Using Oscilloscopes

"The visual respresentation of
an electrical signal against time"

And there are many who use oscilloscopes, yet do not understand what they are looking at. Now, we are not going to enter the realms of teaching of what an oscilloscope does, and how to use them. If you require such education please consult either the manuals or technical institutions.

What we are going to cover is the tricks one can pull when trying to trace power quality issues. Some of the tricks require a certain amount of circuit building and it is assumed that if you can 'drive a scope', you can build simple circuits.

Before We Continue: Please note not all oscilloscopes are suitable for use on the mains. Those built and used primarily for servicing electronic circuits are usually only rated Cat I i.e. protected circuits where the feed impedance is 30W or more (fault currents are limited). Using them for measuring mains (even within a domestic premises) can, during fault conditions, realize fault currents in excess of 5kA. Such currents can result in life-changing burns!

Equally as dangerous are the probes supplied with such instruments. These are usually rated for peak voltages of 100V max. The peak of a 230VAC supply reaches over 3 times that and therefore begs the probe to break down. This will take Live directly to Earth via the probe's shield, which is more than likely to be running through your hand at the time of use.

One of the handiest features of the modern digitally based hand-held 'scopes is the ability to store a waveform and download it to a PC for analysis and/or inclusion in a report. Most units also have an on-screen conversion facility giving Vrms, Vpk, dBm, etc. Such figures are extremely useful but should be used as a guide only as the accuracy is usually, at best, no better than a few percent.

One of the main issues is noise and the advantage of a scope is the ability to display frequencies well into the MHz range. Transformers are selective about the frequency range they will transfer making resistive voltage dividers the better alternative. However, this does raise another issue.....

Do not use mains powered oscilloscopes for this purpose. Doing phase-neutral measurements with a mains powered, single channel input scope will require that the scope's input ground be connected to Neutral. This will lead to serious ground loop currents through the leads.

An isolation transformer or 'soft earth' becomes attractive in these instances, but poses serious safety risks. I am not a fan of the isolation transformer as the casing of the 'scope can be at a dangerous potential without anyone knowing. The 'soft earth' unit described under "measurement aids" has a lamp built in to indicate a dangerous potential with the added advantage of bringing the casing back to a safe potential once the offending voltage is removed, however, using this in the company of any untrained undividual can lead to indemnity claims.

Any person who is seriously considering investigating power quality issues should invest in a decent battery powered oscilloscope with a good Cat rating (600V Cat III minimum), and one that has a long battery life between charges so as to avoid the frequent use of the battery charger. I can recommend the Fluke 43B (using the Scope function) or the Velleman Handheld Scope (mine is the K7105) with a battery life in excess of 8 hours between charges - more than a day's work! Although not as versatile as the more renowned makes of hand test instruments, the battery life more than makes up for only a slight lack of features.

With the issues surrounding the use of a mains powered 'scope (soft earths etc.) one can quickly see why a battery powered one becomes so attractive.

Just a small note on safety here; If you are intending to use battery powered oscilloscopes, such as the Velleman, never use them while powered from an AC adapter. Should the adapter break down during testing then serious current could flow.

Measuring Phase-Neutral  >>
Using Simple Recorders  >>

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© 26.01.03