Comprehensive analysis of RC step-down circuit and its application

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Comprehensive analysis of RC step-down circuit and its application [Copy link]

1 Basic concept of RC voltage reduction

1. What is RC voltage reduction?

RC step-down is a circuit that uses the capacitive reactance generated by a capacitor under an AC signal of a certain frequency to limit the maximum operating current .

The capacitor actually plays a role in limiting the current and dynamically distributing the voltage across the capacitor and the load.

2. What parts does the RC step-down circuit consist of?

The RC step-down circuit is composed of a step-down module, a rectifier module, a voltage stabilizing module and a filter module.

3. Basic design elements of RC voltage reduction

When designing a circuit , you should first determine the maximum operating current of the load, calculate the capacitance value based on this current value, and then select the appropriate capacitance.

The difference here with the linear transformer power supply is that the RC step-down power supply selects the capacitor based on the load current; the linear transformer power supply selects the transformer based on the load voltage and power.

RC voltage drop current calculation

The RC step-down circuit can be equivalent to a step-down capacitor C1 and a load resistor R1, with the resistor and capacitor connected in series to divide the voltage.

The capacitive reactance of capacitor C1 is Zc=-j/wC=-j/2πfC

The impedance of resistor R1 is Zr=R

The total equivalent impedance is Z=Zc+Zr=-j/2πfC+R

So I=U/Z=U/(Zc+Zr)=U/(-j/2πfC+R)

Because the RC step-down power supply is only suitable for small current circuits, the selected capacitance range is generally 0.33UF to 2.5UF, so Zc is -1592j to -9651j. The equivalent load impedance Zr is around 200Ω, and obviously |Zc|>>|Zr|, and the voltage drop of the input power supply voltage on the load is also much smaller than the voltage drop of the capacitor, so: Z≈Zc, and the angle θ of the vector diagram is close to 90°.

From this we can get:

I = U / Z = U / Zc = U / (-j / 2πfC)

=220*2π*f*C*j

=220*2π*50*C*j

=j69000C

I=|I|∠90°, the effective value of current I1=|I|=69000C. When the rectification method adopts half-wave rectification, I1=0.5|I|=34500C.

Design Examples

Given the following conditions: load operating current 15mA, operating voltage 5V. What is the capacitance of the step-down capacitor?

Using half-wave rectification, according to the calculation formula I1=0.5|I|=34500C, we know that C=0.43uF. So a 0.47uF capacitor is selected here, which in turn verifies that the provided current I1=34500C=16.2mA, and the excess current flows through the voltage regulator.

Advantages of RC voltage reduction:

Small size; low cost.

Disadvantages of RC voltage reduction:

Non-isolated power supply, unsafe;

Cannot be used for high power loads;

Not suitable for capacitive and inductive loads;

Not suitable for dynamic loads.

2. The basic principle of resistance-capacitance voltage reduction

1. Capacitor charging and discharging principle

A capacitor is a passive device that stores energy in the form of an electric field. The essence of the capacitor charging and discharging process is the process of two conductive parallel plates acquiring and releasing electrons.

Capacitor charging:

When the electric field strength E inside the capacitor is less than the external power supply voltage U at both ends of the capacitor, the capacitor starts to charge. At this time, the positive electrode of the capacitor continuously loses electrons, the negative electrode continuously gains electrons, and the internal electric field E continuously increases until it is equal to the external voltage U, and the charging ends.

Capacitor discharge:

When the electric field strength E inside the capacitor is greater than the external power supply voltage U at both ends of the capacitor, the capacitor begins to discharge. At this time, the positive electrode of the capacitor continuously gains electrons, the negative electrode continuously loses electrons, and the internal electric field E continuously weakens until it is equal to the external voltage U, and the discharge ends.

DC charging and discharging process of capacitor

As shown in the above figure, the charging process, calculate the time it takes for the voltage of C1 to reach 1V:

Because V0=0V, Vt=1V, V1=5V, R=10K, C=0.1uF, so T= 10000*0.1*0.000001*Ln（5/4)=223uS

AC charging and discharging process of capacitor

The DC charging and discharging of a capacitor is completed in one go, while the AC charging and discharging is a repetitive process.

Full-wave rectifier circuit

Half-wave rectifier circuit

Function and selection of each component

F1: Fuse , for overcurrent protection, 400mA250V model is selected.

RV1: Varistor , for surge protection, generally 10D471K model is selected.

C1: Voltage-reducing capacitor, using larger capacitive reactance to limit the total current of the circuit. Commonly used polyester capacitors (CL21), polypropylene capacitors (CBB21), and safety capacitors (X2), the capacitance value depends on the load requirements. The larger the capacitance, the less safe the circuit is. When designing this circuit, if the capacitance exceeds 2.5uF under 220VAC power supply, and the capacitance exceeds 4uF under 110VAC power supply, then you should abandon the resistance-capacitance voltage reduction and consider other circuits. Here, a 0.56uF safety capacitor (X2) is selected to provide 19mA current.

R2: Discharge resistor, provides a discharge circuit for capacitor C1 after power failure, to prevent the residual voltage on capacitor C1 and the grid voltage from being superimposed on each other to form a high-voltage impact on subsequent devices when the power plug is quickly plugged in or the plug is in poor contact, and to prevent the human body from being touched after the power plug is unplugged, causing harm to personnel. It is generally required that the time for the voltage of C1 to decay to 37% after power failure should be less than 1 second, because T=RC*Ln[（V0-V1)/(Vt-V1）], so T=RC, R=t/C, R<1/C. Here, three 390K 0805 chip resistors are used (to share voltage and power).

R1: Current limiting resistor. This resistor is mainly used to prevent the rectifier diode from being damaged by the high voltage shock generated when the power is first turned on and when the power plug is quickly plugged in or when the plug is in poor contact. If the capacitor C2 happens to touch the peak of the wave when it is first powered on, because C2 is in a short- circuit state (first-order zero-state response) at the moment of power-on, the AC power supply is directly added to R1 and the rectifier tube. There is an instantaneous DC voltage of 220VAC*1.414=311VDC on R1. If the charge of C1 is not fully discharged when the power is turned on, this voltage may be higher. Therefore, R1 should be selected to be a resistor with strong current impact resistance and high voltage resistance. The resistance of R1 should not be too small or too large. If the resistance is too small, the impact current is large, and if the resistance is too large, the power consumption of the entire circuit will increase. The peak current of the rectifier diode is generally large. For example, the peak current of the 1N400X series is 50A, so the resistance of R1 is generally between 10-50Ω.

DZ1: Zener diode, 1N4733 is selected, and the regulated voltage Vz is 5.1 V. The maximum regulated current Iz of DZ1 must be greater than the maximum charge and discharge current of capacitor C1.

R5: forms an RC filter with capacitors E1 and C2 to reduce ripple.

D1: Rectifier diode, which plays the role of half-wave rectification, 1N4007 is selected.

D2: Rectifier diode, which plays the role of half-wave rectification, 1N4007 is selected.

E1: Electrolytic capacitor, filters the voltage after voltage regulation, and provides power to the load in the half cycle when the power is off. Before the next half cycle of the power supply, E1 must ensure that the voltage provided to the load does not decay too much. Here, 1000uF25V model is selected. T=RC*Ln[（V0-V1)/(Vt-V1）]=10mS, so the attenuated voltage Vt=4.8V.

C2: Chip capacitor , for filtering, 0.1uF is selected.

R6: Discharge resistor, provides a discharge circuit for E1 after power failure, generally 5~10K.

R7: Equivalent load.

Pictures of main components

One-time fuse

Resettable fuse

Varistor

Metallized Polyester Film Capacitors (CL21)

Metallized Polypropylene Capacitors (CBB21)

X2 safety capacitor (CBB62/MKP)

3. Application of RC Voltage Reduction

RC step-down is suitable for low power and low current loads due to its small size and low cost. Common applications include energy meters, low power LED lamp drivers, small household appliances and thermostats .

LED lamp driver

Small home appliance applications

Fan Controller

Electric heater controller

Coffee machine