Ohmite EBW Current Sensing

Ohmite EBW Current Sensing

In high current applications such as hybrid power supplies, frequency converters, and automotive electronics, precision current detection resistors are the core components of feedback control and protection circuits. Ohmite's EBW series uses electron beam welding technology to precisely combine different alloys (Manganin or NiCr) with highly conductive copper terminals, achieving extremely low resistance values (starting from 0.2 m Ω), high power density (7 W continuous), and excellent thermal stability. However, engineers often face practical issues such as temperature resistance, pulse capability, welding process, and long-term reliability in selection and application. This article is based on the official technical specifications of the Ohmite EBW series, combined with practical design experience, to systematically sort out the characteristics, application points, and common problem elimination of this series of resistors, helping hardware engineers quickly complete selection and optimize PCB layout.

Overview of EBW Series Core Technologies

1.1 Advantages of Electron Beam Welding

Traditional current detection resistors often use integral alloys or solder connections, which have problems such as high contact resistance, concentrated thermal stress, and high inductance. The EBW series uses electron beam welding to fuse copper terminals with alloy resistors, which has the following advantages:

Extremely low contact resistance: The welding interface has no oxidation or mechanical pressure, and the contact resistance can be ignored.

Low inductance (<3 nH): Suitable for high-frequency switching circuits such as DC-DC converters and SiC/GaN inverters.

High mechanical strength: can withstand reflow soldering (350 ℃ for 30 seconds) or direct soldering on copper conductors.

Long term stability: After 2000 hours of load life testing, the resistance change is ≤± 1%.

1.2 Comparison of Two Alloy Systems

The EBW series offers two alloys, Manganin and NiCr, which are suitable for different TCR requirements and cost considerations.

Model prefix Alloy material Resistance range Power (W) TCR (20-150 ℃) Typical applications

EBWA-M Manganin 0.5, 1 m Ω 5 ± 75 ppm/℃ precision current detection, temperature drift sensitive

EBWA-N NiCr 2, 3, 4 m Ω 5 ± 100 ppm/℃ high resistance, cost optimization

EBWB-M Manganin 0.2, 0.5 m Ω 7 ± 100 ppm/℃ high current (180 A continuous)

EBWB-N NiCr 1, 2, 3 m Ω 7 ± 120 ppm/℃ High power, medium resistance

Key points:

Manganin has a lower TCR, but a lower resistance value (0.2-1 m Ω), making it suitable for applications that require high precision and low temperature drift.

NiCr alloy has a slightly higher TCR (± 100-120 ppm/℃), but can achieve higher resistance values (up to 4 m Ω), and the cost is relatively low.

1.3 Dimensions and Appearance

EBWA：L=10.5 mm，W=5 mm， The thickness varies between 0.32 and 0.88 mm depending on the resistance value.

EBWB：L=15.2 mm，W=7.5 mm， Thickness of 0.3-1.5mm.

The length of the copper terminals is 5.6 mm, which is convenient for welding or bolt fixing.

Key electrical and thermal characteristics

2.1 Power and derating curve

The PDF provides a derating curve chart. The rated power of EBWA and EBWB corresponds to different surface temperatures of resistors:

EBWA: rated power of 5 W at surface temperature below 85 ℃; Linear derating is required when the temperature exceeds 85 ℃, and the power drops to 0 when the temperature reaches 170 ℃.

EBWB: rated power of 7 W below 65 ℃ surface temperature; When the temperature exceeds 65 ℃, the power is reduced to 0 at 170 ℃.

Practical application calculation:

The surface temperature rise of the resistor is Δ T=P × R θ (thermal resistance). Although R θ is not directly given, it can be estimated through typical temperature rise: for example, when EBWB-M 0.2 m Ω passes 180 A, the power P=I ² R=180 ² × 0.0002=6.48 W, which is close to the rated 7 W, and the surface temperature is about 65 ℃. If the ambient temperature is above 65 ℃, it must be downgraded for use.

2.2 Pulse energy capability

The PDF provides pulse energy curves (Pulse Energy/Power) for EBWA and EBWB. This curve presents the relationship between the maximum pulse energy (J) and pulse time (ms) at different resistance values in logarithmic coordinates.

EBWA: The effective area is between 0.5 m Ω (maximum curve) and 4 m Ω (minimum curve). For the intermediate resistance value, the allowed pulse energy is between the two.

EBWB: Similarly, 0.2 m Ω provides the highest pulse tolerance and 2 m Ω provides the lowest.

Engineering interpretation:

The pulse energy capability is mainly determined by the alloy volume and heat dissipation conditions. The lower the resistance, the larger the cross-section of the alloy, and the higher the transient energy it can absorb.

For example, EBWA can withstand about 50 J at a pulse width of 1 ms for 0.5 m Ω, while 4 m Ω can only withstand about 2 J. Therefore, low resistance models should be preferred for high pulse applications such as capacitor pre charging and motor starting.

2.3 Long term stability

According to MIL/industry standard testing, the resistance change of EBW series after the following stress is ≤± 0.2% (load life is ± 1%):