# Current clamp difference question...

Discussion in 'Electronic Design' started by Default User, Jun 26, 2008.

1. ### Default UserGuest

Hi,

I measured the current going to my scope using two clamps. One is a
hall-effect clamp that can do both AC and DC, and the other is just an AC
only clamp although they both output mV.

This first waveform shows the AC only clamp and my question is, why are the
horizontal (no current) areas slanted either upward or downward?

This waveform shows both clamps and you can see that the hall effect one
(light blue) is not distorted and the AC one (yellow) is like the first one:

Is this sort of distortion normal for an AC only transformer based clamp?

Thanks,

Alan

2. ### Phil AllisonGuest

"Default User"

** Yep.

The AC clamp has significant variations in its current sensitivity that
depend on frequency - ie it has a response that falls away at low and high
frequencies.

OTOH, the Hall Effect clamp has response that does not fall away low
frequencies, but only at some fairly high one.

Where there are response changes with frequency - there are also phase
changes with frequency ( all-pass filters notwithstanding).

So, the slanting ( aka tilt ) you see on the scope trace is the result of
phase shift ( generated in the AC clamp) between the frequency components of
that bi-polar pulse waveform.

...... Phil

3. ### Default UserGuest

Hi Phil,

Thanks for explaining it!!! That makes perfect sense!!

Alan

4. ### leggGuest

Depends on the low frequency bandwidth of the sensor, and the
frequency of the current waveform. A transformer has a low-frequency
limit that produces an L/R time constant for waveform droop. Gives the
same effect as a capacitively-coupled RC time constant.

Commercial line frequency current transformers can be very droopy,
particularly clamp-on types with their mechanically-forced gap
(~reduces L); judging from the additionally poor HF definition, that
could be what you're dealing with here.

RL

5. ### JosephKKGuest

Looks like a diode bridge fed into a storage capacitor with a
switching power supply typical current waveform.