Low current measurements with a BNC+ high current clamp

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Steve Smith
Pico Staff Member
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Joined: Sun Aug 25, 2013 7:22 am

Low current measurements with a BNC+ high current clamp

Post by Steve Smith »

Current clamps are without doubt one the most powerful diagnostic tools at our disposal and arguably one of the least intrusive. The results obtained from a current clamp confirms “work done”, so qualifying the integrity of the power and ground circuit whilst consuming only a single channel of our scope.

With the introduction of BNC+, current clamps have now taken on a whole new persona thanks to power via the scope (no internal batteries to worry about) along with auto configuration and zeroing upon connection.

Both Pico BNC+ current clamps (60 A TA473 & 2000 A TA388) have been designed with ease of use in mind such as slim wide jaws, no external switching and additional screening to reduce noise pickup.

For example, the 60 A TA473 provides exceptional performance from 10 mA to 60 A which covers all low current measurement applications.

The 2000 A current clamp (TA388) boasts a resolution of +- 100 mA to 200 A and +- 500 mA above 200 A which can be improved further still using the scaling and filtering features of PicoScope 7 software.

These impressive resolution figures for a clamp designed for high current applications has prompted a discussion about the flexibility of such a clamp for low current measurements.

Given a number PicoScope kits contain a single low and high current clamp, the flexibility of the high current clamp lends itself to both high and low current measurements. Let’s face it, how many times have we required 2 low current clamps during diagnosis and not considered using the 2000 A clamp?

“Need” is the driving force behind improvisation and if a 2000 A clamp is all you’ve got to compliment your low current clamp, then it’s all you’ve got!

So, with that said, let’s test the performance of this superb clamp as an additional low current measurement solution

All the tests below have a BNC+ 60 A and BNC+ 2000 A clamp connected to identical component wiring. E.g. Test 1 has both BNC+ current clamps attached to the same injector wire.

Test 1: Diesel solenoid injector
Image 1
Image 1
Above: The 2000 A clamp (Channel B red) has performed more than sufficiently to confirm the “in-rush”, injector hold and decay current. For absolute accuracy, of course the 60 A clamp demonstrates refined stability at 0 A and a true peak current value of 13.94 A

Test 2: Ignition Coil-on-plug
Image 2
Image 2
Above: I think we can agree there is not a lot to choose between channel B (60 A clamp) and channel C (2000 A clamp) for a relative measurement. I have applied a 400 kHz low pass filter to improve the resolution at the commencement of current flow into the primary circuit where the typical oscillations are clearly detected by the 2000 A clamp

Test 3: O2 sensor heater circuit
Image 3
Image 3
Above, channel B (2000 A clamp) captures the duty control of the O2 sensor heater but with an increased noise level by comparison to the 60 A clamp. Using the reference waveform and filtering features of PicoScope 7, this is easily resolved as can be seen in the pink reference waveform above where a 1 kHz low pass filter has been applied. More information surrounding the features of PicoScope 7 can be found here topic22097.html

Test 4: Gasoline port injection
Image 4
Image 4
Above: Once again, we can see how the noise level somewhat intrudes into our 2000 A clamp capture. However, filtering and scaling of PicoScope 7 has revealed the applicable current ramp associated with this style of port injector and still captured the pintle hump at the point of “needle lift” Please keep in mind the minimum input range for the 2000 A clamp is +- 5 A and here we are capturing peak current at 883 mA

Test 5: Parasitic drain
Image 5
Image 5
Above: I think it’s safe to say we are pushing our luck using a 2000 A current clamp to measure parasitic drain, but I hope you would agree, this clamp has performed exceptionally well? The noise level has improved thanks to a combination of minimal environmental noise (engine off) and the low pass filtering feature of PicoScope 7.

Of course, we cannot rely upon the 2000 A clamp for this style of measurement and the proof is in the pudding given the 60 A clamp returns a pass result (60 mA) whilst the 2000 A clamp indicates a fail @ >80 mA

So, let’s take a step back and look at what we are expecting of this 2000 A clamp in the above tests.

Using Test 4 as a prime example:

We require the 2000 A clamp with a 32 mm jaw to detect the magnetic field about an injector wire of approx. 1.00 mm2 carrying a current of <1 A within an electrically noisy environment!

We then ask the software to display this high frequency, low current using the minimum input range designed for this clamp at +- 5 A!

On paper, that does not seem plausible but the results above speak for themselves.

Whilst we have witnessed a fluttering at 0 A, a manageable increase in noise level and a tendency to read slightly higher than the 60 A clamp in low current applications, I think we can forgive all of these characteristics given the practical gains of having multiple low current measurement solutions. How cool is that?

More information on the BNC+ range of current clamps can be found here https://www.picoauto.com/products/current-clamps

I hope this helps, take care……Steve

martinr
TwoWaves
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Joined: Tue Mar 06, 2018 4:06 pm

Re: Low current measurements with a BNC+ high current clamp

Post by martinr »

Steve,

I’ve just been badly let down by my high amperage current clamp, a Hantek CC650 (topic22448.html). I know: serves me right. So I’m looking to replace it with quality hardware. I know that Pico’s BNC+ hardware is not compatible with my 4425 or 4823, but can you explain the difference between a current clamp that is advertised AC/DC and one that is advertised as DC? For example, the TA473 is listed as AC/DC, and its bandwidth is DC to 20kHz. The TA167 is listed as a DC clamp, but its bandwidth is also DC to 20kHz. To my thinking then, it should be able to measure anything the AC/DC clamp can, so why is it listed only as a DC clamp?

martinr
TwoWaves
TwoWaves
Posts: 53
Joined: Tue Mar 06, 2018 4:06 pm

Re: Low current measurements with a BNC+ high current clamp

Post by martinr »

Digging around, I think I’ve found the answer in a pdf giving advice on how to choose a current clamp.

https://www.chauvin-arnoux.com/sites/de ... alogue.pdf

1. Determine if you are measuring AC or DC (DC current clamps are categorized as AC/DC because they measure both).”

So it was just a slip in the description of the TA167: any clamp described as a DC clamp is an AC/DC clamp. (Whereas any clamp described as an AC clamp almost certainly won’t be able to measure DC current.)

Steve Smith
Pico Staff Member
Pico Staff Member
Posts: 1581
Joined: Sun Aug 25, 2013 7:22 am

Re: Low current measurements with a BNC+ high current clamp

Post by Steve Smith »

Hello Martin, thank you for the feedback and good catch

We can find clarification on AC/DC in the specifications but this requires digging into the clamp data and selecting the specifications tab.

I will ask the website team to include AC/DC within the titles for all DC clamps as this makes perfect sense

Take care......Steve

Kansasdeerslayer
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Joined: Sat Oct 03, 2020 12:41 pm

Re: Low current measurements with a BNC+ high current clamp

Post by Kansasdeerslayer »

Does Pico have any plans to introduce a BNC+ version of the TA 189 low amp large jaw current probe? I am considering purchase of the TA 189 but would wait if there is a newer version coming.

Steve Smith
Pico Staff Member
Pico Staff Member
Posts: 1581
Joined: Sun Aug 25, 2013 7:22 am

Re: Low current measurements with a BNC+ high current clamp

Post by Steve Smith »

Hello and thank you for the post

At present there are no plans for a BNC+ version of the TA189, but its a great suggestion as the clamp jaws are huge and remove the need for battery disconnection and the installation of a link lead for parasitic drain tests

I will add your suggestion as a new feature request for the hardware teams to consider

If jaw size is the deciding factor in your consideration of the TA189, could you take a look here https://www.picoauto.com/products//curr ... rent-clamp at the TA234

I hope this helps, take care.....Steve

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