[ARC5] BC-348-Q Capacitor Questions

[Roy Morgan]

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!