[time-nuts] Re: My AC measurement project & question

[Dave B]

On 23/01/2022 08:30, time-nuts-request at lists.febo.com wrote:
> Stick with the transformer. The use of a capacitive divider is predicated on the line waveform always being a sine wave. Dream on! All it takes is one good spike down the line, maybe only 20-30V amplitude, and your capacitive divider passes it right on to that ADC that has a much lower (3.3V?) limit. Guess what goes poof?
> ------------------------------------------------------------------------

Transformers distort the waveform, unless specifically designed for that 
need.

Many of the commercial monitors, use a fully floating ADC system to 
sample and digitise the incoming AC (via a purely resistive divider and 
protection devices.)Â  Then pass that digital data over a HV isolated 
channel (opto-coupler, or pulse/signal transformer) to other systems for 
parameter measurement recording and display.

The front end samplers and ADC are powered by an internal AC source (at 
10's of kHz) coupled in via a small HV isolated transformer, 
rectified/filtered/regulated etc to power the incoming sampler.  Such 
things are rated for use on industrial 3-phase systems, not just low 
voltage (115/230V) systems with full safety approvals.

(For experimental purposes, a battery would probably do.  But you'd 
still need an isolated data transfer channel.)

If you want to "wing it" there are some USB isolator devices, that even 
pass 5V power (via a built in isolated DC-DC converter.)Â  Conceptually, 
they could be used to run a small USB 'scope front end input device, if 
you trust the far east to build the things with enough 
creapage/clearance distances to maintain isolation in safety...

Or float an entire "sacrificial" laptop, and remote control it via WiFi 
with VNC for example.

I know a US company on the West coast, that used to (maybe still does) 
float entire Tek' scopes at many 100's of KV, to monitor the control 
element waveforms etc in industrial Klystron and TWT test fixtures..

Such 'scopes were "controlled" by extended non conducting control shafts 
and rod's.  115V/60Hz power supplied by an isolating transformer, where 
the secondary had a few inches of air space between it and the 
primary+core assembly.

They were inside the HV PSU rack, behind a non conducting transparent 
panel.   Simple, but worked well.

Regards.

Dave G8KBV



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