Common Mode Choke Impedance
Z = V_noise / I_limit
Quick answer
This calculator solves Z = V_noise / I_limit for Required Impedance. With Noise Voltage 12 V, Noise Current Limit 2 A, the result is 6 Ω. It solves in 3 directions, so you can enter the values you have and get the one you need.
Calculator
Formula
Description
A common mode choke must present enough impedance to suppress common mode noise current below the required limit. This simple relationship helps determine the minimum choke impedance needed at the frequency of concern. Common mode noise appears as identical signals on both conductors of a pair, typically caused by parasitic capacitance coupling to the chassis or ground plane. The choke presents high impedance to common mode signals while passing differential signals with minimal impedance, making it ideal for power line and data cable filtering.
Variables
- Z — Required choke impedance at the noise frequency (Ω)
- V — Common mode noise voltage (V)
- I — Maximum allowable noise current (A)
Practical Notes
Common mode choke impedance is specified at specific frequencies in datasheets. Select a choke whose impedance curve covers your noise frequency range. Typical values range from 100Ω to 10kΩ depending on the application. For USB, Ethernet, and HDMI interfaces, choose chokes with high impedance at the noise frequencies but low insertion loss in the signal passband.
Symbols and units
| Symbol | Meaning | Default unit | Accepted units |
|---|---|---|---|
| Z | Required Impedance | Ω | mΩ, Ω, kΩ, MΩ |
| V | Noise Voltage | V | pV, nV, µV, mV, V, kV |
| I | Noise Current Limit | A | pA, nA, µA, mA, A, kA |
Step by step
- 1. What you know: Noise Voltage 12 V, Noise Current Limit 2 A
- 2. The relation: Z = V_noise / I_limit
- 3. Substitute: V = 12 V, I = 2 A
- 4. Result: Required Impedance = 6 Ω
Related quantities from the same inputs
- P = V × I = 24 W
Nearest standard component values
| Series | Nearest value | Error |
|---|---|---|
| E12 | 5.6 Ω | 6.7% |
| E24 | 6.2 Ω | 3.3% |
Worked examples
Minimum range
Given: Noise Voltage 0.33 V, Noise Current Limit 2 mA
Result: Required Impedance = 0.165 kΩ
Low range
Given: Noise Voltage 3.3 V, Noise Current Limit 20 mA
Result: Required Impedance = 0.165 kΩ
Typical
Given: Noise Voltage 12 V, Noise Current Limit 2 A
Result: Required Impedance = 6 Ω
High range
Given: Noise Voltage 0.23 kV, Noise Current Limit 100 A
Result: Required Impedance = 2.3 Ω
Maximum range
Given: Noise Voltage 2.3 kV, Noise Current Limit 1 kA
Result: Required Impedance = 2.3 Ω
Solving for each variable
Solve for Required Impedance
Noise Voltage 0.33 V, Noise Current Limit 2 mA → 0.165 kΩ
Solve for Noise Voltage
Required Impedance 1 Ω, Noise Current Limit 2 mA → 2 mV
Solve for Noise Current Limit
Required Impedance 1 Ω, Noise Voltage 0.33 V → 0.33 A
How Required Impedance changes with Noise Voltage
| Noise Voltage (input) | Required Impedance (result) |
|---|---|
| 1.2 V | 0.6 Ω |
| 3 V | 1.5 Ω |
| 6 V | 3 Ω |
| 9 V | 4.5 Ω |
| 12 V | 6 Ω |
| 18 V | 9 Ω |
| 24 V | 12 Ω |
| 48 V | 24 Ω |
| 84 V | 42 Ω |
| 120 V | 60 Ω |
The same result in other units
| Unit | Value |
|---|---|
| mΩ | 6000 |
| Ω | 6 |
| kΩ | 0.006 |
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<p><a href="https://vbatpower.com/formulas/cm-choke-impedance/">Common Mode Choke Impedance</a> Calculator by vbatpower.com</p>Frequently asked questions
How do you calculate Common Mode Choke Impedance?
Use Z = V_noise / I_limit. With Noise Voltage 12 V, Noise Current Limit 2 A, Required Impedance works out to 6 Ω.
How do you calculate common mode current choke impedance?
Rearrange for Required Impedance: with Noise Voltage 3.3 V, Noise Current Limit 20 mA, Required Impedance is 0.165 kΩ.
What is the formula for Common Mode Choke Impedance?
The formula is Z = V_noise / I_limit. A common mode choke must present enough impedance to suppress common mode noise current below the required limit.
How do you find Noise Voltage when you know the other values?
Rearrange for Noise Voltage: with Required Impedance 1 Ω, Noise Current Limit 2 mA, Noise Voltage is 2 mV.
How do you find Noise Current Limit when you know the other values?
Rearrange for Noise Current Limit: with Required Impedance 1 Ω, Noise Voltage 0.33 V, Noise Current Limit is 0.33 A.
What units does Common Mode Choke Impedance use?
Required Impedance is shown in mΩ, Ω, kΩ. Values are held in SI base units internally, so you can mix prefixes without converting by hand.
Related Concepts
All EMC / EMI formulas →Need more features?
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