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A center tapped filament transformer, or a 'hum balance' circuit to create an artificial center tap
will do
even better. That circuit was often no more than a 200 ohm pot across the winding, with the wiper
connected to ground, and both filament leads floating above ground. A 2W pot was common for this
use.


Michael A. Terrell

------------------------------

Are there any cheap approximations of the Iprober 520 fluxgate sensor
current probe?
Maybe some sort of IC?

ST

On Sun, Dec 9, 2018 at 8:19 PM David Slipper <softfoot@...> wrote:

I did look at making one with a Hall sensor - the electronics isn't a
problem but the difficulty arises (for me at least) in cutting the slot
in a ferrite ring to put the sensor in or even splitting the core to
make a clamp. I guess it's easier if you have access to the right tools.

Any ways, this works well for me :-)

On 09/12/2018 18:52, stefan_trethan wrote:

If you only need low frequency the LEM brand hall effect sensors, I
believe Allegro also makes chip based ones, can be an option.
Also remember you can put 10 turns through if you need more
sensitivity (insertion impedance permitting).

I did look at making one with a Hall sensor - the electronics isn't a
problem but the difficulty arises (for me at least) in cutting the slot
in a ferrite ring to put the sensor in or even splitting the core to
make a clamp. I guess it's easier if you have access to the right tools.

Any ways, this works well for me :-)

On 09/12/2018 18:52, stefan_trethan wrote:

If you only need low frequency the LEM brand hall effect sensors, I
believe Allegro also makes chip based ones, can be an option.
Also remember you can put 10 turns through if you need more
sensitivity (insertion impedance permitting).

If you only need low frequency the LEM brand hall effect sensors, I
believe Allegro also makes chip based ones, can be an option.
Also remember you can put 10 turns through if you need more
sensitivity (insertion impedance permitting).

I quite often have a need to sense small currents (~10-200mA) and have
always wanted a clamp on current probe but even the used ones for a
scope are out of my budget.

A series resister is one solution but it is rather intrusive and
sometimes the circuit cannot tolerate the voltage drop.

Recently, at a garage sale I came across an old RS current clamp
intended to be used with a multimeter, it is a Hall effect device so
it's good for DC too, so just as an experiment I attached it to my scope
with a "banana<--->BNC" adapter and it works a treat :-)

The sensitivity is 1mV for 10mA and the bandwidth seems adequate for my
needs. If I need better sensitivity I just wind the wire 10 times around
the sensor clamp which gets me 1mV per 1mA.

Since I'm trying to get an Arduino and ESP8266 WiFi adapter to run on
batteries it is proving invaluable.

It may not have the bandwidth of a purpose built 'scope current probe
but it seems OK for my needs.

Dave

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Roger Evans via Groups.Io
Sent: 09 December 2018 08:27
To: [email protected]
Subject: Re: [TekScopes] 466 Channel 2 problem

Colin,

If you press the beam finder and then switch Ch 2 to 'invert' does the position of the channel 2 trace move? This is a quick check on whether the channel 2 imbalance is before or after the >invert circuitry (Q220 - Q226).

I am puzzled when you say that you 'mostly cannot see anything untoward' on channel 2 voltages and then say that the voltages on TP11 and TP14 are very different. Are they the only >places where the DC voltages on the two halves of the paraphase amplifier are noticeably out of balance? Since the amplifier is DC coupled throughout a failed transistor or diode tends to >upset the DC levels through other parts of the chain and finding the failed component can take patience. If the transistors are socketed then just pulling them out for testing, or swapping >with the corresponding channel 1 parts if you don't have a transistor tester is an easy route. If they are soldered in then you need to measure Vbe and Vce to check that they are consistent >with a transistor operating in its linear regime. Don't forget that the switching diodes CR314 - CR318 are part of the DC circuitry so a failed diode will also upset the amplifier balance.

The probablility is that you have a failed semiconductor but it is possible that a resistor has drifted badly over time.

Roger