High Low Voltage Cutout Circuit

The protection of electrical and electronic appliances is the main concern these days. Some electronics appliance is very sensitive to voltage i.e. it only functions when adequate voltage is provided. Some appliances were even destroyed due to high and low voltage supply. To solve this type of issue we are posting a very simpler and more useful project called “High Low Voltage Cutout Circuit”. Previously we had already posted various types of appliance protecting circuits on bestengineeringprojects.com like:

1. Auto Cut Circuit for Over and Under Voltage
2. High Current Regulator Circuit
3. AC Voltage Stabilizer Circuit using 556 
4. Overvoltage Protection Circuit
5. Automatic Voltage Stabilizer Circuit

The difference is, that this circuit is simple and more sensitive than the previous one.

Circuit Description of High Low Voltage Cutout Circuit

The circuit of high-low voltage cutout is shown in Figure 1. The main building block of this circuit is timer IC 555, voltage regulator IC and a few other activities as well as passive electronic components like a transistor, resistor, capacitor, etc. For a better description, we divide the entire circuit into three different reasons.

• Voltage Step-Down and Rectifier Unit | High Low Voltage Cutout Circuit

This unit is a rectifier unit. The mains input is stepped down to 12V-0V-12V using the center tap transformer which is further rectified using diodes D₁ and D₂ (Full wave center tap rectifier). The rectified output is pulsating in nature and needs to filter. Here we have used two capacitors C₁ and C₂ to smoothen the waveform. One-half of rectified output is given to 9V fixed series voltage regulator IC (IC₁) where the other half is left for reference.

Reference Circuit | High Low Voltage Cutout Circuit

This unit is built around voltage-regulated IC and timer IC. The output from diode D₁ is given to the voltage regulator IC, thus output is fixed whereas the output of diode D₂ is fluctuating. This fluctuating voltage (output of diode D₂) is proportional to fluctuating in mains input. When the mains voltage becomes higher than normal, the voltage at pin 6 of timer IC 555 (IC₂) also becomes high. This high voltage at pin 6 drives the output at pin 3 of IC­₂ high. Similarly, when the voltage becomes lower than the voltage at pin 2 of the timer IC becomes low, and pin 7 is also grounded.

Switching Circuit | High Low Voltage Cutout Circuit

The switching circuit is designed around two general-purpose transistors (T₁ = BC108C, T₂ = SL100B). In either case (i.e. low and high voltage) the transistor T₂ becomes off as a result relay RL₁ is de-energized.

Case I: When voltage is high

When the mains voltage becomes high than the normal voltage, the voltage at pin 6 also becomes high. Thus, the output of IC₂ (NE555) becomes high, transistor T₁ starts to conduct, and the base of transistor T₂ is grounded. This led transistor T₂ to off state and the relay was de-energized.

Case II: When voltage is low

When the mains voltage becomes low, the voltage at pin 2 of IC₂ (Timer IC 555) also becomes low, and pin 7 will be grounded. Thus, the base of transistor T₂ is again grounded and the relay drives to de-energized.

Case III: When voltage is normal

The output of the voltage regulator and output from diode D₂ become equivalent as a result neither out of IC₂ become high nor pin 7 of IC₂ will be grounded. Thus, the base of the transistor gets enough voltage to conduct and the relay is energized.

PCB Diagram:

The PCB diagram is designed using Proteus 8.1 design suite. The actual size solder side and component side are shown in the figure below. Download the PCB diagram in PDF format from the link given below.

Figure 2: Solder side PCB of High Low Voltage Cutout

Figure 3: Component Side PCB of High Low Voltage Cutout

Figure 4: 3D View of High-Low Voltage Cutout 

Click here to download the PCB

PARTS LIST OF HIGH LOW VOLTAGE CUTOUT CIRCUIT