Working Principle of DC Voltage Regulator
DC regulated power supply is a 220V industrial frequency alternating current into a regulated output DC voltage device, it needs to be transformed, rectified, filtered, regulated four links to complete. The working principles of the four links are as follows:
(1) power transformer: a step-down transformer, it will grid 220V AC voltage converted to meet the needs of the AC voltage, and sent to the rectifier circuit, the transformer ratio is determined by the transformer's secondary voltage.
(2) rectifier filter circuit: the rectifier circuit converts the AC voltage Ui into pulsating DC voltage. And then filtered by the filter circuit to filter out the larger ripple components, the output ripple smaller DC voltage U1. commonly used rectifier filter circuit has a full-wave rectifier filter, bridge rectifier filter.
(3) filter circuit: the rectifier circuit output voltage in the AC component is mostly filtered out, so as to obtain a relatively smooth DC voltage filter capacitance C to meet the RL-C = (3 ~ 5) T / 2, or T for the input AC signal period, RL for the rectifier filter circuit equivalent load resistance.
(4) voltage regulator circuit: the function of the regulator circuit is to stabilize the output DC voltage, does not change with the AC grid voltage and load changes. Commonly used integrated voltage regulator has a fixed three-terminal regulator and adjustable three-terminal regulator. Commonly used adjustable positive voltage integrated regulator CW317 (LM317) series, their output voltage from 1.25V-37 volts adjustable, the simplest circuit external components only a fixed resistor and a potentiometer. Its chip has a transition, overheating and safe working area protection, the maximum output current of 1.5 A. Its typical circuit is shown below, in which the resistor R1 and potentiometer R2 form the output voltage regulator, the expression of the output voltage Uo: Uo = 1.25 (1 + R2 / R1) in which R1 is generally taken as the 120-240 ohms, the output side of the output side of the voltage difference between the adjusting side is The reference voltage of the regulator (typical value of 1.25V).
Circuit, D5, D6 and C1 together to form a full-wave rectifier filter circuit, the input AC power converted to DC; Q1, Q2 composed of a Darlington shoot-following circuit, Q1 is known as the adjustment tube, Q2 is known as the push tube, the power supply for the load; Q3, D8, R2, R3, RW1 together to form the output voltage sampling negative feedback circuit, Q3 is known as a sampling amplifier tube, D8 is known as the reference regulator tube, this Part of the output voltage is used to stabilize; Q4, R5 output current sampling circuit to provide overcurrent protection for the output; C5 for the output filter capacitor. As the regulator and the load relative to the input is connected in series, so this power supply is called series-type DC regulated power supply.
220V alternating current by the power transformer step-down for low-voltage alternating current, by D5, D6, C1 composed of full-wave rectifier filter circuit rectifier filter into DC, delivered to the Darlington shot with the circuit, and finally by the shot with the circuit to the load to provide a DC voltage VO, the size of the voltage VO by the reference voltage VREF and the sampling branch of the divider ratio decision. When the output voltage VO due to some reason instantaneous increase, the output voltage sampling circuit of the collector current of Q3 will increase, resulting in the base current of the radio-following circuit decreases, the internal resistance of the adjusting tube increases, the tube voltage drop increases, the output voltage VO decreases; when the output voltage VO instantaneously decreases, the collector current of the output voltage sampling circuit of the Q3 will decrease, the base current of the radio-following circuit increases, the tube voltage drop decreases, so that the output voltage VO decreases, and the voltage drop decreases. When the output voltage sampling circuit Q3 collector current will be reduced, the base current of the shot following circuit will be increased, the tube voltage will be reduced, so that the output voltage VO is also increased; the above process makes the output voltage stabilized at a certain voltage value, so as to realize the voltage regulation. By adjusting RW1, the size of the output voltage can be changed.
When the load current exceeds the rated output current, the voltage drop on R5 will make Q4 conductive, forcing the bias voltage of the radio-following circuit to decrease, the internal resistance increases, the tube voltage drop increases, the output current of the radio-following circuit is limited, and the output voltage decreases, thus playing a role in the overcurrent protection, this structure belongs to the current-limiting type protection circuit.
Input 220V AC voltage after transformer step-down, rectifier diode rectifier output for pulsating DC voltage, with a large voltage ripple, after capacitor C1, C2, C3 smooth filter output voltage is relatively smooth, and then input to the regulator circuit, you can get a stable, voltage ripple is very small DC regulated power supply at the output.
