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I am reading the data sheet for the LEM current sensor LAX 100-NP. I find the PCB design a bit confusing when it comes to high speed or low speed design. If you look page 11, left column, high speed design, do you know where the primary current is supposed to enter the sensor in order to have a positive secondary current? In the high speed design, it seems that the primary current enters the sensor in the terminals where the primary current would exit the sensor in the low speed design case. Any opinion? perhaps we can work this out.

Thanks a lot

Lello

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2 Answers 2

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The datasheet asks to create a complete turn for high frequency. They illustrate that with a drawing at the bottom of p.10 . I think, this drawing doesn't show that the sign of the output for the left side would be opposite than that for right side (assuming that the sensor wasn't turned 180° around its vertical axis).

enter image description here

Let's assume that in a low-speed design, the trace that goes to the left edge of the drawing corresponds to upstream and right one is downstream. In a high speed variant, the right hand side of the drawing would be upstream. Alternatively, you can accept that the sign will be inverted.

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  • \$\begingroup\$ Thank you so much. In my case I need a high speed design preserving the signs (i.e. positive primary current = positive secondary current). So, the primary current will enter the sensor from the jumpers on the right (those where the sign says "vias" in the picture above) and will exit from the jumpers on the left. Would you agree with this? \$\endgroup\$
    – Lello
    Commented Oct 20, 2015 at 0:05
  • \$\begingroup\$ @Lello I don't quite agree. I have updated the picture. Hopefully, it will make the answer clearer. \$\endgroup\$ Commented Oct 20, 2015 at 0:21
  • \$\begingroup\$ Thanks Nick :) it is clear. The primary current always enters from the same terminals (those ones 'below' the M terminal) in order to preserve the sign btw primary and secondary current... \$\endgroup\$
    – Lello
    Commented Oct 20, 2015 at 0:46
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All they're saying is that a high-speed design should form a complete loop in the current path, while a low-speed design can be laid out more as a shunt.

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