Simple DC High Low Voltage Cut Off Circuit

If you have a device that needs just the right amount of battery power DC voltage to work safely.

A simple DC cut off circuit acts like a guard for that device.

If the battery gets too weak low voltage, the circuit cuts off the power to protect the device.

If the battery gets too strong high voltage, the circuit also cuts off the power to protect the device.

This way, the circuit helps keep your device safe from harm caused by wonky battery power.

Circuit Working:

Parts List:

Here are the circuit working process from the above diagram.

The circuit utilizes three BC547 transistors each configured as a single transistor amplifier.

The transistors are biased on conducting by default due to the presence of base resistors.

The cut off voltages high and low are adjusted by the two variable resistors marked “set to LOW voltage cut out” and “set to HIGH voltage cutout”.

These resistors are connected to the voltage input through a voltage divider network 4.7k and 5k resistors.

When the DC voltage input rises above the high cut off threshold set by the upper variable resistor the voltage at the base of the corresponding transistor middle one goes low turning it off.

Conversely, if the DC voltage falls below the low cut off threshold set by the lower variable resistor the transistor on the left is turned off.

The relay coil is controlled by the combined output of these two transistors.

When either transistor is off the relay coil loses current and the relay de energizes cutting power to the load.

Limitations of this circuit design:

• The circuit lacks hysteresis meaning the turn on and turn off voltages for the relay are very close.
• This could lead to rapid switching of the relay if the input voltage fluctuates around the cut-off thresholds.
• The circuit does not include any current limiting resistors for the base of the transistors.
• This could potentially damage the transistors if too much current is allowed to flow into their bases.
• There are no components to protect the circuit from reverse polarity input voltage.

Overall, this is a simple design for a high or low voltage cut off circuit, but it could be improved with the addition of hysteresis, base current limiting resistors and reverse polarity protection.

Formulas:

A simple DC high low voltage cut off circuit may be easily constructed using the voltage divider formula.

The formula and its application to the circuit are as follows:

Vout = Vin * (R2 / (R1 + R2))

where,

Vout is the voltage at which the output will be monitored (voltage for cut off)
Vin is the voltage that you are observing as an input (battery voltage)
R1 is the voltage divider circuits initial resistors resistance( here in diagram 1k + upper half of preset)
R2 is resistance of the voltage divider circuits second resistor (here in diagram lower half of preset resistance)

How the formula works for cut off circuit:

Select a reference voltage Vout to stand in for the cut offs high or low voltage threshold.

This might be the lowest safe voltage for low voltage cut off or your preferred fully charged battery voltage for high voltage cut off.

Based on the voltage range of your battery (e.g. the usual battery voltage and its tolerance) choose the Vin range.

Using the above method, select two resistors R1 and R2 to get the desired Vout at the cut off point (reference voltage).

Note:

This is a simple illustration, depending on your unique cut off logic and circuit design, you might need to add other components (such comparators) or change the resistor values.

How to Build:

To build a Simple DC High Low Voltage Cut Off Circuit following are the steps mentioned:

Assemble the circuit on a PCB 

• Following the schematic gather the components.

Set the Cut Off Voltages: 

• Use a multimeter to measure the voltage at the base of the transistors while adjusting the variable resistors.
• Set one for the high cut off threshold and the other for the low cut off threshold based on your application requirements.

Test the Circuit:

•  Connect a DC power supply with adjustable voltage output.
• Gradually increase the voltage and observe the relay activation at the high cut off point.
• Then, decrease the voltage and monitor the relay deactivation at the low cut off point.

Important Safety Reminders:

• Double check all connections before applying power.
• Start with a low voltage on the power supply and gradually increase it while monitoring the circuit.
• Use components with appropriate voltage and current ratings for your application.
• If you are not comfortable working with electronics, seek help from a qualified professional.

Conclusion:

Building a safe and reliable simple DC high low voltage cut off circuit requires incorporating features like hysteresis, current limiting resistors and reverse polarity protection.

It is best to consult more comprehensive resources with improved schematics that prioritize safety before attempting construction.

References:

Automatic High Voltage and Low Voltage Cutoff Circuit

Cutoff voltage