## Wednesday, December 15, 2010

### 【 Weak current College 】 capacitor voltage works 】

Capacitor voltage works and not complicated. His works is the use of capacitance at AC signal frequencies produced by the capacitance to limit the maximum operating current. For example, in the 50 Hz frequency, a 1uF capacitor capacitance of approximately 450 ohms. When 220V AC voltage with both ends of the capacitor, it flows through the capacitance of the maximum current of about 70mA. Although the current flows through the capacitor, but have 70mA capacitors does not produce power on, as if the capacitor capacitance is an ideal, the current flowing through the imaginary part of the capacitance of current, it's work for reactive power. According to this characteristic, if we are in a 1uF capacitor on series a resistive element, then the resistive element ends the resulting voltage and it generates power depends on the characteristics of resistive components. For example, we will a 110V/8W of bulb and a 1uF capacitor series, at 220V/50 Hz AC voltage at the lamp is lit, given normal brightness without being burnt. Because the bulb 110V/8W required by current to 8W/110V = 72mA 1uF capacitor, it and the current limiting characteristics. Similarly, we can also use the bulb 5W/65V and 1uF capacitor series received a 220V/50 Hz AC, lamp will be lit and not be burned. Because the bulb 5W/65V operating current of approx. 70mA too. Thus, the capacitor voltage actually use capacitance current limiting. And a capacitor to actually limit the current and dynamic allocation of capacitors and load voltage.

This type of circuit is typically used for low-cost access to non-isolated low current power supply. Its output voltage is usually a few volts to three dozens of Volt, depends on the Zener voltage tube. The current size is proportional to the current limiting capacity. When using half-wave rectifier, capacitance per micro method can receive current (mean): (international standards organization)

I(AV)=0.44*V/Zc=0.44*220*2*Pi*f*C
=0.44*220*2*3.14*50*C=30000C
=30000*0.000001=0.03A=30mA

If you use full-wave rectification may be double the current (mean) to:
I(AV)=0.89*V/Zc=0.89*220*2*Pi*f*C
=0.89*220*2*3.14*50*C=60000C
=60000*0.000001=0.06A=60mA

In General, this type of circuit full-wave rectification despite current slightly larger, but because the floating, stability and security than half wave rectifier-more poor, so using less.

When using capacitive voltage should note the following:

1, and 220V AC high voltage isolate, please note that security, prevent electric shock!
2. current limiting capacitor shall meet at the firing line, the pressure to be sufficiently large (greater than 400V) and adding series anti-surge shock-resistance and insurance and discharge resistance.
3. takes note of the Zener tube power consumption, no Zener tube disconnected operation.
4, according to the load current size and the AC frequency select appropriate capacitance, rather than load voltage and power.
5, current limiting capacitor must use absolute promise of capacitors, electrolytic capacitors can not be used. And capacitor voltage 400V or above is required. The best capacitor capacitance for iron shell oil immersion.
6, the capacitor voltage cannot be used to power, because of insecurity.
7, the capacitor voltage is not suitable for dynamic load conditions.
8. Similarly, the capacitor voltage is not suitable for capacitive and inductive load.
9, when the need for DC, try to use half-wave rectifier. Bridge rectifier is not recommended. And to meet the conditions of constant load.