Transducers Fail for Understandable Reasons

When a current or voltage transducer appears to misbehave, the cause is usually not a random failure but one of a few systematic problems: an offset from the magnetic environment, a drift from the supply or thermal design, noise from the layout, or an isolation fault from assembly. This article presents a method for diagnosing the common issues in LEM transducers inside motor drives, power supplies and other power electronics, so a technician can move from symptom to root cause quickly.

Offset Problems

The most common complaint is a constant offset in the reading. Start by measuring the output with zero primary current and the sensor in its final position, because an offset measured on the bench can disappear or appear once the board is installed. A nearby magnetic field, a conductor that hugs one side of the aperture, or a current that flows close to the sensor can all add a DC offset. Then check the electrical side: an error in the supply voltage or the measuring resistor appears directly as an offset, so verify both against the datasheet. Measure the offset cold and hot, because a change with temperature points to drift rather than a fixed fault.

Drift with Temperature

Every transducer has a specified offset and gain drift over temperature, and some drift is therefore normal. Excessive drift usually has a deeper cause: an oversized measuring resistor that amplifies the offset current into a large voltage, a supply that itself drifts with temperature, or self-heating from an undersized primary conductor that heats the sensor. Check each in turn, and remember that an open-loop sensor drifts more than a closed-loop part by design, so compare the observed drift against the correct datasheet curve for the technology in use.

Noise and Instability

A noisy current signal is nearly always a layout problem rather than a sensor problem. The secondary output is a small signal that must travel from the sensor to the ADC, and if it runs close to a switching node, a transformer or an inductor, it will pick up interference. Move the measuring resistor close to the ADC, screen the connection where the environment is noisy, and keep the ground return clean. If a filter is needed, size it for the control-loop bandwidth rather than the switching frequency, because a filter that is too aggressive adds phase lag and destabilizes the current loop.

Isolation Faults

An isolation fault is the most serious because it threatens both the equipment and the operator. It usually indicates a layout or assembly problem rather than a transducer defect: insufficient creepage between the primary and secondary, a solder bridge across the barrier, or contamination that reduces the spacing. Check the board against the required clearance for the working voltage, inspect the barrier carefully under magnification, and confirm the spacing on every unit, not only the prototype. A hipot test locates the failure, and any suspect unit should be removed from service until it passes.

A Systematic Method

Work from the simple to the complex. Confirm the supply and the measuring resistor first, because these are easy to check and common sources of error. Then check the magnetic environment and the primary conductor geometry. Then examine the signal routing for noise. Then verify the isolation. In most cases the fault appears in the first two checks. Keep a reference unit that is known good so you can compare a suspect unit against it, and record the offset, gain and noise of each new design on the bench so that any later change is immediately visible.

When to Replace the Transducer

A transducer rarely fails on its own, but when it does, the failure is usually a short or open in the primary or secondary circuit, or a damaged output, often caused by overvoltage on the supply or by a lightning or switching surge. If the checks above are clean and the device still misbehaves, replace it with a known-good unit and re-test. BeiLuo supplies genuine LEM transducers with import declaration, certificate of origin and RoHS documents, and our FAE team can help you diagnose a problem and choose a replacement that resolves the root cause rather than only the symptom.