AC coupled scope poor for 50Hz mains current

[Faringdon]

Hi,
The attached shows how poor our TDS3032 scope is at viewing mains current into an offline PSU (via a tektronix A6303 current probe).

But we can assume  theres pretty well no distortion when viewing output ripple of 50khz SMPS on AC coupled?

TDS3032 manual
https://download.tek.com/manual/071038201web_6.pdf

[Martin72]

Hi,

But we can assume  theres pretty well no distortion when viewing output ripple of 50khz SMPS on AC coupled?

Not really sure what you mean...

You´ve posted the input current of the smps without load, right ?
When you want to see the output (voltage) ripple of the smps, you know what to do then and block the dc-voltage by using AC coupling.
Same when you want to see the ripple of the output current.
Or what is your intention ? 

[Ian.M]

Take an isolation transformer, and a low inductance^* 50A 50mV FSD current shunt.  Connect one end of the shunt to (earth) ground via its sense terminal so your measurement can be ground referenced and you can use an ordinary probe.  Connect the other end's load terminal to one end of the isolation transformer secondary, and the other secondary end to the DUT's Line terminal, via your A6303 probe.  The DUT's Neutral terminal should connect back to Ground at the current shunt's ground side load terminal to complete the circuit.  Now the instantaneous voltage on the the current shunt's transformer end sense terminal in mV should equal the A6303 probe's reading in Amps.   Caution: as the secondary side of the isolation transformer is grounded, the whole primary side DC bus of the DUT is mains live.  You *MUST* use an isolated differential probe if you want to probe any voltage waveforms after the bridge rectifier.

If it doesn't you can start asking questions why not!

One possibility would be that you are exceeding the ratings of the current probe:

The larger A6303 measures higher amplitude, lower frequency currents. It clips onto conductors up to 21.1 mm (0.83 in.) in diameter. Its frequency range is DC to 15 MHz. It will measure continuous currents up to 100 A and peak currents up to 500 A, as long as the amp*second product of 10,000 A*μs is not exceeded.

That's only 4A 1A average over a 50Hz mains half cycle.  Higher average (not RMS) currents are likely to saturate the A6303's core.

* They almost all are low inductance. Look for straight through or meandering construction.  Just avoid any odd ones out with a wound construction for the shunt resistance itself!

Edit: *CANT*/*SHOULDN'T* math before morning Coffee!  10K A.us == 10 A.ms  so the limit is only 1A.10us i.e. 1A average over one half cycle of 50Hz mains.

[Martin72]

We don´t know his settings on the current-amplifier module.
So we don´t know, if the A6303 is already saturated.
Apart from this, when his pics are showing the input current of an offline psu, it´s a remarkable shape of the current.

[bdunham7]

Hi,
The attached shows how poor our TDS3032 scope is at viewing mains current into an offline PSU (via a tektronix A6303 current probe).

But we can assume  theres pretty well no distortion when viewing output ripple of 50khz SMPS on AC coupled?

You have all sorts of issues there.  Yes, it appears that the skewing distortion would be due to the frequency cutoff from the AC coupling, but that is supposed to be much below 50Hz, so I'm not sure why it is so apparent.  If you are using them AM503 amplifier with that probe, then perhaps the fact that it is intended to be used with 50R scope inputs makes some difference.  However, if you use the 50R input on your scope with AC coupling, then your cutoff is 140kHz, which won't work for your 50Hz input nor your 50kHz output.

Typically these probes are used with a 50R DC-coupled input, it's just the way they are supposed to work.  If you want to block the signal resulting from a large DC current so that you can observe the ripple current (I'm not sure why, but...) then you probably need to leave the scope at 1M/AC and use an external 50R terminator.  And, of course, make sure the maximum DC current is well under the probe limit of 100A.

[Martin72]

A

If you are using them AM503 amplifier with that probe

He must use the AM503, when using the A6303.
Scope settings are once done, everything else should be done on the AM503.(Except timebase/triggerlevel, of course)
Scope is set to 10mV/Div, DC coupling, 50 ohm termination, no bandwithlimiter on.
Bandwithlimit, dc or ac coupling, this will be done at the AM503 module.
Or like the manual "say":

NOTE. To maintain accurate readings while using the AM 503B and AM 5030,
the vertical gain of the oscilloscope channel must always remain at 10 mV/Div
and the coupling must remain at DC

Further the scope should have five times the bandwith, the probe got.
In case of the 6303 this shouldn´t be a problem...

[bdunham7]

Bandwithlimit, dc or ac coupling, this will be done at the AM503 module.

I don't have one of these at the moment so I can't experiment.  Do you know if the input overload warning will still indicate DC probe saturation in the AC coupled setting?  IOW, if you set the AM503 to AC coupling with an A6303 probe and run 200ADC through the core of the probe, does the warning light up?

[Martin72]

Hi,

Actually I´m building up a "tek-system" at home, but got only the amplifier module here, without probe and housing.
At work, we got several probes and amplifiers, also the 6303 probe in normal and xl cable lenght.
Can check this on monday when I´m at work.
What I´m missing from the OP here is:
- What to measure
- What are the settings so far
- Clearer description in general what the intention is
- Make pictures from the whole setup

And so on...
But nevertheless, I´ll check the overload thing on monday, when it´s interesting for anyone.
Martin

[Sighound36]

Hi Faringdon

We are UK based as well, he are some screenshots of a filtration device we have been working on before and after with Lecroy CP-31A current probes and HVD3106A high voltage differical probes.

Here are various current loads as well as a zero load.