Current Probe Hall Effect Problems and Questions


 

I don't know the right questions to ask about the Hall Effect devices used
in Tek's current probes like the P6042 and the P6302 but I am struggling to
understand if they are the reason I can get these probes to work.

First, the Hall Effect is a very simple concept. Hats off to the inventor
for realizing it was so simple to do this.

The P6042 I have is getting the High frequency signals that come from the
windings around the current probe's core. I cannot get the DC and low
frequency signals to work.

There are two precision "hand-selected" resistors that bias the Hall Effect
sensor. No matter how I choose them there is always a 0.25V differential
between the output of the Hall Effect Sensor. I can get the bias to be at
the correct +3.0V on one side but as soon as I adjust the other side of the
Hall Effect Sensor to be +3.0V the first side drops to 2.75V.

So now I change the bias resistor to raise the first side up to 3.0V and
that pushes the second side up to 3.25V. I am sure this differential is
causing the differential amplifier IC to saturate. I don't see a way around
this.

Years ago when I opened up the head of one of Tek's current probes I seem to
recall there were one or two hand selected resistors inside the head. If my
recollection is correct were these used to fix that offset problem?

Another question I have is more basic: How do you tell if the Hall Effect
Sensor is dead. It is such a simple concept I cannot imagine how you could
ever blow one out since, at least in the P6042, they can't get any voltages
that might harm them. But I don't know anything about how delicate and prone
to damage they are.

Is there a good source of information somewhere on how they are supposed to
work in Tek current probes, how to test them, and how to bias them?

Dennis Tillman W7pF


snapdiode
 

It's a physical problem, the Hall sensor is some sort of very thin film of secret sauce applied to a piece of ferrite, and then the magnetic circuit is completed by some other piece of ferrite glued to it. And that's the problem, the whole assembly is rigid to the point of brittleness. A mild mechanical shock will somehow crack something and it can not ever be repaired. Ever. You can't fix this electronically.

I took a head apart and you can see the construction both in X-rays and visible light photos. https://groups.io/g/TekScopes/album?id=13145

A dead P6042 is pretty much game over. There is a very nice person who managed to snap up the last in-box replacement head on eBay that I've ever seen. I haven't seen any spares on eBay in a decade +.


Jean-Paul
 

Dear Dennis,

the very thin cross section of the ferrite jaws is so fragile that dropping a probe a few inches onto a hard surface can cause hairline cracks.

Ferrite is a ceramic and has no flexibility, so its super fragile. Either bad hall effet Sensors or cracking of ferrite produce such symptoms as no adjustment of resistance or high differential voltage.

Unfortunately the Tek current probes on epay are often mistreated and impossible to verify that they are not dropped.

All of my probes are working so I have no experience in repair.

Bon courage

Jon


 

Use the later design which instead of two resistors, used a single pot (10k)
as used in the later versions of the probe (don't have manuals here so can't
give more detail).

D.

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Dennis Tillman
W7pF
Sent: 23 May 2021 03:34
To: TekScopes@groups.io
Subject: [TekScopes] Current Probe Hall Effect Problems and Questions

I don't know the right questions to ask about the Hall Effect devices used
in Tek's current probes like the P6042 and the P6302 but I am struggling to
understand if they are the reason I can get these probes to work.

First, the Hall Effect is a very simple concept. Hats off to the inventor
for realizing it was so simple to do this.

The P6042 I have is getting the High frequency signals that come from the
windings around the current probe's core. I cannot get the DC and low
frequency signals to work.

There are two precision "hand-selected" resistors that bias the Hall Effect
sensor. No matter how I choose them there is always a 0.25V differential
between the output of the Hall Effect Sensor. I can get the bias to be at
the correct +3.0V on one side but as soon as I adjust the other side of the
Hall Effect Sensor to be +3.0V the first side drops to 2.75V.

So now I change the bias resistor to raise the first side up to 3.0V and
that pushes the second side up to 3.25V. I am sure this differential is
causing the differential amplifier IC to saturate. I don't see a way around
this.

Years ago when I opened up the head of one of Tek's current probes I seem to
recall there were one or two hand selected resistors inside the head. If my
recollection is correct were these used to fix that offset problem?

Another question I have is more basic: How do you tell if the Hall Effect
Sensor is dead. It is such a simple concept I cannot imagine how you could
ever blow one out since, at least in the P6042, they can't get any voltages
that might harm them. But I don't know anything about how delicate and prone
to damage they are.

Is there a good source of information somewhere on how they are supposed to
work in Tek current probes, how to test them, and how to bias them?

Dennis Tillman W7pF


Twig
 

The offset nulling circuit in the later A6302 probes has a 2K trim pot connected across the +3V supply (orange/white wire to connector pin D) and the -3V supply (red/white wire to connector pin A). The wiper goes to a 20K resistor which then drives the offset signal (brown/white wire to connector pin H).

If you have an AM503, you can look at the output of the Hall device at the output of U110, the 100x preamp stage. Be sure to pull down the gate of Q115 to shut off the feedback, though, so the feedback loop does not null the signal you are looking at. You can connect a 1K resistor across the leads of C124 to pull down the gate of Q115.


 

Hi Snapdiode,

WOW!
Beautiful photographs!
Beautiful job de-potting the head!
Amazing X-Rays!
I am extremely impressed with the job you did dissecting the P6302 head.
It is truly unfortunate that this was once a working current probe.

I knew there was a lot going on just from reading the manual and looking at the schematic, but you brought the entire head to life in a way that made me amazed by what Tek designed. These probes have been a huge success ever they were originally designed and listed in Tek' catalogs.

The P6042 was introduced in 1967. It was designed by Cal Hongel.
The P6302 first appears in the 1977 catalog along with the AM503 and the enormous CT-5 current probe that requires the P6302 to be inserted into it.
The P6302 becomes the A6302 in the 1982 catalog.

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of snapdiode via groups.io
Sent: Saturday, May 22, 2021 10:14 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Current Probe Hall Effect Problems and Questions

It's a physical problem, the Hall sensor is some sort of very thin film of secret sauce applied to a piece of ferrite, and then the magnetic circuit is completed by some other piece of ferrite glued to it.
And that's the problem, the whole assembly is rigid to the point of brittleness.
A mild mechanical shock will somehow crack something and it can not ever be repaired. Ever. You can't fix this electronically.

I took a head apart and you can see the construction both in X-rays and visible light photos.
https://groups.io/g/TekScopes/album?id=13145

A dead P6042 is pretty much game over. There is a very nice person who managed to snap up the last in-box replacement head on eBay that I've ever seen. I haven't seen any spares on eBay in a decade +.







--
Dennis Tillman W7pF
TekScopes Moderator


snapdiode
 

Yes, unfortunate, very much so. It just isn't possible to repair the DC offset problems if the problem is in the transformer. It's pretty good as an AC probe I guess.