What is Over Voltage Protection?
- Over voltage protection is a power supply feature that shuts down or clamps the output if the voltage exceeds a predetermined level.
- To protect electronic components from damage, most power supplies employ an over-voltage protection circuit. Over-voltage conditions have varying effects depending on the circuit, ranging from damaging components to degrading components and causing circuit malfunctions or fire.
- An over-voltage condition may occur in the power supply as a result of internal faults or external causes such as those in the distribution lines.
- When designing effective protection, the magnitude and duration of the overvoltage are important factors to consider. Setting a threshold voltage above which the control circuit shuts down the supply or diverts the extra voltage to other parts of the circuit, such as a capacitor, is part of the protection.
Aspects of Analog Power Supply Failure
- For many analog regulated supplies, the series pass transistor can fail, resulting in a short circuit between the collector and emitter.
- If this occurs, the full unregulated voltage can be seen at the output, and many analog series regulators require an input voltage that is 1.5 to 2 times the output voltage.
- If this occurred, an unacceptably high voltage would be applied to the entire system, causing many ICs and other components to fail.
- When considering the voltages involved, it is easy to see why overvoltage protection is so critical. A typical power supply may deliver 5 volts of stabilized power to logic circuitry. The input voltage to the power supply regulator may be in the range of 10 to 15 volts to provide adequate stabilization, ripple rejection, and the like.
- Even 10 volts would be enough to destroy many modern chips, especially the more expensive and complicated ones. As a result, preventing this is critical.
Crowbar Circuit Diagram
- A crowbar circuit’s circuit diagram is very simple and easy to build and implement, making it a cost-effective and quick solution. The crowbar circuit diagram in its entirety is shown below.
- The input voltage (blue probe) is the voltage that must be monitored in this case, and the circuit is designed to cut off the power supply if the supply voltage exceeds 9.1V. In the working section that follows, we will go over each component’s function.
Crowbar Circuit Operation
- When the input voltage exceeds the limit, the Crowbar circuit creates a short circuit across the power lines and blows the fuse. When the fuse blows, the power supply is disconnected from the load, protecting it from high voltage. The circuit operates by establishing a direct short circuit across the power lines as if a crowbar were dropped between the circuit’s power lines. As a result, it is known as the crowbar circuit.
- The voltage over which the circuit should short is determined by the Zener voltage. The circuit consists of an SCR that is directly connected across the circuit’s input voltage and ground, but this SCR is kept turned off by grounding the SCR’s gate pin. When the input voltage exceeds the Zener voltage, the Zener diode begins to conduct, supplying a voltage to the SCR’s Gate pin, causing it to close the connection between the Input Voltage and Ground, resulting in a short circuit. This short circuit will draw the maximum amount of current from the power supply and blow the fuse, disconnecting the power supply from the load.
- The Zener Diode in this case is rated for 9.1V, but the input voltage has exceeded that value and is now at 9.75V. As a result, the Zener Diode opens and begins to conduct by applying a voltage to the SCR’s Gate pin. The SCR then begins to conduct by shorting the Input voltage and Ground, causing the fuse to blow due to maximum current draw.
- Fuse: In this circuit, the fuse is essential. The fuse rating should always be less than the SCR’s maximum current rating but greater than the current consumed by the load. We should also ensure that the power supply can supply enough current to blow the fuse if it fails.
- 0.1uF Capacitor: This is a filtering capacitor; it removes spikes and other noise like harmonics from the supply voltage to keep the circuit from triggering incorrectly.
- 9.1V Zener Diode: This diode determines the overvoltage value; because we used a 9.1V Zener diode, the circuit will respond to any voltage greater than 9.1V. The Designer can select the value of this resistor based on his requirements.
- 1K Resistor: This is simply a pull-down resistor that connects the SCR’s Gate pin to the ground, keeping it turned off until the Zener begins to conduct.
- 47nF Capacitor: Every power switch, such as an SCR, requires a snubber circuit to suppress voltage spikes during switching and prevent false triggering of the SCR. In this case, we simply used a capacitor to accomplish the task. The capacitor value should be just enough to filter the noise because a high capacitance value will increase the delay before the SCR begins to conduct after the Gate pulse is applied.
- Thyristor (SCR): The Thyristor is in charge of causing a short circuit between the power rails. Care should be taken to ensure that the SCR can handle such high current values without blowing the fuse or damaging itself. The SCR’s gate voltage should be less than the Zener breakdown voltage.
- Schottky Diode: This diode is optional and is only used for protection. It ensures that no reverse current flows from the load side, which could damage the protection circuit. Because it has less voltage drop across it, a Schottky diode is used instead of a regular diode.
Crowbar Circuit Limitations
Even though this power supply overvoltage protection circuit is widely used, it has some limitations.
- A simple Crowbar Circuit is extremely useful for overvoltage protection and is thus an essential component of bench power supplies. Even though this circuit is extremely useful, some limitations must be noted.
- The trigger voltage in a Thyristor-based design is set by the Zener Diode and is usually not adjustable. As a result, selecting the proper Zener Diode is critical. The circuit’s trigger voltage should be slightly higher than the power supply’s output voltage so that spikes and noise do not accidentally trigger it.
- When the power supply is used in RF Designs, such as an RF Transmitter, proper power line filtering before and after the transmitter is required.
- Overvoltage causes the circuit to trip and blow the fuse. As a result, fuses must be replaced whenever an overvoltage occurs.
