2 * 3.
000001 = 0.
3 a = 30 ma if the full-wave rectifier can get double the current (
As: I (
89 * 220 * 2 * 3.
000001 = 0.
6 a = 60 ma generally, although such a full-wave rectifier circuit current is larger, but because the float, stability and safety to less than half wave rectification type, so use less.
Using this circuit, need to pay attention to the following: 1, or 220 v ac high voltage isolation, please pay attention to safety, beware of electric shock!
2, current limiting capacitance must connect to the wire, pressure has to be big enough,
More than 400 v)
And string of anti surge shock resistance and discharge resistance and insurance.
3, circuit design, should be used to determine accurate value of load current, and then the reference sample to choose step-down condenser capacity.
Because by step-down capacitor C1 for the load current Io, is actually through the C1 Ic charge and discharge current.
The smaller the capacity of C1, the greater the capacitive reactance Xc, the charge and discharge current is flowing through C1 is larger.
When the load current Io less than the charge and discharge current of C1, excess current will flow through the pressure tube, if stabilivolt Idmax is less than the maximum permissible current Ic -
Easy create stabilivolt burned when the Io.
4, in order to ensure the C1 and reliable work, the choice of pressure should be greater than twice the power supply voltage.
5, the choice of the bleeder resistor R1 must ensure that the discharge within the required time off the charge on the C1.
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