DC current measurement is always challenging and the “whacked” condition is more common than any
of us care to admit. Whacked can mean a current reading when the unit is off or just a strange reading
you can’t explain or correct for. If you measure widely varying DC current levels, or have recently
experienced a current surge, hysteresis is the likely culprit.
Understanding and correcting the problem takes some explanation and then some work. You must also
believe in the power of ElectroMagic (or Electromagnetics if you insist on using the proper term). Those
of us who studied EM in an early morning class still believe it’s magic, but, I digress…
Nearly all current measurement devices have a metallic core to transform the operating current down to
an easily measurable signal level. The problem is, transformation follows a curve:
The curve above shows magnetic Hysteresis, where the loading curve is different from the unloading
curve. If you can’t sleep until you understand this, The Electric Academy has an excellent video here.
For our purposes, we need to recognize the retention magnetism imposed as the current rises and then
returns twice per cycle. When measuring AC, we complete the curve and return to our starting point,
which effectively washes out most retained or residual magnetism. The effect is still there, but it’s
manageable and definable.
BUT, if we suddenly interrupt the current, some residual magnetism remains. Residual magnetism
causes the core to take a “set”, which manifests in measurement as an offset.
When measuring DC, a variable (hence the need to measure) current flows in one direction and doesn’t
reverse. Residual magnetism is not “washed out”, remains, and establishes an offset error. The error is
non-linear and unpredictable. Reversing direction doesn’t wash it out. It just changes the direction by
some other indeterminate value. All we can do is capture the maximum impact.
It’s not a threshold effect – it has some impact across the measurement range.
Most manufacturers (at least the ones that care) will publish a specification of residual impact over the
normal operating range. A well-designed device shouldn’t get “whacked” within its normal range.
“Whacked” typically requires a current surge outside the rated range. In your case, we’d bet a surge
created an offset that makes the device appear “whacked”. Now you know what it is. What can you do
to correct it?
Ideally, we recommend eliminating the residual magnetism. If the magnetic core is accessible,
degaussing is possible and your best initial course. NOTE: Do not operate any degaussing device if you
have any medical electronics on or in you. The results can be fatal.
If it’s buried inside a can, or within some other ferric material, you may have to wait for the residual
magnetism to bleed off on its own, which it will eventually do. If, however, you need to continue
measuring over the next few weeks, you may need to replace the device, but that’s another topic.
Check out, “Measuring DC Current with 50X Surges” for examples and insights.
