This circuit is a 555 time base circuit to protect the load from overvoltage and overcurrent.
1. The working principle of the circuit
The circuit principle is shown in Figure 44.
When the load is working normally, the power supply VDD, transistor VT3, load and resistor R6 form a loop, and the power supply supplies power to the load. When there is an overcurrent phenomenon on the load, the increase of the load current makes the potential on the resistor R6 increase to 0.65-0.7V, and the increased potential on the resistor R6 is added to the base of the transistor VT1 to make VT1 turn on. At this time, pin 6 and pin 2 of the 555 time base circuit get a low level, the 555 time base circuit sets immediately, pin 3 outputs a high level, and the light-emitting diode LED lights up. At the same time, the discharge tube in the 555 time base circuit Cut off, that is, pin 7 is suspended, the transistor VT3 is cut off, and the power supply and load are disconnected. After the power supply and the load are disconnected, the power supply charges the capacitor C3 through the resistor R2. When the voltage across the capacitor C3 rises to 2/3VDD, the 555 time base circuit is reset again, the transistor VT3 is turned on, VT1 and VT2 are turned off, and the power supply restarts. Added to both ends of the load, if it is still under the condition of overload current, the above process will be repeated until the current on the load drops to the normal value. In this way, the overcurrent protection function of the circuit to the load is achieved.
If the voltage on the load is overloaded, the overvoltage on the load is added to the resistor R2 and the variable resistor RP, which increases the potential of the positive pole of the voltage regulator tube VS, resulting in breakdown of the voltage regulator tube, making the transistor VT2 conduction, 555 The time base circuit will be in the set state, which also makes the transistor VT3 cut off, achieving the function of overvoltage protection.
2. Selection of components
The 555 circuit uses NE555, μA555, SL555 and other time-base integrated circuits; the transistors VT1 and VT2 use 9014-type silicon NPN medium-power transistors, and the transistor VT3 uses 8550-type silicon PNP medium-power transistors, requiring a current amplification factor β≥100; LEDs use φ5mm red led. R1—R6 selects RTX—1/4w carbon film resistors; RP can use WSW organic solid fine-tuning variable resistors; C2 selects CT1 ceramic capacitors; C1 and C3 select CD11—25V electrolytic capacitors; VS selects 3.6 V, 1W 2CW105 silicon Zener diode.
3. Production and debugging method
When this circuit is in use, the magnitude of the overcurrent can be controlled by adjusting the size of the resistor R6, where the relationship between R6 and the minimum overload current IS can be estimated by the formula R6=(0.65~0.7)V/IS. At the same time, the size of the overvoltage can be set by adjusting the size of the variable resistor RP.