* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Circuit Protection Solutions for Security and Fire Alarm Systems
Telecommunications engineering wikipedia , lookup
Electromagnetic compatibility wikipedia , lookup
Power over Ethernet wikipedia , lookup
Fault tolerance wikipedia , lookup
Electrical ballast wikipedia , lookup
Variable-frequency drive wikipedia , lookup
Power inverter wikipedia , lookup
Electric power system wikipedia , lookup
Immunity-aware programming wikipedia , lookup
Electronic engineering wikipedia , lookup
Power engineering wikipedia , lookup
Ground (electricity) wikipedia , lookup
Current source wikipedia , lookup
History of electric power transmission wikipedia , lookup
Voltage regulator wikipedia , lookup
Distribution management system wikipedia , lookup
Electrical substation wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Stray voltage wikipedia , lookup
Semiconductor device wikipedia , lookup
Buck converter wikipedia , lookup
Power MOSFET wikipedia , lookup
Alternating current wikipedia , lookup
Electrical wiring in the United Kingdom wikipedia , lookup
Voltage optimisation wikipedia , lookup
Network analysis (electrical circuits) wikipedia , lookup
Earthing system wikipedia , lookup
Mains electricity wikipedia , lookup
APPLICATION NOTE Circuit Protection Solutions for Security and Fire Alarm Systems Security and fire alarm systems are designed for operation within specified current and voltage ratings. If these ratings are exceeded, due to short-circuit or voltage transients, components may sustain permanent damage and the equipment may fail. Power supplies and circuit traces must also be protected against faults that may occur during installation or in the case of a shorted back-up battery. Modems are frequently utilized in security systems to automatically alert the fire or police department in an emergency. These telecom lines must be protected from damage caused by lightning strikes, power-line crosses or AC power induction. Alarm systems must comply with the UL864 standard, which mandates that soldered-in fuses are not allowed to qualify a power supply as inherently limited. If the system has provisions for connection to a telephone line it must comply with UL60950 and TIA 968-A in North America, as well as ITU K.21 in Europe and other parts of the world. Overcurrent Protection Options Fuses have typically been used for overcurrent protection in Many equipment manufacturers prefer resettable polymeric security and alarm systems. However, UL864 and UL60950 positive temperature coefficient (PPTC) device protection. pose difficult challenges for these devices, which can fatigue Unlike fuses, these devices generally do not require under certain test conditions. More importantly, fuses are replacement after a fault event, and allow the circuit to return single-use components that must be replaced after a fault to normal operating condition after the power has been occurs. cycled and the overcurrent condition is eliminated. PPTC Device Principle of Operation PPTC devices are made from a composite of semi-crystalline The resistance typically increases by three or more orders of polymer and conductive particles. At normal temperature, magnitude. This increased resistance helps protect the the conductive particles form low-resistance networks in the equipment in the circuit by reducing the amount of current polymer (Figure 1). However, if the temperature rises above that can flow under the fault condition to a low, steady-state the device’s switching temperature (Tsw) either from high level. The device remains in its latched (high-resistance) current through the part or from an increase in the ambient position until the fault is cleared and power to the circuit is temperature, the crystallites in the polymer melt and become cycled – at which time the conductive composite cools and amorphous. The increase in volume during melting of the re-crystallizes, restoring the PPTC to a low-resistance state in crystalline phase separates the conductive particles resulting the circuit and the affected equipment to normal operating in a large non-linear increase in the resistance of the device. conditions. www.circuitprotection.com Figure 1. PPTC devices protect the circuit by going from a low-resistance state to a high-resistance state in response to an overcurrent or overtemperature condition. Because PPTC devices transition to their high-impedance current to the equipment to very low levels, or as an indicator state, based on the influence of temperature, they help to the control system that the equipment is overheating. The provide protection for two fault conditions – overcurrent and control system can then determine what action is appropriate overtemperature. Overcurrent protection is provided when the PPTC device’s internal temperature rises due to I2R to protect equipment and personnel. heating within the device. High current levels through the PPTC device heat it internally to its switching temperature causing it to “trip” and go into a high-impedance state. Tyco Electronics’ PolySwitch PPTC devices are employed as series elements in a circuit. Their small form factor helps conserve valuable board space and, in contrast to traditional fuses that require user-accessibility, their resettable The PPTC device can also be caused to trip by thermally functionality allows for placement in inaccessible locations. linking it to a component or equipment that needs to be Because they are solid-state devices, they are also able to protected against overtemperature conditions. If the withstand mechanical shock and vibration. equipment temperature reaches the PPTC device’s switching temperature, the PPTC device will transition to its highimpedance state, regardless of the current flowing through it. In this way, the device can be used either to reduce the The PolySwitch LVR series of devices includes components that are rated for line voltages of 120 VAC and 240 VAC, for up to 2A of operating current at 20°C. They offer low resistance, fast time-to-trip, and a low-profile form factor. Overvoltage Protection Considerations A variety of methods can be used to help protect security devices remain at the clamping voltage. Therefore, the power and fire alarm systems from overvoltage conditions caused dissipated in the foldback device is much lower than in a by switching or lightning transients. There are two major clamping device. This allows the use of a much smaller device categories of overvoltage protection devices – clamping to conduct the same amount of surge current. devices and foldback, or crowbar, devices. Clamping devices, such as metal oxide varistors (MOVs) and diodes, allow voltages up to a specified clamping level to pass through to the load during operation. Foldback devices, such as gas For many applications, Tyco Electronics’ SiBar thyristor surge suppressor is the preferred solution. The results of recent testing by Tyco Electronics comparing the behavior of a TVS diode with that of a SiBar thyristor are shown in Figure 2. discharge tubes (GDTs) and thyristor surge suppressors, operate as shunt devices in response to a surge that exceeds SiBar thyristor surge suppression devices meet the immunity the breakover voltage. and test requirements for PoE (Power over Ethernet) equipment, and their low on-state voltage allows for smaller In some applications, foldback devices have an advantage over clamping devices because in the foldback state very little voltage appears across the load while it conducts harmful surges away from the load; whereas clamping form factor devices – as compared with clamping devices of comparable energy-handling capability. The relatively low capacitance of SiBar thyristors also makes them useful for high-data-rate circuits. TVS: 1.2/50, 8/20ms Combination Waveform, 40 Ohm Source Impedance 1 kVoc, Surge 10 Voltage [SiBar] Spare SiBar: 1.2/50, 8/20ms Combination Waveform, 40 Ohm Source Impedance 1 kVoc, Surge 10 Current [Through Circuit] Voltage [SiBar] Spare Current [Through Circuit] 140 25 140 25 120 120 60 10 Voltage [V] 15 Current [A] Voltage [V] 80 40 100 80 15 60 10 Current [A] 20 20 100 40 5 5 20 20 0 0 50 100 150 0 0 250 200 0 50 100 Time [ µseconds] 150 200 0 250 Time [ µseconds] TVS Diode Behavior SiBar Thyristor Behavior Device Pk (I) Pk (V) Avg (I) Avg (V) TVS Diode 23.4 124.8 4.37 74.19 SiBar TVB058SA-L 23.8 89.6 5.41 25.33 Figure 2. Performance comparison of TVS diode and SiBar thyristor. MOVs also offer high-current-handling and energy- Security and fire alarm equipment must also be protected absorption capability, fast response, and low cost, making against damage caused by electrostatic discharge (ESD). To them suitable for overvoltage protection in line voltage help shunt ESD away from sensitive circuitry in electronic applications. Pairing Tyco Electronics’ ROV device with a devices, Tyco Electronics’ PESD devices offer a very low PolySwitch PPTC device can help provide a completely capacitance of 0.2 pF, which is useful in high-frequency resettable circuit protection solution for power supplies and applications. control board transformers. Hybrid Circuit Protection Options A generation of polymer-enhanced Zener diodes can also In operation, the PPTC layer responds to either diode heating help protect sensitive electronics from damage caused by or overcurrent events by transitioning from a low to high inductive voltage spikes, voltage transients, use of incorrect resistance state. In the event of a sustained high-power power supplies and reverse bias. As shown in Figure 3, Tyco overvoltage condition, the tripped PPTC element limits Electronics’ PolyZen device incorporates a stable Zener diode current and generates voltage drop to help protect both the for crisp voltage clamping and a resistively non-linear PPTC Zener and the follow on electronics – effectively increasing layer. the diode’s power-handling capability. Power Supply PolyZen Protected Electronics (External or internal) VIN + 2 1 PolyZen Device GND VOUT 3 Regulated Output Figure 3. Polymer-enhanced Zener diode helps provide input power protection for portable electronics. RLOAD Protected downstream electronics This device is particularly effective at clamping and Figure 4, Tyco Electronics’ 2Pro device incorporates PPTC smoothing inductive voltage spikes. In response to an overcurrent technology with an MOV component into one inductive spike, the Zener diode element shunts current to thermally protected device that helps provide current limiting ground until the voltage is reduced to the normal operating during overcurrent events and voltage clamping during range. In the case of a wrong voltage power supply, the overvoltage events. device clamps the voltage, shunts excess power to ground, and eventually locks out the wrong supply. This hybrid device approach helps reduce component count and improve equipment reliability by preventing thermal The relatively flat voltage vs. current response of the runaway and maintaining varistor surface temperature at less polymer-enhanced Zener diode helps clamp the output than 150°C, thereby preventing the device from reaching voltage, even when input voltage and source currents vary. unsafe temperatures that may result from overvoltage Simply put, the polymer-enhanced device helps provide transients. The device’s small footprint, resettable coordinated protection with a component that protects like a functionality and coordinated protection capabilities make it Zener diode, but is capable of withstanding very high-power suitable for a wide range of telephony and security system fault conditions without requiring any special heat sinking applications. structures beyond normal PCB traces. Tyco Electronics offers a broad range of devices that help Integrating PPTC technology with an MOV has resulted in provide overcurrent and overvoltage protection in security another circuit protection device that can help equipment and fire alarm systems. All of the circuit protection devices manufacturers meet the UL60950, TIA-968-A, and ITU K.20 discussed in this paper are employed in the circuit diagram and K.21 telecom protection requirements. shown in Figure 5. Figure 4. Tyco Electronics’ 2Pro device helps protect against damage caused by both overcurrent and overvoltage events. As shown in Figure 5. Coordinated circuit protection options for security and fire alarm systems. Raychem Circuit Protection Products 308 Constitution Drive, Building H Menlo Park, CA USA 94025-1164 Tel : (800) 227-7040, (650) 361-6900 Fax : (650) 361-4600 2Pro, PolySwitch, PolyZen, Raychem, SiBar, TE logo and Tyco Electronics are trademarks. All information, including illustrations, is believed to be reliable. Users, however, should independently evaluate the suitability of each product for their application. Tyco Electronics Corporation makes no warranties as to the accuracy or completeness of the information, and disclaims any liability regarding its use. Tyco Electronics’ only obligations are those in the Tyco Electronics Standard Terms and Conditions of Sale for this product, and in no case will Tyco Electronics be liable for any incidental, indirect, or consequential damages arising from the sale, resale, use, or misuse of the product. Specifications are subject to change without notice. In addition, Tyco Electronics reserves the right to make changes without notification to Buyer—to materials or processing that do not affect compliance with any applicable specification. © 2008 Tyco Electronics Corporation. All rights reserved. RCP0046E.1008 www.circuitprotection.com www.circuitprotection.com.hk (Chinese) www.tycoelectronics.com/japan/raychem (Japanese)