Author Topic: Using a current transformer as an oscilloscope probe  (Read 339 times)

0 Members and 1 Guest are viewing this topic.

Offline cvanc

  • Frequent Contributor
  • **
  • Posts: 538
  • Country: us
Using a current transformer as an oscilloscope probe
« on: June 21, 2019, 06:23:26 am »
Hi all-

Have any of you ever used a current transformer as a scope probe?  Like in the attached spec sheet.  Did it work OK?

 

Online tautech

  • Super Contributor
  • ***
  • Posts: 15337
  • Country: nz
  • Taupaki Technologies Ltd. NZ Siglent Distributor
    • Taupaki Technologies Ltd.
Re: Using a current transformer as an oscilloscope probe
« Reply #1 on: June 21, 2019, 06:32:12 am »
Yep, one from an old PC switchmode PSU so to hopefully have some range into the 10's of KHz.

Current waveform reproduction and accuracy good enough for low frequencies when checked against a Tek P6022.
We had to trim it some with shunting a resistor across the outputs so to get meaningful measurements from it.

Not sure I'd wanna use one that was rated to only 400 Hz.
Avid Rabid Hobbyist
 
The following users thanked this post: cvanc

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 13266
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Using a current transformer as an oscilloscope probe
« Reply #2 on: June 21, 2019, 06:33:19 am »
How do you use a current transformer "as" a probe?  You can certainly connect it up with a burden resistor and thread it on a wire; but it's not exactly clip and go like a split-core probe is.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
The following users thanked this post: cvanc

Online tautech

  • Super Contributor
  • ***
  • Posts: 15337
  • Country: nz
  • Taupaki Technologies Ltd. NZ Siglent Distributor
    • Taupaki Technologies Ltd.
Re: Using a current transformer as an oscilloscope probe
« Reply #3 on: June 21, 2019, 06:38:39 am »
How do you use a current transformer "as" a probe?  You can certainly connect it up with a burden resistor and thread it on a wire; but it's not exactly clip and go like a split-core probe is.

Tim
Yep, this ^
TBH it's a right PITA and you get real sick of breaking connections to insert the thing.
Wasn't long before I bought a Tek P6021.....well waited until there was one on eBay that I could afford.
Avid Rabid Hobbyist
 
The following users thanked this post: cvanc

Offline cvanc

  • Frequent Contributor
  • **
  • Posts: 538
  • Country: us
Re: Using a current transformer as an oscilloscope probe
« Reply #4 on: June 21, 2019, 08:06:44 am »
Thanks everyone.  Here's a bit more detail.

I do a lot of audio amp testing.  Some amps have a single ended output and some are balanced.  So if I'm, say, looking to see where they clip using my scope across the load resistors I need to take care about that unbalanced scope input.

I do most of my bench testing (all the simple 'triage' stuff anyway) with a 400Hz oscillator.  I have big 8Ω non-inductive loads.  I want to use transformers to sniff the current in these loads so my scopes' unbalanced input is isolated from the DUT.

I would hook the transformer secondaries directly to the scope inputs; no shunt or other load would be added.  The 8Ω loads and associated transformer would be mounted onto a small piece of wood and permanently paired to each other.

This is solely to see the shape of the waveform, I have nice bench meters across the loads to tell me what the voltage is.  I do not need any amplitude accuracy from the scope.  Just need to see that 400Hz sine wave, and see when it flattens out.

These transformers are rated for 400Hz, so... why wouldn't this work?  I think it should be a decent, cheap solution to one (and only one) measurement issue.  Right?

Thanks again.
« Last Edit: June 21, 2019, 08:09:20 am by cvanc »
 

Online tautech

  • Super Contributor
  • ***
  • Posts: 15337
  • Country: nz
  • Taupaki Technologies Ltd. NZ Siglent Distributor
    • Taupaki Technologies Ltd.
Re: Using a current transformer as an oscilloscope probe
« Reply #5 on: June 21, 2019, 07:52:04 pm »
For that use I'd give them a go for sure cvanc.

You can always confirm their signal fidelity is more or less correct by using math and 2 channels for scope isolation from the amp.
Avid Rabid Hobbyist
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 13266
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Using a current transformer as an oscilloscope probe
« Reply #6 on: June 22, 2019, 06:56:30 am »
Yeah, that's fine for measuring output current.  And as long as your load resistor is resistive at the relevant frequencies (I would guess it's good up to ~low MHz), current or voltage will be an equivalent measurement.

You can solve the endedness problem by using two probes, with the scope set to difference (usually MATH menu).  You still need the grounds connected; it should be okay to connect these to chassis or signal or power ground.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf