Cheap air compressor time delay circuit

This is a cheap air compressor time delay circuit. We especially design Air conditioning or refrigerators that use compressor.

The function of this circuit is a time delay do not apply the power to the compressor suddenly, when the power outage renewed within 3-5 minutes.

In case the power outage then go back suddenly, will be to cause the compressor caused burns easily damaged. So we have to use such the delay circuit.

This circuit has many advantages over General circuits. The first open the machine, the circuit will connect power to the compressor immediately. Makes no need to wait. and if the power outage longer than the allotted time. When the AC line come back, the circuit will connected the power to compressor too.
In Figure 1 is the circuit diagram of this projects. The load in circuit is magnetic switch or relay to cut the power supply to the compressor.

How it works

The AC line through the load and the bridge circuits are the voltage and maintain a constant voltage to 7.5V for this circuit.

Schematic diagram of air compressor time delay
Figure 1 Schematic diagram of air compressor time delay circuit

In the event that apply the power to the circuit for the first time, There will be, a DC voltage across the diode D1 charges capacitors C1 and Meanwhile, the DC voltage through R3 causing Q2 work.

So there are the voltage to drive transistor Q3 work. The voltage at emitter Q4 Q4 will be higher than the base makes it work. So there are the voltage to drive a gate of SCR1 works, Like the ON switch to load fully.

The properties of the SCR, the circuit will maintain the switch until the power go off. The SCR circuit to stop working.

While the circuit is running, There will be voltage to charges C1 are constantly working to make the transistor Q1 is conductor.

In the power outage the voltage at C1 will discharges out complete within 3-5 minutes. But if the power back in the mentioned period, the voltage would cause transistor – Q1 immediately. The voltage at base of Q2 low enough Q2 do not works. The voltage will partially charges through R4 to the capacitors C2 the charging voltage will appear at emitter Q3 to emitter Q4.

Which will need use times in charging about 3-5 minutes. The voltage at emitter of Q4 so will higher than voltage at base Q4 also works, cause there are the voltages to drive SCR1 works. Then, the voltage to load and to control compressor again.

It can be seen that this circuit is a very simple circuit, because use fewer equipment and also Avoid to use the transformers as the power supply for this circuit.

For those who want to change over time, slow or fast, may change the R4, C2. If you add value to rise, then the time will be even longer. So You may try to change it as needed.

How to builds

This circuit can assemble all components into the PCB as Figure 2, when completed, will try to stop using it immediately.

Copper PCB layout-components layout
Figure 2 the actual-size of Single-sided Copper PCB layout (left) and the components layout (right)

Caution is building a terminal diode and capacitors have to be really sure.

To build this circuit. Easy to use. Because the circuit has only two terminals use only. You just install Virtual Load the normal switches as Figure 3. Legs A-B may switch interchangeably.

Installing the application of this projects
Figure 3 Installing the application of this projects

Parts you will need

Q1,Q2,Q3______C945 or C1815 NPN transistors
Q4____________BC557 NPN transistors
SCR1__________C106 SCR
D1,D2________1N4148____75V 150mA Diodes
D3,D4,D5,D5___1N4007___1000V 1A Diodes
ZD1__________7.5V 1W zener Diode
Resistors 0.25W 5%
R9___________30K 5W
C1___________47uF 16V Electrolytic
C2___________100uF 16V Electrolytic
C3____________10uF 16V Electrolytic
C4____________0.1uF 63V Metallized Polyester Film Capacitor


  1. Harshith HM January 23, 2016
  2. momename January 25, 2016

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