One, capacitance step-down principle

 

The working principle of capacitance step-down is to use capacitance to issue the capacitive reactance of generation to restrict electric current of the biggest job in certain communication signal frequency. For example, below the labour frequency condition of 50Hz, the capacitive reactance that the capacitance place of a 1uF produces is 3180 ohm about.

 

Add in the two end of capacitor when the alternating voltage of 220V, the biggest electric current that has shed capacitance is 70mA about. Although had shed the electric current of capacitance to have 70mA, but do not produce power comsumption on capacitor, if capacitance is an ideal capacitance,should be, the electric current that has shed capacitance is empty ministry electric current, the work that it makes is without result power.

 

According to this characteristic, if we establish ties again on the capacitor of a 1uF component of sex of a block, criterion the tension that two end get block sex element and the power comsumption that it produces depend on completely the character of component of this block sex.

 

For example, we establish ties the bulb of a 110V/8W and a capacitance of 1uF, on the alternating voltage that receives 220V/50Hz, bulb is illumed, give out normal brightness and won’t by burn down. The electricity that needs because of the bulb place of 110V/8W is 8W/110V=72mA, it and the current limliting character that 1uF capacitance place produces photograph be identical.

 

Manage together, we are OK also on the alternating current that receives the bulb of 5W/65V and 1uF capacitance series 220V/50Hz, bulb can be illumed likewise, and won’t by burn down. Because the working electric current of the bulb of 5W/65V also is 70mA about.

 

Accordingly, capacitance step-down is to use capacitive reactance current limliting actually. And capacitor rises to restrict electric current and trends allocation capacitor and load to actually the part of two upright voltage.

 

Next graphs permit the representative application of step-down for block, c1 is step-down capacitance, r1 to disconnect the discharge of the C1 when power source discharges block, d1 is half commutate diode, d2 is in city of report lose half Zhou Wei C1 to provide discharge return, otherwise capacitance C1 is full of report not to work, z1 is stabilized voltage diode, c2 is filter wave capacitance. Output is stabilized voltage diode the stable voltage value of Z1.

 

 

In applying actually, can replace with next graphs on graph, here used Z1 to be being mixed to character retrorse character, its are retrorse and characteristic (namely its stabilized voltage is characteristic) will stabilize voltage, its are using report to lose half weeks to provide discharge return to C1 to character.

 

 

In bigger voltaic application, can use complete wave commutate, pursue as follows:

 

 

When commutate of little tension complete wave is outputted, electric current of the biggest output is namely:

 

Capacitive reactance: Xc=1/(2πfC)

Electric current: Ic = U/Xc=2πfCU

 

The following should notice when using capacitance step-down:

 

1, the working frequency according to laden voltaic size and alternating current chooses proper capacitance, is not the voltage of basis load and power.

 

2, current limliting capacitance must be used without polarity capacitance, cannot use absolutely electroanalysis capacitance. And the compression of capacitance must be in 400V above. Optimal capacitance is capacitance of iron carapace oily dip.

 

3, capacitance step-down cannot be used at high-power condition, because insecure.

 

4, capacitance step-down does not fit dynamic and laden requirement.

 

5, same, capacitance step-down does not suit to allow gender and perceptual load.

 

6, when should needing dc to work, use half commutate as far as possible. Do not suggest to use bridge-type commutate. And should satisfy constant and laden requirement.

 

2, choice of parts of an apparatus

 

When 1. circuit is designed, should determine first the exact value of load current, consult give typical examples will choose the capacity of step-down capacitor next. Because pass the voltaic Io that step-down capacitance C1 offers to load, it is to shed those who cross C1 to fill Ic of discharge electric current actually. C1 size is larger, capacitive reactance Xc is smaller, shed classics C1 fill, discharge electric current is bigger. Be less than C1 when load current Io when filling discharge electric current, redundant electric current can have shed stabilized voltage canal, those who be like stabilized voltage canal is the biggest when allowing voltaic Idmax to be less than Ic-Io easy cause stabilized voltage to be in charge of burn down.

 

2. works to make sure C1 is reliable, its compression chooses to should be more than the power source voltage of double.

 

The choice of R1 of 3. release resistor must make sure the release inside the time that asking drops the charge on C1.

 

3, devise give an example

 

Foregone C1 is 0.33μF, communication input is 220V/50Hz, beg circuit to be able to furnish laden the biggest electric current. C1 is in the capacitive reactance Xc in circuit:

 

Xc=1 / (2 πf C) = 1/ (2*3.14*50*0.33*10-6) = 9.65K

 

The charging current that has shed capacitor C1 (Ic) for:

 

Ic = U / Xc = 220 / 9.65 = 22mA

 

Normally the relation of the capacity C of step-down capacitance C1 and load current Io can think approximately: C=14.5 I, among them the unit of capacity of C is μF, the unit of Io is A. Power source of capacitance step-down type is power source of segregation of a kind of blame, special attention wants to keep apart on application, prevent to get an electric shock.