How to use lm317 as a current regulator
LM317 working principle
As an integrated three-terminal voltage regulator block with variable output voltage, LM317 is a convenient and widely used integrated voltage regulator block.
There are many models of the 317 series voltage stabilizing block: for example, LM317HVH, W317L and so on. Electronic hobbyists often use the 317 regulator block to make a regulated power supply with variable output voltage (the basic form of the circuit is shown in the figure below). The output voltage of the regulated power supply can be calculated by the following formula, Vo = 1.25 (1 + R2/R1). Just looking at the formula itself, the resistance values of R1 and R2 can be set at will.
However, as a formula for calculating the output voltage of a regulated power supply, the resistance values of R1 and R2 cannot be set arbitrarily. First of all, the output voltage range of the 317 regulator block is Vo=1.25V-37V (the 317 regulator block with high output voltage, such as LM317HVA, LM317HVK, etc., has an output voltage range of Vo=1.25V-45V), so R2/R1 The range of ratio can only be 0-28.6. Secondly, the 317 stabilizing block has a minimum stable operating current, some materials are called minimum output current, and some materials are called minimum discharge current. The value of the minimum stable working current is generally 1.5mA. Due to the different manufacturers and models of the 317 stabilizing block, the minimum stable working current is also different, but it is generally not greater than 5mA. When the output current of the 317 stabilizing block is less than its minimum stable working current, the 317 stabilizing block cannot work normally. When the output current of the 317 stabilizing block is greater than its minimum stable working current, the 317 stabilizing block can output a stable DC voltage. If you do not pay attention to the minimum stable working current of the 317 voltage stabilizer block when you use the 317 voltage stabilizer block to make a stabilized power supply (as shown in the figure), the following abnormal phenomena may occur in the stabilized power supply you made: Output of the stabilized power supply There is a big difference between the on-load voltage and the no-load voltage.
To solve the problem of the minimum stable working current of the 317 stabilizing block, the resistance value of R1 and R2 can be set so that the output current of the 317 stabilizing block is greater than or equal to its minimum stable working current when there is no load, so as to ensure the 317 stabilization The pressure block can work stably when there is no load. At this time, as long as Vo/(R1+R2)≥1.5mA, the 317 voltage regulator block can be guaranteed to work stably when there is no load. 1.5mA in the above formula is the minimum stable operating current of the 317 voltage regulator block. Of course, as long as the 317 regulator block can work stably under no load, the value of Vo/(R1+R2) can also be set to any value greater than 1.5mA.
After calculation, it can be known that the maximum value of R1 is R1≈0.83KΩ. And because the maximum value of R2/R1 is 28.6. So the maximum value of R2 is R2≈23.74KΩ. When using the output voltage calculation formula of the 317 stabilizing block to calculate its output voltage, it is necessary to ensure that the two inequalities R1≥0.83KΩ and R2≤23.74KΩ are established at the same time to ensure that the 317 stabilizing block can work stably under no load. Of course, connecting the bleeder resistor R in parallel at the output of the 317 stabilizing block (as shown in the figure) can also provide the minimum stable working current for the 317 stabilizing block. However, because the parallel bleeder resistance cannot change with the change of the output voltage, if it is necessary to ensure that the output current of the 317 stabilizing block when the output voltage is 1.25V, its output current is greater than its minimum stable working current, then the output of the 317 stabilizing block When the voltage is 37V, the current flowing through the bleeder resistor is too large, which not only wastes electric energy, but also increases the burden of the 317 regulator block, which is not a proper method.
LM317 power supply circuit principle
In the above schematic circuit, a three-terminal voltage regulator LM317T is mainly used, whose function is mainly to stabilize the voltage signal in order to improve the stable performance and reliability of the system. LM317T is a kind of device: After the working voltage is provided by the Vin terminal, it can keep its +Vout terminal (pin 2) higher than the voltage of its ADJ terminal (pin 1) by 1125V. Therefore, only a very small current is needed to adjust the voltage of the ADJ terminal, and a relatively large current output can be obtained at the Vout terminal, and the voltage is higher than the ADJ terminal by a constant 1125V. The output voltage can also be changed by adjusting the resistance value of the ADJ end (end 1) (LM317T will ensure that the voltage on the part of the resistance connected to the ADJ end and the +Vout end is 1125V). Therefore, when the resistance value of the ADJ terminal (terminal 1) increases, the output voltage will increase.
The output voltage of LM317T can be adjusted continuously from 1125V to 37V.
LM317 adjustable regulated DC power supply circuit
DC stabilized power supply is a device that converts 220V power frequency alternating current into a regulated output direct current. It needs four steps of voltage transformation, rectification, filtering, and stabilization to complete. It is generally composed of a power transformer, a rectifier filter circuit and a voltage regulator circuit.
1, step-down part
The power transformer is a step-down transformer, and its function is to transform the 220V AC voltage into the AC voltage Ui required by the rectifier filter circuit. The transformation ratio of the transformer is determined proportionally by the secondary side of the transformer, and the power ratio between the secondary side and the primary side of the transformer is P2/P1=n, where n is the efficiency of the transformer.
2, rectification part
The design uses a single-phase bridge rectifier circuit. It is composed of four diodes, and its composition principle is to ensure that the voltage and current direction on the load remain unchanged during the entire cycle of the secondary voltage u of the transformer. In order to achieve this, it is necessary to correctly guide the current flowing to the load within the positive and negative half cycles of Uz so that its direction remains unchanged. Set the two ends of the transformer secondary side to be a and b respectively, then a is "+" and b is "a". When "", there should be current flowing out of point a, when a is "one" and b is "+", there should be current flowing into point a; on the contrary, when a is "+" and b is "one", there should be current flowing into point b, so a and Two diodes should be connected to point b to guide current. The specific circuit principle is shown in Figure
If the midpoint of the secondary side of the bridge rectifier transformer is grounded, the two load resistors should be connected and the midpoint of the connection should be grounded. According to the working principle of the bridge rectifier circuit, when point a is "+" and point b is "one", D1 and D3 are turned on, D2 and D4 are turned off, U01=U2, U02=one U2; and when point b is "1" +" When point a is "one", D2 and D4 are turned on, D1 and D3 are turned off, U01=one U2, U02=U2, so that the two loads get positive and negative voltages respectively.
If the transformer secondary voltage u2=U2sinwt, U2 is the effective value. When u2 is a positive half cycle, the current flows out of point a and flows into point b through Dl, RL, and D3, so the voltage on the load resistance RL is equal to the voltage on the secondary side of the transformer, that is, uo=u2, the reverse voltage that the D2 and D4 tubes bear Is a u2. When u2 is a negative half cycle, the current flows from point b and flows into point a through D2, RL, and D4. The voltage on the load resistance RL is equal to one u2, that is, uo=one u2, and the reverse voltage D1 and D3 bear is u2. In this way, since the two pairs of diodes D1, D3 and D2, D4 are turned on alternately, the load resistance RL has current through the whole cycle of u2, and the direction is unchanged, the output voltage uo=|U2sinwt|.
3, filter circuit
The amplitude of the rectified direct current changes greatly, which will affect the performance of the circuit. The characteristics of "passing AC and blocking DC" of the capacitor can be used to combine two parallel capacitors in the circuit as a capacitor filter to filter out the AC component.
Capacitor filter circuit is the most common and simplest filter circuit. A capacitor is connected in parallel at the output end of the rectifier circuit (that is, both ends of the load resistance) to form a capacitor filter circuit. The filter capacitor has a large capacity, so electrolytic capacitors are generally used. Pay attention to the positive and negative electrodes of the electrolytic capacitor when wiring. The capacitor filter circuit uses the charging and discharging effects of the capacitor to smooth the output voltage. If the two filter capacitors are connected and the connection point is grounded, the positive and negative power supplies with smooth output voltage can be obtained at the same time.
Under ideal conditions, there is no loss on the secondary side of the transformer, and the diode conduction voltage is zero, so the voltage across the capacitor is equal. When it rises to the peak value and then begins to fall, the capacitor is discharged through the load resistance, and its voltage also begins to drop. The trend is basically the same as the voltage across the capacitor. But because the capacitor discharges exponentially, when it drops to a certain value, the capacitor will continue to discharge through the load, and the voltage across the capacitor will slowly decrease exponentially.
LM317 typical application circuit
1,Between 2 feet is a 1.25V voltage reference. To ensure the output performance of the regulator, R1 should be less than 240 ohms. The value of the regulated voltage can be adjusted by changing the resistance of R2. D1, D2 are used for protection.
How to use lm317 as a current regulator
1, clarify the input and output terminals of LM317
Control 3 LED lights to emit light through LM317. The rated voltage of LED lights is 3V.
4. In the third step, why use LM317 as a stabilizer instead of a voltage regulator?
The reason is: if one of D1, D2, D3 is short-circuited, and LM317 is made of a voltage regulator, the remaining two good LED light-emitting diodes will also be damaged due to the higher voltage.
But if LM317 is used as a steady current tube, even if one of D1, D2, D3 is short-circuited, LM317 will automatically adjust the current to keep the current constant, thus playing the role of protection circuit
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