Why are current probes for scopes so expensive

[icpart]

Hi guys.
I think to buy some type of oscilloscope  current probe. But I am not sure which type of current probes are more suitable for non isolated DC/DC or isolated AC/DC switching power supplies.  It is possible to use pure AC current probe like TEK p6021 for example on buck (or bust converter) to test input ripple current or inductor current, or switching transistor currents current?
Are the AC+DC current probes are more preferable for examination of switching power supplies?

[HighVoltage]

Hi guys.
I think to buy some type of oscilloscope  current probe. But I am not sure which type of current probes are more suitable for non isolated DC/DC or isolated AC/DC switching power supplies.  It is possible to use pure AC current probe like TEK p6021 for example on buck (or bust converter) to test input ripple current or inductor current, or switching transistor currents current?
Are the AC+DC current probes are more preferable for examination of switching power supplies?

As long as you have only AC components in your waveform, you can get away with a pure AC current probe
As soon as you have some constant DC parts in your waveform, you need to use a DC current probe, to get an accurate reading

[mikerj]

The Labdevice current probes are not expensive.  Around 300USD. They measure AC and DC from the low uA range up to 1A. Perfect for electronic development.
www.labdevice.ch

Those are not proper current probes.  If you read the datasheet it's just a shunt resistor with a diff amp across it so you don't get any isolation, you have to break the circuit you are measuring, they have a limited usable voltage range and you have to deal with the burden voltage of the shunt.

[lukier]

Recently I scored A6302 for fair price and then I bought the old AM503 amplifier.

In principle it works, but it is really vintage, crusty, drifty and likes to pickup all kinds of noise. People say that AM503A or AM503B are better, but these things are stupidly expensive for what they are (especially to what I paid for the probe).

I was thinking about building my own amplifier, a bit of DIY, I could learn something and it shouldn't be extremely expensive. I can always use the old AM503 if it doesn't work.

I've analyzed AM503A service manual and schematic and also watched tesla500 videos on TCPA300 repairs. In these amplifiers the signal path seems to be as follows (simplified):

Hall Sensor (probe) -> Hall PreAmp -> Power Amp for bucking current -> AC coil (probe) -> Attenuator (rotary switch or a bunch of relays) -> Output Amplifier (gain, switchable BW limit etc) -> 50 Ohm Output

TCPA300 seems to be made out of off the shelf parts, AM503A (according to the schematic) still has Tek favorite in-house hybrids : attenuator an output amp. Not sure about AM503B.

However, new probes don't have these separate amplifiers at all! TCP202A or Agilent N2893A that HighVoltage repaired recently (https://www.eevblog.com/forum/testgear/agilentkeysight-n2893a-current-probe-(teardown-and-repair)/) seem to only have Hall PreAmp + bucking current Power Amp, no switchable attenuator or amplifier.

How does this work then? Are the designers relying on the scope's front end attenuation & amplification circuitry? What voltages can be expected out of the probe transformer (also fed with bucking current)? AFAIR the output from the probe transformer is 50 Ohm, but some scopes with built-in 50 Ohm input impedance have relatively low input voltage range (like 5V RMS) so it worries me a bit.

If I can too skip attenuator & output amplifier then it would greatly simplify my DIY efforts.

[tautech]

How does this work then? Are the designers relying on the scope's front end attenuation & amplification circuitry? What voltages can be expected out of the probe transformer (also fed with bucking current)? AFAIR the output from the probe transformer is 50 Ohm, but some scopes with built-in 50 Ohm input impedance have relatively low input voltage range (like 5V RMS) so it worries me a bit.

If I can too skip attenuator & output amplifier then it would greatly simplify my DIY efforts.

AC probes like the Tek P6021/22 are terminated into either a amplifier (134) or a 1Mohm  termination that encloses a compensation network and switchable attenuation, it is an entirely a passive module that allows for 2 levels of output.
IIRC 2mA/mV and 10mA/mV.
Modern DSO's have front end attenuation to match.

The format and circuitry is outlined in an old Tek booklet that has been linked here several times that I'm aware of.
http://www.davmar.org/TE/TekConcepts/TekProbeCircuits.pdf
19Mb

P70 is the circuit specific to P6021.

[lukier]

AC probes like the Tek P6021/22 are terminated into either a amplifier (134) or a 1Mohm  termination that encloses a compensation network and switchable attenuation, it is an entirely a passive module that allows for 2 levels of output.
IIRC 2mA/mV and 10mA/mV.
Modern DSO's have front end attenuation to match.

The format and circuitry is outlined in an old Tek booklet that has been linked here several times that I'm aware of.
http://www.davmar.org/TE/TekConcepts/TekProbeCircuits.pdf
19Mb

Thanks for this document, it might be very helpful. I was talking about DC probes, but I suppose the circuits for the AC ones will be useful as well, as the output of the DC probe comes from the AC transformer anyway.

Now it is a bit more clear why probes like Agilent N2893A don't have the attenuator + amplifier. They probably just have the 1M passive termination, similar to the one shipped with P6021 (even not switchable) and it terminates 25/50 Ohm (I forgot off the top of my head what is the impedance of the transformer output, but AFAIR something like that) output line from the AC transformer and then just uses oscilloscope 1M front end with its AC/DC/BW limit/attenuation/gain. Not bad!

So it poses another question. What was then the advantage of external attenuator + amplifier before the scope front end that AM503(A/B) and TCPA300 employed. From the P6021 TekWiki page I see that if it is used in combination with 134 amplifier it has lower low frequency cut off - this doesn't apply for the DC probe I suppose.

[chlan]

The Labdevice current probes are not expensive.  Around 300USD. They measure AC and DC from the low uA range up to 1A. Perfect for electronic development.
www.labdevice.ch

Those are not proper current probes.  If you read the datasheet it's just a shunt resistor with a diff amp across it so you don't get any isolation, you have to break the circuit you are measuring, they have a limited usable voltage range and you have to deal with the burden voltage of the shunt.

Hi mikerj

You are right. You have to open the circuit and you don't have isolation. But in our lab it's the probe everybody is using. Reasons are the low offset voltage, accurate measurements and the high bandwidth. We often need to measure low currents (below 1mA) and fast  current peaks. Compared to the Tek and LeCroy probes we have, this cheap tool works much better.

[nctnico]

I recently made a differential amplifier board for this purpose so I can attach a current shunt sitting at a couple of volts over or under the scope's ground.

[D3f1ant]

Took a gamble and ordered a labdevice 100mA unit. Its quite a bit cheaper than the keysight N2821A I was going to buy 

[H.O]

Since you're apperently working for them it would be good form to say that you are instead of just spamming the forum with advertisment for your companys product(s) pretending to be a happy customer. (6 posts so far - all of them "recommending" their various products)

But now that you have, do us a favour and post some photos of the internals of the probe (something you'd gladly do if you ARE a happy customer). My guess is there are two LEM LA 100-P transducers in it.

EDIT: Now the post I'm referring to has been deleted so the above looks a bit out of context...