Best practices for inexperienced users

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1 Feb 2010
Posts:

I just bought a USB Cleverscope with the 14 bit digitizer and 8 probe digital pod. I was hoping for a basic tutorial for dummies might be included in the docs that would instruct how to safely use the scope although if it is there I haven't yet found it.

This is my first scope and I'm learning electronics through reading and trial. I want to work on a great variety of potentially hazardous items, old television sets & computer monitors, switched mode power supplies, high voltage brushless DC motor controllers, etc. and I'm concerned about the risk of doing something stupid like shocking myself, something I'm trying to fix or even worse frying my new scope.

I don't yet have a differential probe and would like to avoid the expense if it isn't needed. As I understand it, the stock probe tips are high impedance 1 MOhm/10 MOhm depending on setting, but the ground clips are very low impedance. So one of the biggest risks is when attaching the ground clips. Is there a fuse in the probe ground clip lead or in the Cleverscope in case of a goof up? If not are there fused probes available suitable for use with the Cleverscope?

So if I'm unsure of the continuity/voltage of a point where I wish to attach a ground clip what is the best approach? Should I use my cheap handheld DMM to first do a continuity test and if there is continuity, set it to it's highest voltage range to reprobe the potential ground point, and then gradually dial down to the lowest voltage range to confirm no potential? If there is no continuity between a given point in a device under test and the scope probe's ground clip is it essentially safe/ok to use the passive probe as a differential probe, ie can I be moving the ground clip around to test across individual components?

I'm guessing it is generally better to be operating the scope and my computer on mains power through the same grounded outlet and if I need isolation to put the device under test on an isolation transformer? I don't yet have an isolation transformer, but I do have a battery powered pure sine wave inverter which I'm expecting I can use for the same purpose? Any risks?

Also, eventually I'd like to be using the Cleverscope in mobile situations with a netbook computer and making an adapter cable to power the Cleverscope from a separate battery. Is it ok to have these two devices floating with respect to each other only to be connected by the USB cable? Any risk of a static shock on connection? Would I want to use an isolating USB cable in this case and let the two devices both float? Or should I try to make a ""ground"" connection between the scope and netbook power connectors? If so, what should the impedance of the connection be?

I apologize if these questions are stupid or already answered elsewhere. If so, just direct me to where I can find the answers instead of taking the trouble to go into all the details.

Thanks,
Todd Allen

3 Feb 2010
Posts: 395

Hello Todd,
I think these are very reasonable questions.
I think a white paper on this is a good idea, and we will write it in the next couple of days, and put it in the resources section, and add it to the manual.
We are using standard scope probes and they don't come with a fuse. My suggestions are:
1. Run all your equipment off mains power, but put a ground fault interrupter (or residual current detector) in the power lead to plug-box you run your gear off. Any ground current will then trip the power supply, and save you and your equipment.
2. If you know you will be possible to probe higher voltage lines, and you don't want the power to be disconnected, then use an isolation transformer as you say. You still have to be aware that if you have one ground clip attached to one location and you clip the other ground clip to a live terminal current will flow between the ground clips.
We don't have a fuse, but we do have two thin sacrificial tracks under the PCB going from the BNC ground lead to the main system earth. If you connect two clips to different potentials, then one of these tracks will blow. This is not the end of the earth, as it is fairly simple to repair.
3. You could buy a differential probe, as you say. This will give you much greater versatility.
4. You could buy the Ethernet upgrade, which is isolated (we will credit the USB card cost), and isolate the power supply. This provides protection for you and the PC, but does not prevent two ground clips at different potentials.

The idea of using a cheap voltmeter to check potentials is a good one.

Yes you can use a small 12V gel cell battery to power the cleverscope with the USB being the common earth. No need for other connections.

We do plan to do a few small videos showing you how to do simple tasks. Check out the videos page every now and then :-)

5 Feb 2010
Posts:

Bart, thanks for the info and suggestions. I went out and bought two GFCI outlet adapters. I'm currently just using one with a power strip and everything (PC, Scope, DUT) plugged into it. I may use the second one just for the device under test when I need to power it from the inverter for isolation or if I find myself tripping off my computer too often when ground faulting on the DUT.

Actually I hope to never trip it because I'm not convinced it will offer adequate protection, especially when dealing with devices that have high energy caps and inductors.

I'm increasingly thinking about getting a differential probe. The flexibility and safety advantages are looking increasingly desirable. I haven't yet screwed up, but I've come close a couple times.
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