DIY Compressor time delay circuit

This is a DIY Compressor time delay circuit. It is suitable for designing in Air conditioning or refrigerators that use a compressor.

Basic working.

Suppose that the power outage. The power-on again. Fortunately, we have this circuit. It will do not turn on the power to the compressor suddenly.

But it will wait about 3-5 minutes before start again.


In case the power outage. then the power is back on suddenly. It will be to cause the compressor caused burns easily damaged. So, we have to use this delay circuit like this.

This circuit has many advantages more than 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 set time. When the AC line come back, the circuit will connect the power to compressor immediately, too.

How it works

Figure 1 is the circuit diagram of this projects. The load in the circuit is a magnetic switch or relay to cut the power supply to the compressor.

The AC Main flows through the load and a bridge diodes, R9, C4 and Zener diode circuit. They are the DC voltage regulator to maintain a constant voltage to 7.5V for this circuit.

Schematic diagram of air compressor time delay

Figure 1 Schematic diagram of DIY compressor time delay circuit

First power comes

There will be a DC voltage across the diode D1 charges capacitors C1 and Meanwhile, the DC voltage through R3 causing Q2 work.

Then, there is the voltage to drive transistor Q3 work. Next, The voltage at the emitter of Q4 is higher than the base, Q4 runs too.

And importance, It can 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 goes 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 a conductor.

In the power outage, the voltage at C1 will discharge out complete within 3-5 minutes. But if the power back on in the above period.

It will cause transistor – Q1 runs immediately. And, the voltage at the base of Q2 is lower not enough. So, Q2 does not work.

Next, the voltage will slowly charge a capacitors C2 through R4. to This voltage will appear at emitter Q3 to emitter Q4.

Which it will need to use times in charging about 3-5 minutes. The voltage at the emitter of Q4 so will higher than base Q4. So it causes has the voltages to drive SCR1 works. Then, the voltage to load to control the compressor again.

We will see that this circuit is a very simple circuit. Because use fewer parts 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. You may change the R4, C2. If you add value up, the time will be longer. So, you may try to change it as needed.

How to build

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)

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

Caution: In building a terminal diode and capacitors have to be really correct.

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

Install time delay to compressor
Figure 3 Installing the application of this project


The coil voltage of the relay

I think the relay is 220V AC coil like this.

Because SCR works well more 4A load. So, the coil may be more 1.5W coil.
For a contact of the relay. Please check the power of load or Air conditioning before. For example 900Watts, the current is 3A at 220V.

And I think this circuit may use under AC 120V, too.

Enjoy with your project.

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, D6: 1N4007, 1000V 1A Diodes
ZD1: 7.5V 1W Zener Diode
Resistors 0.25W 5%
R1: 500K
R2: 1M
R3: 100K
R4: 680K
R5, R8: 2.2K
R6: 33K
R7: 1K
R9: 30K 5W
C1: 47uF 16V Electrolytic
C2: 100uF 16V Electrolytic
C3: 10uF 16V Electrolytic
C4: 0.1uF 63V Metallized Polyester Film Capacitor

Related Posts


I always try to make Electronics Learning Easy.

I love electronic circuit. I will collect a lot circuit electronic for teach my son and are useful for everyone.
Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Inline Feedbacks
View all comments
Close Menu