RC resistance capacitance step-down, application of a coup
Recently seen a lot of people do a resistance capacitance step-down things, see the online information, in fact is not so simple, a lot of places should pay attention to. Check the data, also made a small night lamp aging, a week if there is no problem.
One, the basic concept
The working principle of capacitor is not complicated. Its working principle is the reactance of AC signal generated in the frequency of the use of capacitive to limit the maximum working current.
For example, the frequency of 50Hz, at a 1uF capacitor produced about 3180 ohms. When both 220V and AC voltage in capacitor, the maximum current through the capacitor is about 70mA (68mA more). Although the current flowing through the capacitor in the capacitor 70mA, but does not generate power, as if the capacitor is a capacitor, current flowing through the capacitor for imaginary current, it did work for wattless power. According to this characteristic, we if the capacitor of a 1uF is in series connection with a resistive element, the voltage at the two ends of the resistive element and the resulting power depends entirely on the characteristics of the resistive element.
Capacitance: Xc=1/ (2* π *f*C
Current: Ic=U/Xc
Two, for example calculation
In Figure 3, the known C1 was 0.33 μ F, AC input to 220V/50Hz, for the circuit to the maximum of the current supplied to the load. C1 in the circuit reactance of Xc
Xc=1 / (2 pi f C) = 1/ (2*3.14*50*0.22*10-6) = 14.476K
The charging current through the capacitor C1 (Ic) for:
Ic = U / Xc = 220 / 14.476 = 15mA.
Relationships are usually step-down capacitor capacity of C1 C and Io load current can be considered: C=14.5*Io, wherein the unit capacity of C is F, Io is a unit of mA.
Three, the practical application
Figure 1 is a LED constant current, voltage drop minus BE pressure drop is E pole resistance to voltage, constant current. Because of the linear constant current power greatly in the triode, so this circuit is suitable for a plurality of LED series, in order to reduce the power consumption of the triode.
Figure 1:
Figure 2 light LED Nightlight, B pole to ground resistance is a photosensitive resistor, the daytime P tube LED is not bright; night P pipe cut-off LED conduction.
Figure 2:
Figure 3 is a common LED Nightlight
Circuit of R2 for resistance C1 discharge; prevent the rapid plug or plug bad contact of C1 capacitor residual voltage and grid voltage superimposed to form high impact on the subsequent devices and prevent unplug the power plug after exposure to human body harm to personnel, so the RC time constant in the ideal state (≤ T T=1/F, F=50Hz), but in the actual use of R1 should not be too small, otherwise R1 power is too large, we generally take RC time constant is smaller than 300mS.
R4 R3, the current limit resistor circuit; the resistance is mainly the damage of high voltage impact prevent first power on and produced in fast plug or plug bad contact of the rectifier diodes, capacitors C2 in the first power if just touch on the peaks, due to C2 in a short instant power state, the AC the power supply directly on the R2, R3 and rectifier tube, 220VAC × 1.414=311VDC transient voltage of R3, R4; C2 fast capacitance in the plug or plug bad contact, if the capacitor C1 in the n+1 peak power, electricity peak in n+2, capacitance C1 let Δ V in n+1 to n+2 time (C1, R2), 220VAC * 1.414+ R3, R4 (220VAC × 1.414- at =622VDC- Δ V Δ V) instantaneous DC voltage, if considering the fluctuation of network voltage, this voltage will be higher, so R3, R4 to select the current shock resistance and high pressure resistance, general choose high power oxide film resistors or metal film resistors, R3, R4 total resistance can not be too small, not too big, the resistance is too low impact current, resistance is too large and the whole circuit power consumption increases, the peak current of rectifier diode in general will be relatively large, such as the 1N400X series The peak current is 200A, so R3, R4 between the total =40-50 resistor. Considering the main goal of power is not the most for the DIY board below, resistance of 200R, impact current is very small to ensure the long term reliability. But its power only 0.045W quite small
C2 filter capacitor, general cost is slightly higher than the load voltage selection. For example, 50V can, but considering that LED production inserted opposite or long-term use of LED open and other abnormal situations, 310 DC will be filled into the electrolytic capacitor, to prevent the explosion accidents. Select the 400V electrolysis is not the risk.
In those rechargeable LED Nightlight online buying is added a lead-acid battery 4V, no pressure limiting device, so is the charging time marked on the shell, the current used for fever, severe electrolyte consumption, short service life. Can not be directly into charge lithium batteries, especially not with protective plate of lithium can not replace, otherwise the explosion or fire. |
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