MOV Surge Energy

E = 0.5 × C × (V_clamp² − V_supply²)

Quick answer

This calculator solves E = 0.5 × C × (V_clamp² − V_supply²) for Surge Energy. With Effective Capacitance 1 mF, Clamping Voltage 120 V, Supply Voltage 56.4 V, the result is 5.61 J. It solves in 2 directions, so you can enter the values you have and get the one you need.

Calculator

Surge Energy (J)
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Formula

E = 0.5 × C × (V_clamp² − V_supply²)

Description

Metal oxide varistors (MOVs) absorb surge energy by clamping the voltage across them and dissipating the energy as heat. The energy absorbed during a surge event is approximated by the capacitive energy difference between the clamping voltage and the normal supply voltage. This calculation helps select an MOV with sufficient energy rating for the expected surge environment. The MOV must absorb this energy without exceeding its single-pulse or cumulative energy rating, which degrades over multiple surge events.

Variables

  • E — Energy absorbed by the MOV (J)
  • C — Effective circuit capacitance during the surge (F)
  • V_clamp — MOV clamping voltage (V)
  • V_supply — Normal supply voltage (V)

Practical Notes

For AC mains protection, MOVs are rated by their maximum continuous operating voltage (MCOV), which must exceed the highest expected line voltage including tolerances (e.g., 275V MCOV for 230V mains). MOV energy ratings range from 1-10 J for signal-level protection to 100-1000+ J for mains surge protection. MOVs degrade with each surge event; GDTs (gas discharge tubes) and TVS diodes are preferred where longevity matters.

Symbols and units

Symbol Meaning Default unit Accepted units
E Surge Energy J µJ, mJ, J, kJ, Wh, kWh
C Effective Capacitance µF pF, nF, µF, mF, F
Vclamp Clamping Voltage V pV, nV, µV, mV, V, kV
Vsupply Supply Voltage V pV, nV, µV, mV, V, kV

Step by step

  1. 1. What you know: Effective Capacitance 1 mF, Clamping Voltage 120 V, Supply Voltage 56.4 V
  2. 2. The relation: E = 0.5 × C × (V_clamp² − V_supply²)
  3. 3. Substitute: C = 1 mF, Vclamp = 120 V, Vsupply = 56.4 V
  4. 4. Result: Surge Energy = 5.61 J

Worked examples

Minimum range

Given: Effective Capacitance 1 µF, Clamping Voltage 3.3 V, Supply Voltage 1.55 V

Result: Surge Energy = 4.244 µJ

Low range

Given: Effective Capacitance 10 µF, Clamping Voltage 33 V, Supply Voltage 15.5 V

Result: Surge Energy = 4.244 mJ

Typical

Given: Effective Capacitance 1 mF, Clamping Voltage 120 V, Supply Voltage 56.4 V

Result: Surge Energy = 5.61 J

High range

Given: Effective Capacitance 10 mF, Clamping Voltage 2.3 kV, Supply Voltage 1.08 kV

Result: Surge Energy = 5.727 Wh

Maximum range

Given: Effective Capacitance 100 mF, Clamping Voltage 23 kV, Supply Voltage 10.8 kV

Result: Surge Energy = 5.727 kWh

Solving for each variable

Solve for Surge Energy

Effective Capacitance 1 µF, Clamping Voltage 3.3 V, Supply Voltage 1.55 V → 4.244 µJ

Solve for Effective Capacitance

Surge Energy 100 µJ, Clamping Voltage 0.33 V, Supply Voltage 0.726 V → -0.4783 mF

The same result in other units

Unit Value
µJ 5610000
mJ 5610
J 5.61
kJ 0.00561
Wh 0.001558

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Frequently asked questions

How do you calculate MOV Surge Energy?

Use E = 0.5 × C × (V_clamp² − V_supply²). With Effective Capacitance 1 mF, Clamping Voltage 120 V, Supply Voltage 56.4 V, Surge Energy works out to 5.61 J.

What is the formula for MOV Surge Energy?

The formula is E = 0.5 × C × (V_clamp² − V_supply²). Metal oxide varistors (MOVs) absorb surge energy by clamping the voltage across them and dissipating the energy as heat.

How do you find Effective Capacitance when you know the other values?

Rearrange for Effective Capacitance: with Surge Energy 100 µJ, Clamping Voltage 0.33 V, Supply Voltage 0.726 V, Effective Capacitance is -0.4783 mF.

What units does MOV Surge Energy use?

Surge Energy is shown in µJ, mJ, J, kJ, Wh. Values are held in SI base units internally, so you can mix prefixes without converting by hand.

Related Concepts

All Safety / Protection formulas →
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