Exhaust fan delay switch

In real life, everyone has this experience: sometimes you want the exhaust fan to run for a while after you leave the bathroom. But if you forget to turn off the exhaust fan when you come back, it will waste energy. It would be great if the exhaust fan had a delay function.

Here we introduce an exhaust fan delay switch that is easy to modify and use. Its advantages are as follows:

1. Do not change the wiring, and do not change the appearance of the existing wall switch. Usually, the wall switch only introduces the live wire into the wall switch dark box, while many exhaust fan delay switches need to connect the neutral wire and the live wire at the same time, which brings many difficulties to the transformation, especially when the bathroom heater and the exhaust fan are integrated. If this design is adopted, the original wiring will not be changed, and the appearance of the existing wall switch will not be changed at all.

2. Easy to install, no sequence requirements. There is no sequence distinction between the live wire input and output, and there is no problem of positive and negative connection. However, if the delay switch has a wiring sequence requirement, once the user connects it incorrectly, it may cause a safety accident.

3. It can be manually switched on for a long time. The operation method is the same as that of an ordinary wall switch. If the switch is closed for a long time, the exhaust fan will work for a long time. It is especially suitable for the situation where the bathroom heater and the exhaust fan are integrated.

4. No standby loss, safe and energy-saving. When turned off, the standby power consumption of the entire circuit is zero, there is no standby power consumption, and there is no safety hazard. Moreover, the entire circuit has no relay mechanical contacts and has a long service life.

The schematic diagram is shown in Figure 1. 1 and 2 are followed by the time delay switch, which acts as an equivalent switch to control the on and off of the exhaust fan.

1. Circuit composition

The main circuit is composed of bridge rectifier block BR1 and thyristor Q3. R10 and C3 act as buffers.

The power switch circuit is composed of transistors Q1, Q2, diode D4, and switch SW, which provide the working power of the operational amplifier and the trigger signal of the thyristor. R8 and R11 provide a path for the weak leakage current of Q1 and Q2 to avoid false triggering.

The delay control circuit is mainly composed of R4, C1, and operational amplifier U1. The output voltage of the operational amplifier controls transistor Q2.

2. Working Principle

At the beginning, since Q1 and Q2 are not turned on, the op amp has no working power, the thyristor has no trigger signal, the exhaust fan does not rotate, and the entire circuit consumes no power.

When the exhaust fan needs to be started, the switch SW is closed, the cathode potential of the diode D4 is zero, and the PNP transistor Q1 forms a base current through R7 and D4 and enters a saturation state. The 12V working power required by the operational amplifier is generated by R5 current limiting, D1 voltage stabilization, and C2 filtering.

At the same time, when the potential of the collector of Q1 exceeds 20V, D2 breaks down in reverse, generating a trigger current for the gate of thyristor Q3 through R9. Thyristor Q3 is turned on, which is equivalent to connecting terminals 1 and 2, and the exhaust fan is powered on and starts to rotate.

Since the voltage across capacitor C1 is basically zero at this time, the potential of pin 2 of op amp U1 is lower than that of pin 3. The op amp outputs a high level, providing base current to Q2 through R2, causing Q2 to enter a saturation state, thereby providing a path for the base current of Q1.

Because there is a positive feedback resistor R1 between pins 1 and 2 of the operational amplifier U1, the potential of pin 3 is higher, reaching 10.8V, ensuring that the operational amplifier continues to output a high level and maintain the exhaust fan's rotation.

When the switch SW is turned off, Q1 continues to conduct, generating the 12V operating power required by the operational amplifier, and sending a trigger current to the gate of the thyristor Q3, causing the thyristor Q3 to continue to conduct, and the exhaust fan continues to rotate.

C1 obtains charging current through R4, and the voltage at both ends continues to rise. When the potential of pin 2 of op amp U1 is higher than that of pin 3, the op amp outputs a low level, Q2 enters the cut-off state, and then Q1 is cut off, the op amp loses its working power supply, the thyristor Q3 is also cut off, and the exhaust fan stops rotating.

The length of time the exhaust fan rotates depends on the charging time constant of R4 and C1. The values ​​in the figure can make the exhaust fan rotate for more than ten minutes. The component parameters can be set according to actual needs.

This completes the exhaust fan delayed closing process.

When the exhaust fan needs to be started again, the switch SW is turned on. At this time, C1 is rapidly discharged through R6, D3, and SW, and the voltage at both ends drops to almost zero, and the previous working process is repeated.

If the exhaust fan needs to work for a long time (for example, the bathroom heater and the exhaust fan are integrated into one), the switch SW is closed for a long time, and the exhaust fan can work for a long time.

3. Installation and debugging

1. Installation

The printed circuit board is shown in Figure 2. Its area is small and suitable for installation in a wall switch.

The installation is very simple. Disassemble the wall switch, remove the two wires originally connected to the wall switch contacts, and connect them to the IN terminal at the upper left corner of the printed circuit board. Lead out two wires from the SW terminal at the lower right corner and connect them to the original wall switch contacts. After fixing them, the installation is complete.

2. Debugging

Since the circuit board is connected to the mains, it is recommended to use an isolation transformer for debugging to ensure safety.

The resistance value of R4 is set to be small in advance, so that the charging time constant of R4 and C1 is only a dozen seconds. This makes it easier to debug quickly.

Close switch SW, then open switch SW, the exhaust fan should rotate. If abnormal, check Q1, Q2, Q3 and auxiliary components according to the above working principle.

The exhaust fan should stop after running for a while. If abnormal, check U1 and the delay element according to the above working principle.

Then, mainly adjust the resistance value of R4 to make the delay time reach a satisfactory value.