[ARC5] BC-348-Q Capacitor Questions

[Dennis Monticelli]

The problem with Hall Sensors is getting decent absolute accuracy and
sensitivity.  As a measure of relative magnetic fields that are moderately
strong, they are quite useful.  Your auto ignition system probably uses a
Hall to sense engine rotation.

Tektronix makes some nice DC+AC probes such as their pioneering P6042 and
its many offspring.  These use a combination of Hall Sensor and magnetic
core and can be quite costly.  The probe itself is a little bulkier than the
428 which make it a bit harder to get around the wires buried in our
radios.  The BW of the P6042 is 50MHz vs only 400Hz for the old HP.  Roy
gave a good description of the operation of the HP 428.  I will only add
that the circuit architecture is VERY clever.  My hat is off to the
designer.  It uses the "flux gate" principle in which a magnetic chopper
up-converts the input current signal (which has been converted to flux) to
40KHz, amplifies it greatly in a 40KHz IF, then synchronously demodulates
the signal for use as feedack to the head assembly to restore balance.  The
feedback signal drives the meter and scope output as it represents a near
perfect scaled replica of the input current being measured. There are 9
ranges which cover from 1mA FS to 10A FS.

No, I'm afraid you cannot homebrew one of these gizmo's.  There is, however,
a modern solid state equivalent made by FW Bell and others.  It is the same
basic instrument as the 428, only transistorized and housed in a small
plastic box with a similar (though much fatter) external probe.  I have one
of these too, also courtesy of eBay.  They can run off batteries which is
quite handy, but the fat probe that was designed for industrial & automotive
use is not as effective poking around a chassis as the slimmer 428 probe.
As Roy noted, 428's are often found without their probe.  That's how I
bought mine very cheaply. But then I had to chase down a probe.  It's easier
if you buy one with its probe, though you will have to pay much more that
way.  HP 428's with probe in good operating condition go for $100 or less.
Ditto for the FW Bell version.

Dennis AE6C

On Sun, Aug 22, 2010 at 11:25 PM, Roy Morgan <k1lky at earthlink.net> wrote:

>
> On Aug 23, 2010, at 2:08 AM, Henry Mei'l's wrote:
>
> > Hello Dennis
> >
> > How does this work?I don't have an HP 428 but is there some way I
> > can rig up
> > something using standard VTVMs/VOMs &/or scopes to do the same thing?
> >
> > ----- Original Message -----
> > From: "Dennis Monticelli" <dennis.monticelli at gmail.com>...
> >> I find that it is easier and less invasive to use an old HP 428
> >> clip-on
> >> current meter to check caps.
>
> Basically, here is how the HP clip-on milliammeter works:
>
> The clip on probe contains a magnetic core that is placed over the
> wire being tested.  The core is operated at a certain level of
> magnetic flux by the instrument.  The DC current in the wire under
> test, changes the flux in the core.  This change is detected by the
> instrument and there is a corresponding reduction of the flux in the
> core to return it to the original balanced state.
>
> So, the important parts of the system are:
>
> A unique clip-on core
> A source of magnetism in the core that is monitored by the instrument
> A feedback system to reduce the magnetism to restore the original flux
> level
> A metering system to display the detected current.
>
> Note: if you buy one of these HP meters without the clip-on probe, you
> have a useless piece of equipment.
>
> There are clip-on probes meant for use with oscilloscopes that detect
> AC currents in a wire, and may well be some that detect DC currents.
> Tektronix, naturally, had/has such AC detecting probes. The one I know
> of is quite old now, and had a little power supply and amplifier as
> part of the setup.  It's frequency response was modest, but I think it
> worked at line current and up to perhaps some couple of hundred kc.
>
> I have a couple of devices that are called "Hall Effect sensors".
> These things are based on relatively new creations of modern physics
> that I don't understand, but I think they detect current in a wire
> that is passing through the opening in the thing.  Search via Google
> for "hall effect sensors" and you'll get lots to read. Apparently, you
> can buy them at Digi-Key.  This seems to be a promising avenue to
> explore.
>
>
> Roy
>
> Roy Morgan
> k1lky at earthlink.net
> K1LKY Since 1958 - Keep 'em Glowing!
>
>
>
> ______________________________________________________________
>  ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>