Viewing Currents on a Scope

[Julian Barnes]

Guys,

I'd like to view the phase shift between currents and voltages testing
various combinations of inductor, resistor and capacitor networks. The
voltages should be no problem, but how can I display the currents? Do I
have to have some kind of special probe?

TIA,

jules.

 

[whit3rd]

On Tuesday, November 17, 2015 at 12:09:09 PM UTC-8, Julian Barnes wrote:

I'd like to view the phase shift between currents and voltages testing
various combinations of inductor, resistor and capacitor networks. The
voltages should be no problem, but how can I display the currents?

If it's a current to ground, you can use an inverting op amp to
generate a voltage signal (sometimes called 'transimpedance amp').
For some situations, a diamond transistor is a good sensor (and
it'll be faster than most op amps); ground the 'base', current sense
pin is the 'emitter', and output from 'collector' to ground through
a resistor generates the voltage output.

<http://www.ti.com/lit/ds/symlink/opa861.pdf>

If it is a current NOT to a midlevel power point (ground), the amplifier
situation gets tricky, of course. Usually that means a sense resistor
and an instrumentation amplifier are required. Sometimes, though,
you can just connect two o'scope probes to the sense resistor and
display the difference.

 

[Phil Hobbs]

On 11/17/2015 03:06 PM, Julian Barnes wrote:

Guys,

I'd like to view the phase shift between currents and voltages testing
various combinations of inductor, resistor and capacitor networks. The
voltages should be no problem, but how can I display the currents? Do I
have to have some kind of special probe?

TIA,

jules.

The simplest method is to put a small resistor in series with it, and
measure the voltage drop. This is how a multimeter does it. This
perturbs the circuit, of course--the voltage drop in the meter is called
the "burden voltage", which is an apt name.

The second simplest is to use a current probe. I have a classic
Tektronix P6042 that cost me $175 a few years back. It uses a
combination of a current transformer and a Hall sensor to cover DC-50
MHz. Interestingly, clamping the probe over your signal wire adds
series inductance. That causes a voltage drop at AC, almost like the
DMM's burden voltage.

The most complicated is to build a transimpedance amplifier. This is
basically an op amp inverter with the input resistor left off, so that
you dump current straight into the inverting input. Ideally the op amp
will force the voltage swing at its input to be small, so there's
effectively much less burden voltage.

The circuit itself is conceptually simple--the parlour trick is getting
it to work fast enough without oscillating when you hang a cable on its
input.

I'd go with the resistor.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

 

[M Philbrook]

In article <n2g1ck$r4b$1@dont-email.me>, jb9889@notformail.com says...

Guys,

I'd like to view the phase shift between currents and voltages testing
various combinations of inductor, resistor and capacitor networks. The
voltages should be no problem, but how can I display the currents? Do I
have to have some kind of special probe?

TIA,

jules.

Yes, or, just put a R in series with your circuit and measure it
with a secondary channel on the scope. Make sure the commons of your
probes (BNC) are isolated on the scope. I know My cheap hand held
Owen is. Not sure about the Rigol etc..

The other tool is to get yourself a low current DC clamp meter with
a BNC or adapter to plug into your scope, I have 2 60 amps and 1 650
amp.

Thinking of getting a much larger one, soon.

Jamie

 

[Jasen Betts]

On 2015-11-17, Julian Barnes <jb9889@notformail.com> wrote:

Guys,

I'd like to view the phase shift between currents and voltages testing
various combinations of inductor, resistor and capacitor networks. The
voltages should be no problem, but how can I display the currents? Do I
have to have some kind of special probe?

you can. eg: VCR heads have in the past be re-purposed as current probes.

OTOH you may be able to add a low resistance and probe the voltage across
that.

or you can spend real money on a current clamp probe.

http://www.ebay.com/bhp/oscilloscope-current-probe

--
\_(ツ)_

 

[George Herold]

On Tuesday, November 17, 2015 at 3:09:09 PM UTC-5, Julian Barnes wrote:

Guys,

I'd like to view the phase shift between currents and voltages testing
various combinations of inductor, resistor and capacitor networks. The
voltages should be no problem, but how can I display the currents? Do I
have to have some kind of special probe?

TIA,

jules.

What's the highest frequency you are hoping to look at.
If you are staying well away from ~1MHz,
and are good at making your own circuits.
Then I took the TIA at the bottom of your post to be a sign
that you should make a TIA (Trans Impedance Amp.) I made a
little one of these on some copper clad to look at I-V curves
of diodes and things. One opamp, bypass caps, some R's and a switch.
I was going to post a picture, but it's "on loan" to someone.

George H.

 

[amdx]

On 11/17/2015 2:06 PM, Julian Barnes wrote:

Guys,

I'd like to view the phase shift between currents and voltages testing
various combinations of inductor, resistor and capacitor networks. The
voltages should be no problem, but how can I display the currents? Do I
have to have some kind of special probe?

TIA,

jules.

Here's how I learned to do it, with a current sense resistor.
The pcb* was built to accept scope probes and will work reasonably well
up to 2 MHz.

http://s395.photobucket.com/user/Qmavam/media/TheBoardwithnotes.jpg.html

http://s395.photobucket.com/user/Qmavam/media/TheBoardDrawing.jpg.html

Current and voltage waveform

http://i395.photobucket.com/albums/pp37/Qmavam/Scope90DegreePhaseShift.jpg

http://s395.photobucket.com/user/Qmavam/media/CurlyQforprobe.jpg.html

You can add a few extra solder points for you resistor, as you see in green,
I have add connections for a capacitor.

If you have more questions about the pcb or use, ask.


*The pcb was designed to plug in a piezo ceramic transducer, a variable
inductor was adjusted until the current and voltage were in phase. Used
for measuring specs of our transducers to then calculate drive inductor
and transformer ratio.
Mikek

 

[Julian Barnes]

On Wed, 25 Nov 2015 06:28:40 -0600, amdx wrote:

You got lots of advice, did you put it to positive use.
Feedback is always nice.

Yes, thanks all. I'm going to go with the el-cheapo option and adopt the
current sensing resistor suggestion. The current probes cost rather a
lot, having taken a look at the prices online!

 

[amdx]

On 11/18/2015 11:29 AM, amdx wrote:

On 11/17/2015 2:06 PM, Julian Barnes wrote:
Guys,

I'd like to view the phase shift between currents and voltages testing
various combinations of inductor, resistor and capacitor networks. The
voltages should be no problem, but how can I display the currents? Do I
have to have some kind of special probe?

TIA,

jules.

You got lots of advice, did you put it to positive use.
Feedback is always nice.

Mikek

 

[Baron]

Julian Barnes prodded the keyboard with:

On Wed, 25 Nov 2015 06:28:40 -0600, amdx wrote:

You got lots of advice, did you put it to positive use.
Feedback is always nice.

Yes, thanks all. I'm going to go with the el-cheapo option and adopt
the current sensing resistor suggestion. The current probes cost
rather a lot, having taken a look at the prices online!

Ought to pull a floppy disk drive apart. The heads make great current
probes !

--
Best Regards:
Baron.

 

[amdx]

On 11/25/2015 8:48 AM, Baron wrote:

Julian Barnes prodded the keyboard with:

On Wed, 25 Nov 2015 06:28:40 -0600, amdx wrote:

You got lots of advice, did you put it to positive use.
Feedback is always nice.

Yes, thanks all. I'm going to go with the el-cheapo option and adopt
the current sensing resistor suggestion. The current probes cost
rather a lot, having taken a look at the prices online!

Ought to pull a floppy disk drive apart. The heads make great current
probes !

Are they position sensitive or can you pass a wire through them?
ie, had do you make it work?
Mikek

 

[amdx]

On 11/25/2015 6:31 AM, Julian Barnes wrote:

On Wed, 25 Nov 2015 06:28:40 -0600, amdx wrote:

You got lots of advice, did you put it to positive use.
Feedback is always nice.

Yes, thanks all. I'm going to go with the el-cheapo option and adopt the
current sensing resistor suggestion. The current probes cost rather a
lot, having taken a look at the prices online!

Thanks for the input, what is your frequency of interest?

Mikek

 

[Baron]

amdx prodded the keyboard with:

On 11/25/2015 8:48 AM, Baron wrote:
Julian Barnes prodded the keyboard with:

On Wed, 25 Nov 2015 06:28:40 -0600, amdx wrote:

You got lots of advice, did you put it to positive use.
Feedback is always nice.

Yes, thanks all. I'm going to go with the el-cheapo option and
adopt the current sensing resistor suggestion. The current probes
cost rather a lot, having taken a look at the prices online!

Ought to pull a floppy disk drive apart. The heads make great
current probes !

Are they position sensitive or can you pass a wire through them?
ie, had do you make it work?
Mikek

If you pull one apart you will see that it is a very tiny coil on a
ferrite former. If you want to detect the current flowing in a pcb
trace or wire you just align it with the trace and look at the output
with a scope. A times 10 op amp helps since the output voltage is
quite small.


--
Best Regards:
Baron.

 

[Jasen Betts]

On 2015-11-26, amdx <nojunk@knology.net> wrote:

On 11/25/2015 8:48 AM, Baron wrote:
Julian Barnes prodded the keyboard with:

On Wed, 25 Nov 2015 06:28:40 -0600, amdx wrote:

You got lots of advice, did you put it to positive use.
Feedback is always nice.

Yes, thanks all. I'm going to go with the el-cheapo option and adopt
the current sensing resistor suggestion. The current probes cost
rather a lot, having taken a look at the prices online!

Ought to pull a floppy disk drive apart. The heads make great current
probes !

Are they position sensitive or can you pass a wire through them?
ie, had do you make it work?

yes, they are position sensitive. ok for relative-time measurments and
fox-hunts, not much good for abolute measurements, or compating two
different circuits.

for that you need a current transformer which goes right round the
conductor.


--
\_(ツ)_

 

[Julian Barnes]

On Thu, 26 Nov 2015 07:58:46 -0600, amdx wrote:

On 11/25/2015 6:31 AM, Julian Barnes wrote:
On Wed, 25 Nov 2015 06:28:40 -0600, amdx wrote:

You got lots of advice, did you put it to positive use.
Feedback is always nice.

Yes, thanks all. I'm going to go with the el-cheapo option and adopt
the current sensing resistor suggestion. The current probes cost rather
a lot, having taken a look at the prices online!


Thanks for the input, what is your frequency of interest?

Mikek

DC - 200Mhz primarily. I struggle with the physical challenges of
anything much beyond 200!

 

[amdx]

On 11/29/2015 7:05 AM, Julian Barnes wrote:

On Thu, 26 Nov 2015 07:58:46 -0600, amdx wrote:

On 11/25/2015 6:31 AM, Julian Barnes wrote:
On Wed, 25 Nov 2015 06:28:40 -0600, amdx wrote:

You got lots of advice, did you put it to positive use.
Feedback is always nice.

Yes, thanks all. I'm going to go with the el-cheapo option and adopt
the current sensing resistor suggestion. The current probes cost rather
a lot, having taken a look at the prices online!


Thanks for the input, what is your frequency of interest?

Mikek

DC - 200Mhz primarily. I struggle with the physical challenges of
anything much beyond 200!

ooh! 200MHz, You better put a Curly Q as tight across a resistor as
you can. Mount your scope probe right against your resistor.
> http://s395.photobucket.com/user/Qmavam/media/CurlyQforprobe.jpg.html

Mikek

 

[Cursitor Doom]

I was just thinking that there must be current sensing circuitry in many
SMPSs - the newer ones that provide for power factor correction anyway -
so I guess it should be possible to obtain a suitable current sensing
element for free from, say an old scrap computer for example?