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Q: What Are the Functions of Varistor?
A: Varistors are voltage-controlled resistors. They are common in outlet strips, Uninterruptible Power Supplies (UPS) and on electrical power lines.
Function
Varistors protect devices such as computers, entertainment centers, communications equipment and consumer applications, from current surges caused by lightning, switching and bad power from the electrical line.
Features
Varistors have extremely high resistance for normal voltages. When the voltage increases, this resistance falls, acting like a short. This causes the current surge to bypass the equipment.
Fact
Under normal conditions, varistors do not change the electrical characteristics of the circuit.
Considerations
If an outlet strip suppresses a surge, it should be replaced, as the varistors may be zapped (nonfunctional). Outlet strips equipped with surge protection have LED indicators that indicate if they need to be replaced.
Alternative
Names Other names for varistors include current suppressors, lightning arrestors, surge arrestors and surge suppressors.
Q: How do you determine correct size varistor on a 24vac coil?
A: You want the varistor to be several volts greater than the peak voltage of the signal across which it is connected. 24 volts is probably the RMS value of the coils voltage. Your varistor must be selected to be greater than the peak voltage. When the voltage exceeds the clamping voltage of the varistor, it conducts current, preventing the voltage from reaching a dangerous level. If the elevated voltage is maintained for too long a time, the varistor will heat to a point where the metals in it melt and fuse together, causing a short. This also poses the posibility of a fire risk, as the current is now conducting heavily through the varistor. This is what often happens to a varistor in a power strip when lightening strikes nearby. It renders the power strip useless. But the connected devices are usually spared.
Q: How you determine correct size varistor for protect system which work with 220v AC?
A: You use varistor wich has the max voltage for that aplication example: vacuumcleaner max allowed voltage is 240v ac so you chose aprox. 250 V varistor.
Q: How can a radial MOV meet the requirements for temperature cycle and 125℃ operating temperatures?
A: On request, Littelfuse radial MOVs can be encapsulated with a special phenolic material that withstands these harsh conditions. Special part number designations will be assigned. ML, AUML, MLE, CH and RA series parts are designed to operate from -55℃ to 125℃ without derating.
Q: Can MOVs be connected in parallel?
A: Yes. The paralleling of MOVs provides increased peak current and energy-handling capabilities for a given
application. The determination of which MOVs to use is a critical one in order to ensure that uniform current sharing occurs at high transient levels. It is recommended that we perform this screening and selection process.
Q: Can MOVs be connected in series for special voltage applications?
A: Yes. MOVs can be connected in series to provide voltage ratings higher than those normally available, or to provide ratings between the standard offerings.
Q: How are MOVs connected for single-phase and three-phase protection?
A: FOR SINGLE-PHASE AC: The optimum protection is to connect evenly rated MOVs from hot-neutral, hot-ground and neutral-ground. If this con?guration is not possible, connection between hot-neutral and hot-ground is best. FOR THREE-PHASE AC: This depends upon the 3-phase configuration.
Q: How do you determine correct size varistor on a 24vac coil?
A: You want the varistor to be several volts greater than the peak voltage of the signal across which it is connected. 24 volts is probably the RMS value of the coils voltage. Your varistor must be selected to be greater than the peak voltage. When the voltage exceeds the clamping voltage of the varistor, it conducts current, preventing the voltage from reaching a dangerous level. If the elevated voltage is maintained for too long a time, the varistor will heat to a point where the metals in it melt and fuse together, causing a short. This also poses the posibility of a fire risk, as the current is now conducting heavily through the varistor. This is what often happens to a varistor in a power strip when lightening strikes nearby. It renders the power strip useless. But the connected devices are usually spared.
Q: What is lightning protection system ?
A: A lightning protection system is a system designed to protect a structure from damage due to lightning strikes by intercepting such strikes and safely passing their extremely high voltage currents to "ground". Most lightning protection systems include a network of lightning rods, metal conductors, and ground electrodes designed to provide a low resistance path to ground for potential strikes.
Q: How lightning systems works?
A: Lightning protection systems are used to lessen damage to structures by lightning strikes. Lightning protection systems mitigate the fire hazard which lightning poses to structures. A lightning protection system provides a low-impedance path for the lightning current to lessen the heating effect of current flowing through flammable structural materials.
Q: How to Protect Your Building from Lightning:
A: 1. Install a Lightning Protection System that complies with current nationally recognized codes. Lightning protection systems consist of air terminals (lightning rods) and associated fittings connected by heavy cables to grounding equipment, providing a path for lightning current to travel safely to ground.
2. Install surge arresters at your service and telephone equipment to prevent surges from entering the home or other buildings oil power or telephone lines. Surges are diverted to ground, and both wiring and appliances are protected.
3. Install transient voltage surge suppressors in receptacles to which computers and other electronic equipment are connected in order to limit the voltage to 11/2 times the normal (maximum for solid state devices).
Misconceptions About Lightning Protection Systems:
Lightning protection systems do not attract lightning, eliminate lightning strike risk, dissipate thunderstorm charge, or provide full protection to electronic equipment. Surge protectors provide protection from more distant lightning strikes that cause system voltage surges, but can not eliminate risk to sensitive electronics from a direct structure strike.
It will definitely help to know about lightning protection systems and remove misconceptions from your mind.
Q: How to choose the right varistor for my application?
A: Let us look at an example:
Maximum Continous voltage:
10V rms and 14Vdc: can you see why? (RMS will hav 1.41 peak-to-peak voltage and this will be equivalent to 14Vdc)
Clamping voltage:
42V @ Ip 5A Ths will be voltage at which the varistor will be able to conduct up to 5A pulse (destructive if: see next parameter)
Transient energy:
5J over 1000us : This 5J can be absorbed by the VDR but only over 1000us.
and the last
Varistor voltage at 1mA: in this case it will be 18Vdc.
So, as you can see this element will be used to protect dc voltages of 14Vdc or RMS 10Vrms and will clamp pulses of up to 42V but for not longer than 1000us.
If you forse to work this MOV above these maximum parameters you have good chances to see and smell some smoke...