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doctor electrons

Sat Mar 23 2013, 09:55PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

I have a question regarding a scope i recently purchased. I am not new to oscilloscopes by any means, i have been
using them for some years now. The scope i just picked up is a new Tektronix MSO2024 mixed signal scope.
My question is this! Under the probe setup options there is an option to select the probe type. Voltage or current;
these two options have their own sub categories when one is selected. When i select the probe type as voltage a sub category opens for measurement type, again voltage or current. (this category is not available when you select current as the probe type.) If you select "measure current" another category opens that has two settings, default they are as follows.
10.00 A/V and 100.0mV/A and both can be adjusted. I have never seen this on any scope before and it is nowhere to be found in the scopes manual! The way i have always measured current is the old series resistor and two channels method.
The math for that is pretty simple.

Does anyone have any experience with settings like this? I can't wrap my brain around how this could give you an accurate current measurement if it is user adjustable. I can only guess that they expect you to know the resistance of the circuit under test, or maybe the resistance of the probe itself is part of the equasion.

Any help would be appreciated on this one, i played around with it and nothing i could get it to read was anywhere near accurate.

Thanks in advance!!

Ash Small

Sat Mar 23 2013, 10:40PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I'd use 'the old series resistor and two channels method'...

radiotech

Sat Mar 23 2013, 10:44PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

With 10A/V the scope is measuring conductance, and 100mV/Amp the scope is
measuring resistance.

Its one or the other, when Amps and Volts are involved.

Ohms/cM or Siemens/cM showing on the scope?

doctor electrons

Sat Mar 23 2013, 11:08PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

When the settings are turned on and set to current, the scope reads amps (A) or milliamps (mA).
(as shown on the display) Where the scope when set to "measure voltage" would normally show volts or (V).
With the A/V and mV/A settings, they are tied together and you can only adjust A/V. When you do the mV/A changes
with it. Here is an example of what it reads when hooked to the probe comp pins. 975.072Hz square wave. 400us/DIV.
The volts/DIV setting changes to current. What would normally be 2V/DIV turns to 20A/DIV and the trigger level which normally is in volts also turns to amps. Trigger level set to 50% reads 24.8A. More or less saying that my 5volt 1khz
square wave has a max amplitude of 49.6 amps. That reading will change if i modify the A/V setting. How would i determine what to set it at!!??

Thanks for the replies guys!!

ConKbot of Doom

Sun Mar 24 2013, 04:59PM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329

That setting is for connecting a current probe and putting in a scale factor/sensitivity. They just let you enter one or the other without having to invert the number. If you use a 1 ohm shunt resistor in series to ground with a load, you'd put 1A/V or 1000mV/A in for that.

If you use the math function to multiply a channel with amperage and a channel with voltage, the scopes typically spit out watts as the units too.

Jrz126

Sun Mar 24 2013, 05:10PM

Registered Member #242 Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210

How are you measuring the current? Most current probes will convert the measured current into a voltage. That A/V number you set in the scope is the scaling factor on your current probe. I have several rogowski coils at work which are 2mV/A, or 500A/V
Using this setting and measuring 500A, it would display as 500A instead of 1V.

doctor electrons

Sun Mar 24 2013, 09:01PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

From what i have determined by changing the settings, this IS ONLY for a voltage probe. When you select "current"
under the probe type these settings are unavailable. When "voltage" is selected under probe type they become available.
First, after selecting voltage probe, and choosing "measure current" instead of "measure voltage". These settings are only available when "measure current" is selected under the voltage probe type. Nothing of the sort appears when current probe is selected. There must be something missing from this puzzle. Using a resistor wouldn't need these settings. Simply measuring the voltage drop across a series resistor with a second channel and doing the math would give you the current.
For $4000 dollars i think i'm missing something.

Is it possible these settings are for connecting a current transformer to the voltage probe and reading current from it?
The Tek current probes actually plug in and change all of the settings automatically so there would be no need to adjust settings like this on this scope. In addition they're not available when one is attached to the scope! Still puzzled here!

...

Sun Mar 24 2013, 09:29PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

You are not missing something, it is just there to make the scale read correctly like the 'label' function (and like others have mentioned, it should carry though with other math operations). It is just doing the 'doing the math' part of your method, and lets you input the scale factor of the resistor/current sensor/etc that you are using.
Ie, for your 'measure the voltage drop across a series resistor with a second channel and doing the math' method you would:

set channel 1 and channel 2 to be a voltage probe, measure current, set the scale factor to match your resistor (ie, 1A/V for a 1ohm resistor). Clip channel 1 and channel 2 across the resistor, and in the math menu subtract channel 1 and channel 2. The scope now reads the current flowing through your resistor directly, in amps.

There is not much difference between it and just simply setting the probe type to 'current' and dialing in the scale factor there, but for $4000 you shouldn't have to play tricks telling your scope that you are connecting a current probe when you are actually connecting a voltage probe reading current

doctor electrons

Sun Mar 24 2013, 09:48PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Thanks ... !! That makes sense to me! Sadly, im right back where i was with the resistor method, just don't need a pencil and paper anymore! I suppose that's about all Tek can do for you other than sell you the 1200 dollar current probes.
I bet thats why i have never seen it before. I appreciate all the input and will do some further testing with a resistor in the mix. Looking for it to match up with the math of course. If its accurate i guess its worth having.
I also fully agree, for the price i shouldn't have to chant spells in binary to the thing! Now to create a spreadsheet that has the different scale factors for multiple resistances, then i should be able to make proper use of the option on a real circuit.

You guys are the best! There is always an answer here on 4HV!!

Edit:
So if i understand this correctly that A/V value should be set to the resistor values reciprocal and the current reading will be correct.

Erlend^SE

Tue Mar 26 2013, 12:30AM

Registered Member #1565 Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159

Even generic current transformers with shunt does the same A/V trick.
Maybe you should look into them? given the right resistor it may match up to one of the scope-settings.

(mostly only useful for non-PCB applications)

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