Varistor
A model of a voltage-dependent resistor.
Description
The Varistor block is a voltage dependent resistor (VDR) . This component is also commonly known as a metal oxide varistor (MOV). The block has high resistance at low voltages and low resistance at high voltages.
You can protect parts of an electrical circuit from voltage spikes by placing this block in parallel with them. When a voltage surge occurs, the resistance of the varistor drops significantly, causing current to flow through the varistor instead of through the circuit.
Use the Parameterization parameter to select one of two options for the behaviour of this block. The Linear option is based on the on and off states of the varistor and uses a linear relationship between current and voltage in both regions. The Power-law option uses an exponential relationship between current and voltage in the initial on state. This option also adds a third, linear region at higher voltages.
Linear parameterization
This parameterization option divides the voltage-current relationship into two linear regions:
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Off region - the resistance is high and the current slowly increases with increasing voltage.
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On region - the resistance is low and the current increases rapidly with increasing voltage.
This figure shows the voltage/current relationship in the on and off regions.
Use linear parameterization in one of these scenarios:
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You are modelling voltage spikes near a threshold voltage.
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You expect your varistor to behave linearly in all areas.
The voltage-current relationship for a linear varistor is as follows:
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Where:
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and are the varistor voltage and current, respectively.
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- is the threshold voltage separating the two areas of operation. Set this value using the Clamping voltage parameter.
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and - resistances in the switched on and switched off regions. Set these values using the On resistance and Off resistance parameters respectively.
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- constant used to ensure current continuity between the two regions:
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Power law parameterization
This parameterization variant divides the voltage-current dependence into three regions:
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Leakage region - the resistance is high and the current increases slowly with increasing voltage.
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Normal region - resistance decreases exponentially with increasing voltage.
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Rise area - resistance is low and current rises rapidly with increasing voltage.
This figure shows the three areas of operation on a logarithmic scale.
Use power law parameterization in one of these scenarios:
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You are modelling voltage spikes over a large voltage range.
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You expect your varistor to behave exponentially in the first turn-on region.
The voltage-current dependence for a power law varistor is as follows:
where:
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and are the varistor voltage and current, respectively.
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- power-law exponent, which defines the rate of current increase with voltage increase in normal-mode. Set this value using the Normal-mode power-law exponent parameter.
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and are the threshold voltages corresponding to the leak-normal-mode and normal-mode-rise transition points. Set these values using the Leakage to normal voltage transition and Normal to upturn voltage transition parameters respectively.
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and - resistances in the leakage and upturn regions. Set these values using the Leakage-mode resistance and Upturn-mode resistance parameters respectively.
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, and are constants used to ensure current continuity between regions:
,
,
и
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Parameters
Main
Parameterization - varistor operating mode
Linear (by default) | Power-law
Select how the varistor resistance changes with increasing voltage:
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Linear- two regions. The low voltage region has a high resistance and the high voltage region has a low resistance. -
Power-law- three regions. The leakage region has high resistance. Normal region has exponentially decreasing resistance. The rise region has a low resistance.
Clamping voltage - threshold voltage
260 V (by default) | positive number.
Transition point voltage, , between the off and on states of the linear varistor.
Dependencies
Used when the Parameterization parameter is set to Linear.
Off resistance - resistance of the switched off area
3e8 ohms (by default) | positive number.
Resistance at low voltage, , of varistor in off state.
Dependencies
Used when the Parameterization parameter is set to Linear.
On resistance - resistance of the included area
1 ohm (by default) | positive number.
Resistance at high voltage, , of varistor in on state.
Dependencies
Used when the Parameterization parameter is set to Linear.
Leakage to normal voltage transition - first threshold voltage
130 V (by default) | positive number
Transition point voltage, , between the leakage region and the normal region of a power law varistor.
Dependencies
Used when the Parameterization parameter is set to Power-law.
Normal to upturn voltage transition - second threshold voltage
300 V (by default) | positive number.
Transition point voltage, , between the normal and upturn regions of the varistor with power law.
Dependencies
Used when the Parameterization parameter is set to Power-law.
Leakage-mode resistance - resistance in the leakage area
3e8 ohms (by default) | positive number.
Leakage-mode resistance, , of the varistor.
Dependencies
Used when the Parameterization parameter is set to Power-law.
Normal-mode power-law exponent - resistance in the normal region
45 (By default) | positive number.
An exponent that determines the rate of current increase as the varistor voltage increases in the normal-mode power-law region.
Dependencies
It is used if the Parameterization parameter is set to Power-law.
Upturn-mode resistance - resistance in the rise region
0.07 ohms (by default) | positive number.
Resistance at high voltage, , of the varistor in the rise region.
Dependencies
Used when the Parameterization parameter is set to Power-law.
Terminal resistance - terminal resistance
100e-6 ohms (by default) | non-negative number.
A small permanent resistance in series with the varistor. Set this value to zero to remove the resistance from the equivalent circuit.
Capacitance - parallel capacitance
4.4 nF (by default) | non-negative number.
Parasitic capacitance in parallel with the varistor. Set this value to zero to remove the capacitor from the equivalent circuit.