P6042A - Tek current probe - replacement circuits - AN-61 LinearTechnology

[Calvin]

Hi,

my Tek P6042A current probe is working ... needs just some adjustments .... but I´m thinking designing a replacement circuit as a future project.
Now there´s the AN-61 from LinearTechnology written by Jim Williams as a start.
So I began to hack the circuit into LTSpice.
Wondered though about the high gain values especially of the LT1228s current feedback amp part.
The old Tek manuals and memos state a sensitivity of 50µV/1mA of the Hall Element and a 1mV/1mA output after the buffer stage that feeds ito the sensing coil of the probe.
The buffer needs to maximally supply for +-10V/200mA into the 50R load at a current of +-10A.
The ac sensor is a a 50 turns coil (0.5mH) which will also result in 1mV/1mA sensitivity.
So the Hall-Element amplifier requires a gain of 20 to fit the sensitivity of the ac-coil sensor.
AN-61 shows a circuit by Jim Williams which is intended as a direct replacement of the Hall-element amplifier of the P6042A.
It seems to me that there is a multiple of errors in that design.
- the gain of the transconductance part of the LT1228 is already high, giving ~10mV@50µVin
- the gain of the current feedback part of the LT1228 varies around 1.000x .. as such it is impossible to measure +-10A curent as the amplifier saturates much much earlier.
- the value of the set current resistor of the transconductance part is way too low, resulting in ~7.5mA (datasheet recimmends between ~100µA and 1mA)
- the datasheet mentions a temperature compensation through the Iset pin of the LT1228 by adding two resistors which is missing
Has anyone already made experiences with this circuit?
Attached is a modded schematic that at least in simulation results in a clean transistion between low-frequency and high-frequency part.

regards
Calvin

 

[David Hess]

- the gain of the transconductance part of the LT1228 is already high, giving ~10mV@50µVin
- the gain of the current feedback part of the LT1228 varies around 1.000x .. as such it is impossible to measure +-10A curent as the amplifier saturates much much earlier.

Does that take into account that the LT1228 is itself inside of a feedback loop which includes the sensor?  The gain of the LT1228 nulls the DC current through the sensor.

- the value of the set current resistor of the transconductance part is way too low, resulting in ~7.5mA (datasheet recimmends between ~100µA and 1mA)

The datasheet also says that the maximum set pin current is 15 milliamps.

- the datasheet mentions a temperature compensation through the Iset pin of the LT1228 by adding two resistors which is missing.

Jim Williams just did not bother with temperature compensation assuming instead that the operating temperature range would be limited.

[Calvin]

Hi,

"Does that take into account that the LT1228 is itself inside of a feedback loop which includes the sensor?"
Actually no.  why I and my colleague missed to ´see´ the fb loop 
Still though the low-frequency Hall-amplifier has a bandwidth of several kHz.

regards
Calvin

[David Hess]

"Does that take into account that the LT1228 is itself inside of a feedback loop which includes the sensor?"
Actually no.  why I and my colleague missed to ´see´ the fb loop 

The gain inside the loop still matters for equalization to produce a flat response.

Still though the low-frequency Hall-amplifier has a bandwidth of several kHz.

It does, and the DC correctly loop can only operate that quickly although I am not sure what limits the hall effect response in this case.

[Jay_Diddy_B]

Calvin,

There is an important link missing in your model. You need to drive G1 and G2 with the current flowing in T3.

This is a link in the magnetic circuit.

Jay_Diddy_B

[Jay_Diddy_B]

Hi,

Here is an LTspice mode that represents the circuit on AN61:



The flux in the core is modelled by summing the ampere turns in the two windings.

The bridge in AN-61 is modelled by the Thevenin equivalent circuit.

The circuit works by nulling the flux in the core.

Modeling Results

AC response



Step Response - Transient Domain



Fast Step Response




I have attached the LTspice model.

You can play with the parameters in the model to determine the sensitivity to change.

Regards,
Jay_Diddy_B

 AN61 model.asc (2.61 kB - downloaded 76 times.)

[Tantratron]

I've a few questions.

Q1: could you post a picture of your P6042A probe sensor and amplifier because unless to be wrong, I cannot find any datasheet about tektronix P6042A.

Q2: There are legacy bulky P6042 units designed long ago. Later tektronix miniaturized the DC-AC probe (hall + current transformer + amplifier) as TCP202 which be an upgrade of the specific flat band nulling amplifier

[Jay_Diddy_B]

Hi,

You are correct there is no 'A' version of the P6042.

Link: http://w140.com/tekwiki/wiki/P6042

It is an error made in the OP's title that has been copied in later posters.

Jay_Diddy_B

[Tantratron]

Hi,

You are correct there is no 'A' version of the P6042.

Link: http://w140.com/tekwiki/wiki/P6042

Hello Jay, would you happen to know the road map or evolution through time made by tektronix about these DC-AC probes ?

It seems the probe head (hybrid hall sensor and current transformer) including the probe black body has not changed physically so initially the amplifier was separate. There must have been other designs improvements through time so I wonder if the actual TCP202 which has probe head and amplifier inside its termination box does not offer say similar circuit @Clavin wanted to do.

Thanks, Albert