Ohmite EBW Current Sensing

Countermeasure:

Check the furnace temperature curve to ensure that the actual temperature of the resistor does not exceed 300 ℃ (with margin).

Verify using a temperature measuring board. If the temperature cannot be lowered, the EBW series with higher temperature resistance can be used instead (which is already good, or consider using other thick film resistors).

Pulse capability and surge protection

5.1 Interpretation of Pulse Energy Curve

The vertical axis of the double logarithmic curve in the PDF is Pulse Energy (J), and the horizontal axis is Time (millisecond). The curve provides the maximum allowable pulse energy (single, non repetitive) for different resistance values.

Actual calculation example:

Assuming the peak current surge in the circuit is 500 A, lasting for 2 ms, flowing through the EBWA-M 0.5 m Ω resistor. Pulse energy E=I ² × R × t=500 ² × 0.0005 × 0.002=0.25 J. Query curve: Approximately 30 J is allowed at 2 ms for 0.5 m Ω, which is much greater than 0.25 J and safe.

If it is a 4 m Ω model, under the same 500 A surge, E=500 ² × 0.004 × 0.002=2 J. However, the 4 m Ω curve allows for approximately 2 J at 2 ms, which happens to be at the boundary and requires caution.

5.2 Repetitive pulse

The PDF curve does not specify the ability to repeat pulses, but engineering experience shows that the average power of repeated pulses should not exceed the rated power (after derating), and the energy of each pulse should be less than 50% of the single pulse limit to ensure longevity.

5.3 Common faults: Surge causing resistance explosion

Phenomenon: At the moment of power on, the current detection resistor explodes or has an infinite resistance value.

Reason: The charging of the front-end capacitor generates extremely high peak currents (such as thousands of amperes), and although the time is very short, the energy exceeds the absorption limit of the resistor.

Countermeasure:

Use an oscilloscope in conjunction with a Rogowski coil to measure the actual surge current.

Calculate energy and compare it with the curve. If it exceeds the limit, NTC thermistor or pre charging resistor can be connected in series to limit the surge.

The EBWB 0.2 m Ω model is selected, which has the highest pulse energy capability.

Engineering Application Q&A

Q1: Can the EBW series be used for AC current detection (such as 50/60 Hz)?

A: Okay. Manganin and NiCr alloys are both non-magnetic materials with inductance<3 nH, and have no inductive impedance effect on power frequency and kHz level currents. However, it should be noted that the effective value of AC current corresponds to heating, and the temperature rise should be calculated based on the derating curve.

Q2: How to improve the accuracy of current measurement?

A：

Adopting Kelvin connection.

Select low bias voltage and low temperature drift models for operational amplifiers (such as ± 0.5 μ V/℃).

Pay attention to the resistance TCR: For Manganin (± 75 ppm/℃), the resistance changes by 0.75% at a temperature rise of 100 ℃. If higher accuracy is required, temperature compensation can be done in the software.

Perform single point calibration before use (such as adding 50 A current and recording ADC values).

Q3: Can EBWA and EBWB be used in parallel to expand current?

A: Parallel connection is possible, but due to the dispersion of resistance values, it is recommended to independently connect a small inductor (such as a magnetic bead) in series with each resistor or use a matching resistor. The simplest method is to use a higher power single resistor, EBWB-M 0.2 m Ω already supports 180 A continuous current, which is usually sufficient.

Q4: Is the resistance stable after being stored for many years?

A: The EBW series uses metal alloys with no aging mechanism. However, long-term exposure to high humidity and corrosive gases may cause oxidation of copper terminals, affecting weldability. It is recommended to store in a dry, sulfur free environment.

Alternative and upgrade suggestions

When a certain model of the EBW series is discontinued or difficult to purchase, the following alternative solutions can be considered:

Recommended Precautions for EBW Model Replacement

EBWA-M 0.5 m Ω Vishay PMA2415 or Bourns CSM2F inspection package size and TCR

EBWA-N 2-4 m Ω Isabellenh ü tte BVS or PBV series, pay attention to power and pulse capability

EBWB-M 0.2 m Ω Ohmite's own LVK series (lower power) or Riedon's SSA series ensures continuous current capability>180 A

Re evaluation is required when replacing:

PCB pad compatibility

Pulse energy curve

Thermal resistance difference