Security and fire alarm systems are designed to operate over a specific range of currents and voltages. If the voltage and current are out of the normal range due to a short circuit or transient overvoltage, the components in the system will be permanently damaged and the device will not function properly. Power supply and circuit traces must also be protected to avoid failure during installation or when the backup battery is shorted.
Modems in security systems are often used to alert fire departments or police in the event of an emergency. These telephone lines must be protected from lightning, power lines, or AC induced currents. The warning system must also comply with the UL864 standard, which requires that the welded fuses are not allowed to be current limited in the power supply circuit. If the security system is connected to a telephone line, it must comply with UL60950 and TIA968-A standards in North America and 21 standards in Europe and elsewhere. In security and alarm systems, fuses are typically used for overcurrent protection. However, UL864 and UL90950 are clearly over-demanding for these devices, which may cause equipment fatigue under certain test conditions. More importantly, the fuse is a one-time device that must be replaced after it has been burned.
Many device manufacturers prefer to use reusable polymer PTC devices for circuit protection, such as devices. Unlike fuses, PPTC devices do not need to be replaced after a short-circuit fault, and allow power to be re-energized after overcurrent conditions are removed to return the circuit to normal operation.
PPTC devices are composed of a composition including conductive fillers, such as carbon black fillers, etc., which make the entire device conductive. At normal temperature, the conductive particles form a very low on-resistance in the polymer. However, if the temperature rises to the switching temperature of the device, either caused by a large current through the device or caused by a sudden increase in ambient temperature, it will cause the crystallites in the polymer to melt and be in an amorphous state.
During the melting phase of the crystal, an increase in the amount of melting separates the conductive particles, resulting in a non-linear increase in the on-resistance of the device. The on-resistance is typically increased by three or more orders of magnitude. The increase in on-resistance reduces the current through the device under fault conditions to a very low, stable value, thus helping to protect the equipment in the circuit.
Since the on-resistance of the PPTC device is changed from low resistance to high resistance based on temperature changes, this device can achieve overcurrent and overtemperature protection. When current flows through the PPTC device, the current is converted to thermal power according to the power consumption formula I2R and the device is heated. When the heated temperature reaches the switching temperature, the PPTC device "trips" and enters a high on-resistance state.
The PPTC device can also be placed next to a heat-generating device that requires overheat protection or at the junction of the device. If the temperature of the device reaches the switching temperature of the PPTC device, the PPTC device will transition to a high on-resistance state. In this way, the PPTC device can be used to reduce the current to a very low level or as a system indication that the device is over temperature. The control system can then decide which appropriate action to take to protect the device or personnel.
A PPTC device is a device that is connected in series to a circuit. Their small size helps save board space compared to conventional fuses. Traditional fuses must be installed in a location that is accessible and replaceable by the user. Since PPTC devices are solid-state devices, they are able to withstand mechanical shock and vibration.
There are many ways to protect your security and fire alarm systems from transient overvoltages caused by switches or lightning. Two types of overvoltage protection methods that are commonly used are voltage clamp devices and voltage monitor devices, or devices called "". Clamping devices, such as metal oxide varistors and diodes, allow voltages up to a certain clamp level to pass through the load. Voltage monitor components, such as gas discharge tubes and thyristor surge suppressors, operate when a surge voltage that exceeds the shutdown voltage occurs.
In applications where the supply line requires overvoltage protection, the metal oxide varistor can withstand large currents, absorb energy, and respond quickly. Pairing metal oxide varistors and PPTCs allows for a complete circuit protection scheme to protect the transformers in the power supply lines and control boards.
Security and fire equipment must also be protected from electrostatic discharge. ESD protection devices can dissipate static electricity accumulated by sensitive circuits in electronic devices. PESD devices offer ESD protection and have very low capacitance, making them ideal for high frequency applications.
A new generation of reinforced polymer Zener diodes can also protect sensitive electronic devices from induced voltage spikes, transient overvoltages, incorrect power supplies, and reverse polarity. The device includes a regulated Zener diode for voltage clamping and a nonlinear PPTC thin film device.
When the device is in operation, the PPTC film is switched to a high-pass state through a low-pass state to achieve diode overheating or overcurrent protection. Under continuous high-power overvoltage conditions, the startup of the PPTC component limits the current and creates a voltage drop to effectively protect the Zener diode and subsequent electronics, as well as increasing the power capability of the diode.
The device is effective in clamping and smoothing induced voltage spikes. For induced voltage spikes, the Zener diode shunts current to ground until the voltage reaches the normal operating range. In the case of abnormal supply voltage, this type of device can clamp the voltage and shunt additional power to the ground to achieve the lock on the abnormal voltage.
The relatively flat voltage and current response of the device helps clamp the output voltage, even when the input voltage and current are completely asymmetrical. Simply placing one such device provides the same protection as a Zener diode, but can withstand very high power anomalies and eliminates the need to place any special heat sink on a typical PCB trace. Integrated devices can help device manufacturers meet the needs of UL60950, TIA-968-A, 20, and K.21 telecommunications protection in terms of circuit protection. The 2Pro device includes PPTC overcurrent protection technology and integrates a metal oxide varistor into a thermal protection device that provides current limiting under overcurrent conditions and voltage clamping under overvoltage conditions.
Overcurrent protection is achieved through the device. Overvoltage protection is achieved through metal oxide varistors and PESD devices, and integrates the protection device I/O ports and 2Pro devices described previously. Security and fire alarm systems are designed to operate over a specific range of currents and voltages. If the voltage and current are out of the normal range due to a short circuit or transient overvoltage, the components in the system will be permanently damaged and the device will not function properly. Power and circuit traces must also be protected to avoid failure during installation or when the backup battery is shorted.