Re: Scope grounding and generators

Robert Simpson

First, a question came up about my background.
With a year of military training I was a radio repair tech in the Army, both tube and transistor equipment. I worked my way through college (business degree) working with the university electric shop under the direction of certified electricians. We did ever thing from wall outlets, motors to building main panels. I still have my half inch conduit bender. I have been repairing Tek scopes, computer monitors, TVs etc. as a hobby for over ten years now including HV supplies in 7603s and a 7904. I have appropriate HV probes. My professional life was in Information Technology from early DEC mini computers to finally leading development teams building custom web data base business applications as a PMI certified project manager. Generators are new to me.

Moving on,
A little bit of progress. I have good ground wire set up. After the comment about the ground being 3/16”, I found a short piece of copper tubing with a 3/16 OD. By soldering the end of an insulated wire into a short section of tubing, I can push the tubing into the grounding hole part of a socket with no open wire exposed. As the tubing fits snugly, I have a protected secure connection. The other end of my wire is screwed into the ground lug of the generator.

Since the generator puts out distorted AC, I am curious about max volts. I have a Fluke DMM, model 8024B (the one with buttons along the left side) that has a max reading save option. So I am thinking of some experiments. Such as what is max volts when first starting the generator. What is the max volts when running with no load. What is the max volts when under some load.

The suggestion about isolating the scope through a transformer got me thinking about a voltage spike damaging the scope. I did probe my house power using a connected socket the scope is plugged into at my bench. I used a P6006 with 6 foot cable. Checking the P6006 on the calibrator signal showed a very clean square wave. Then using the 50V setting I checked house power. Not surprising, there is a very clean sine wave of about 350V P-P and a 60Herz frequency.

Also, would an isolation transformer (which I have somewhere around the house, Haven't used it in awhile) be just as good as a filament transformer (which I don't have)?

Note: All this effort is only meant for a couple of test runs. I have no plans of ongoing monitoring. I think my generator is running a little fast as my DMM shows 132V as compared to my house voltage of about 121V. Would a wonky sine wave of the generator affect the DMM reading?

So the scope can show me the frequency. If the generator is running fast, I will need to adjust the governor. If the frequency is correct then maybe the voltage regulator is off. And of course I am curious about just what kind of sine wave the generator puts out. I don't have a frequency meter as I haven't needed one until now.


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