[chbot] Earth issues

[Richard Jones]

When the oscilloscope mains earth is not connected you may expect your
scope probe ground lead to achieve half mains potential through the
potential divider formed by the capacitors in the typical mains input
filter circuit shown here:

https://interferencetechnology.com/wp-content/uploads/2015/09/Woodgate-Fig-2-768x407.jpg

The solution is to connect the oscilloscope to mains earth and design your
way around the measurement issues. I have witnessed accidents (bangs,
flashes, smoke and embarrassment) caused by unearthed oscilloscopes and a
moments inattention.

I much prefer to disconnect circuits completely before soldering. You have
less likelihood of damage from static and connected equipment delivered
voltages that way.

Richard Jones


On Tue, Nov 22, 2016 at 2:45 PM, Marshland Engineering <
marshland at marshland.co.nz> wrote:

> Ok not Kaiakora type but electrical ones.
>
>
> I have a digitally controlled soldering iron, PSU and Scope. For the past
> XX
> years, I never connected the scope earth to the plug earth thinking that
> the
> probe ground would therefore float with reference to my power supply earth.
>
> Any way, when soldering components on my test PCB, a few LED's started to
> glow. I measured the voltages on the test equipment and I had 90 volts
> between
> my soldering iron and scope probe earth. A few more measurements and I
> found
> scope probe earth was the problem.
>
> What is normal? Earth the scope, disconnect all cables to the board before
> soldering, run an isolating transformer?
>
> Often USB powered boards get supply from the PC and some laptops only have
> 2
> pin plugs so all sorts of earth issues can occur?
>
> Any suggestions ?
>
> Just ordered a battery pocket scope from Aliexpress. That will solve one
> issue.
>
> Cheers Wallace.
>
>
>
>
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