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Ideas to improve a zero-field transformer based circuit
Ideas to improve a zero-field transformer based circuit
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Old 28th June 2020, 10:40 PM   #11
1audio is offline 1audio  United States
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Ideas to improve a zero-field transformer based circuit
If you have the necessary SNR with 60 Ohms maybe just adding another amp stage would be an easier way to get there.
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Old 29th June 2020, 02:41 PM   #12
Elvee is offline Elvee  Belgium
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I remade the noise sims to leave no stone unturned, but the result is punishing even with a LT1028: depending on the frequency, the penalty is comprised between 5 and 20.

I have made an attempt at 2mA FS (with the 60K, of course), and it looks practicable, at least from a noise POV.
The hum is atrocious, because the magnetic circuit is long and flimsy and catches every bit of field leaked in the vicinity, but it could be usable in the right environment (my lab as it is does not qualify).
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Old 29th June 2020, 08:00 PM   #13
BSST is offline BSST  United States
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Hi Elvee,

I'm not seeing modeled in your schematic the 104 ohm series resistance you mentioned in your first post describing the ~0.5H transformer secondary. Am I missing something?

Best,

Steve
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Old 29th June 2020, 08:45 PM   #14
Elvee is offline Elvee  Belgium
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It is not shown explicitly, but it is included in the minor parameters of the inductor:
Ideas to improve a zero-field transformer based circuit-acclampscrsh-png
Attached Images
File Type: png ACclampscrsh.png (47.6 KB, 61 views)
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Old 1st July 2020, 07:53 PM   #15
Elvee is offline Elvee  Belgium
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I have made my choice: I will use two frontends with the 60 ohm one using Steve's proposition, and the 60K one my initial circuit with the caps in the positive feedback path. They are sufficient to make the amplifiers DC-stable.
The DC point is not very well defined, but since the rest is AC-coupled, it does not really matter.

Of course, using a single, DC-coupled frontend would be better if possible, but although the equalizations for the two situations are similar, they cannot be fused in a single network and would need to be switched anyway.

The secondary winding has a relatively nasty behavior, which is not easily modeled with lumped parasitics, and the length of the winding causes transmission line effects which are not simple to compensate with lumped components, but it is possible to arrive at an acceptable result using simple means, if some ripples are tolerated.
This is the uncorrected 10kHz squarewave response:

Ideas to improve a zero-field transformer based circuit-acclampw1-jpg

It is corrected with this network:

Ideas to improve a zero-field transformer based circuit-acclampeq-png

And the (provisional) result is this:

Ideas to improve a zero-field transformer based circuit-acclampw2-jpg

At this stage, I am not going to refine the circuit too much, as the final implemention will have some effect
Attached Images
File Type: jpg ACclampW1.jpg (204.3 KB, 55 views)
File Type: png ACclampEq.png (6.7 KB, 49 views)
File Type: jpg ACclampW2.jpg (222.4 KB, 55 views)
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Old 2nd July 2020, 01:52 AM   #16
trobbins is offline trobbins  Australia
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Elvee, just taking a step back to the range of applications you may have had in mind, was this aimed at non-contact audio current measurement, as per some of the cheap LEM current sense modules that are nowadays in common use in switchmode power supply pcbs?
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Old 2nd July 2020, 07:12 PM   #17
BSST is offline BSST  United States
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Hi Elvee,

Very nice work!

At this inconveniently late date, I have another thought--- a so-called “DC servo” sometimes wrapped around audio power amps.

On the notion that a sketch is worth a thousand words, I’ve pasted one possibility into your schematic. There would be an identical network around U1 but I didn’t bother to sketch it. Since it forces 0 gain at DC (sub-Hz bandwidth), the servo would probably behave similarly to the circuit in my first picture and may not offer any advantage. But with the servo scheme, I thought you might devise the unified, gain-switched design you prefer.

BTW, I have a couple Amprobes inherited from my dad, one with a stuck meter movement. Maybe it will become another project.

Best,

Steve
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Old 2nd July 2020, 08:24 PM   #18
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by trobbins View Post
Elvee, just taking a step back to the range of applications you may have had in mind, was this aimed at non-contact audio current measurement, as per some of the cheap LEM current sense modules that are nowadays in common use in switchmode power supply pcbs?
What I have in mind is a general-purpose, bench instrument mainly aimed at audio, PSU and similar applications: visualizing the waveforms of the collectors current in an OP stage, the charging pulses in the smoothing capacitor of a conventional DC supply, the inrush current of a transformer, etc.
I have had this thing lying around for many years, and I thought that the wide jaw opening combined with the relatively slim core would make it handy for practical use.
The fact that its parameters allow for a relatively large max current (200A rms) together with a good sensitivity and dynamic range (minimum FS range 20mA or even 2mA) was also a factor that decided me.
I already have a number of other probes, but I thought this one would add something else.

Quote:
Originally Posted by BSST View Post

At this inconveniently late date, I have another thought--- a so-called “DC servo” sometimes wrapped around audio power amps.

On the notion that a sketch is worth a thousand words, I’ve pasted one possibility into your schematic. There would be an identical network around U1 but I didn’t bother to sketch it. Since it forces 0 gain at DC (sub-Hz bandwidth), the servo would probably behave similarly to the circuit in my first picture and may not offer any advantage. But with the servo scheme, I thought you might devise the unified, gain-switched design you prefer.
I have already considered and tested this option in sim, but with a plain-vanilla scheme, it requires to push the pole of the servo at impossibly low-frequencies, resulting in time-constants in the tens of seconds.
There could be way to avoid that, like a higher order filter, but I don't feel I am competent enough to tackle the resulting stability issues.
The unused components on the schematic are the remains of another, local DC servo scheme I also tested -but abandoned in the end-
Quote:
BTW, I have a couple Amprobes inherited from my dad, one with a stuck meter movement. Maybe it will become another project.
If they belong to an instrument similar to the one I pulled mine off, the stuck meter is the normal condition: it is part of the printing process.
Most of the time, the writing needle is pushed against the paper, and as a result the galvanometer is stuck.
Regularly, the pen is lifted, and the movement is allowed to move to another position.
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Last edited by Elvee; 2nd July 2020 at 08:28 PM.
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Old 2nd July 2020, 09:13 PM   #19
1audio is offline 1audio  United States
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Ideas to improve a zero-field transformer based circuit
Correct to better explain the stuck needle models- some had a mechanism that trapped the meter except when making a measurement (using a button or trigger) so you could measure something without being able to see the meter. But it may just be stuck.
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