[chbot] Earth issues

[Volker Kuhlmann]

On Tue 22 Nov 2016 14:45:25 NZDT +1300, Marshland Engineering wrote:

> 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.

Somewhat dangerous, the scope has a metal case requiring earthing. Probe
ground then has a low-resistance connection to earth. Yes it's possible
to write off probes and scopes with that, I've heard, measuring mains
circuits.

The scope's HV generator probably causes its circuit to float up.

> What is normal? Earth the scope, disconnect all cables to the board before
> soldering, run an isolating transformer?

Are you talking about measuring, or soldering?

Measuring: Earth scope chassis (safety). If you measure in USB type
situations, use differential mode (halves your inputs) or an isolating
transformer. With the latter you can probably still only measure with
probe ground connected to signal ground.

Soldering? Trivially simple, use a decent soldering iron with
transformer and the element going on 12V or 24V, with an ESD connector
(connected to iron tip through some 100kHohm) that you connect to your
ESD work matt. If you have one of those braindead iron jobs from Weller
(lots of American ones seem to suffer from this) with an earthed tip,
get a useful one instead. ERSA has some nifty ones, but boy do they take
it out of your pocket.

> 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?

PC upplies are earthed by requirement, yes. The laptop situation
shouldn't be so bad, you only have ground issues, not earth issues. At
least you don't blow things up as soon as you touch it with your
soldering iron. Oh come off it, who has time for powering down the
circuit each time...

Volker

-- 
Volker Kuhlmann
http://volker.top.geek.nz/	Please do not CC list postings to me.