I will provide you with a detailed analysis of the selection parameters for varistors, with original content and clear logic. The core selection parameters of varistors mainly include the following seven aspects:
1、 Varistor Voltage
Definition: The voltage value at both ends of a varistor under a specific current (usually 1mA DC) is the threshold voltage that triggers protection.
Key selection points:
Calculation benchmark: It needs to be higher than the peak value of the maximum operating voltage of the circuit.
Communication system: Voltage sensitive ≥ (2.2~2.5) × AC effective value (e.g. 430V~550V for 220V AC system).
DC system: Voltage sensitive ≥ (1.6~2.0) × DC rated voltage.
Margin design: It is necessary to consider power grid fluctuations (usually taking a safety factor of 1.2-1.4 times).
2、 Maximum Continuous Operating Voltage
Definition: The allowed long-term application of AC effective value or DC voltage determines the lifespan of the device.
Key selection points:
The actual working voltage needs to be lower than this value, otherwise it will accelerate aging.
The selection level of poor grounding scenarios needs to be increased (such as phase to ground protection being higher than phase to phase protection).
3、 Maximum Limiting Voltage/Residual Voltage
Definition: The peak voltage at both ends when subjected to a large surge current (such as an 8/20 μ s waveform).
Criticality:
The residual voltage must be less than the withstand voltage value of the protected equipment, otherwise the protection will fail.
The smaller the residual voltage ratio (residual voltage/varistor voltage), the better the nonlinear characteristics (ideal value close to 1).
4、 Surge Current Capacity
Definition: The ability to withstand a single surge current (standard 8/20 μ s waveform test).
Selection strategy:
Redundant design: When it is difficult to accurately calculate actual surges, a margin of at least 2 times should be reserved.
Parallel expansion: Connecting multiple units of the same specification in parallel can increase the flow rate (ensuring consistent characteristics).
5、 Energy Absorption
Definition: The energy absorbed by a single pulse, calculated as W=k · I · V · T (where k is the waveform coefficient).
Scene adaptation:
High energy type: used for inductive energy storage and release (such as motor coils).
High power type: Suitable for high-frequency repetitive pulses (such as switching power supplies).
6、 Leakage current and static parameters
Leakage current:
The DC current under 75% voltage sensitivity affects standby energy consumption, usually requiring<100 μ A.
Static capacitance:
Inherent capacitance value (pF~nF level), high-frequency circuits require low capacitance models (such as SMD varistors).
Temperature coefficient:
The temperature coefficient of voltage/current affects high temperature stability, and industrial environments require<0.1%/℃.
7、 Response time and frequency adaptability
Response time: nanosecond level (usually<25ns), but lead inductance may prolong the actual response time.
Installation requirements: The lead wires should be extremely short (<10cm), and PCB layout should be prioritized for mounting.
Frequency limit:
Power frequency (50/60Hz): applicable to all types.
High frequency>1MHz: Low capacitance models or TVS diodes should be selected.
Summary of Selection Process
Determine circuit parameters: operating voltage (AC/DC), surge source type (lightning strike/switch surge).
Calculate core parameters: voltage sensitivity, residual voltage requirements, and current requirements.
Adapt to environmental factors: temperature range, installation space (size Φ 5mm~Φ 53mm).
Verify secondary parameters: leakage current (energy-saving design), capacitance (high-frequency circuit).
Key design principles: Varistor voltage is the foundation, residual voltage is the core criterion for protection effect, and current capacity determines reliability. When using multi-level protection, varistors are often used as a buffer in the front stage, and fast response devices (such as TVS tubes) are required in the back stage.