[sdiy] Who apart from me things the whole MIDI board needs replacing ?

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Thu Jan 27 11:46:59 CET 2022


Ouch!  Yes, Class-II devices like laptop chargers can output some pretty 
high voltages relative to earth/ground.  I was surprised only yesterday 
to find that the aluminium cladding on the back of my work Toshiba 
laptop's screen had 280 volts peak-to-peak on it relative to ground when 
the mains charger was plugged in!  I could always tell when the charger 
was plugged in by the feel just running my hand over the metal case.  It 
doesn't quite feel like an electric shock, just like it is 
buzzing/vibrating or as if it feels rough.  As soon as you unplug the 
charger it feels totally smooth when you run your hand over it.  Same 
thing with the touch dimming lamp at my bedside.

I can imagine how these voltages don't play nicely with modern 
low-voltage logic and dev kits though.  To be honest I'm in the habit of 
always unplugging the laptop charger to reduce noise before I connect 
any sig gen or scope probes anyway, so that's probably why I haven't 
blown anything up yet ;-)  Class-I and Class-II appliances don't play 
nicely together!

-Richie,



On 2022-01-27 09:40, Roman Sowa wrote:
> For that purpose I've got permanently connected 20k resistor between
> my test bench power supply black terminal (GND or negative whatever
> you call it) to Earth terminal. This way PSU lines are not floating on
> hundreds of volts whenever they want but are close to 0 while not
> causing short with PC chasis when I use for example a programmer. It
> was enough to keep my IDC3 in good condition for years, but when I
> took it home, where I use floating PSU (laptop brick feeding DC/DC) I
> have fried my ICD after 3 days :(
> 
> Roman
> 
> W dniu 2022-01-26 o 14:16, Mike Bryant pisze:
>> Good idea.  Might stop people lifting the grounds on equipment they 
>> really shouldn't do so on !!
>> 
>> 
>> -----Original Message-----
>> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of 
>> Gordonjcp
>> Sent: 26 January 2022 12:12
>> To: synth-diy at synth-diy.org
>> Subject: Re: [sdiy] Who apart from me things the whole MIDI board 
>> needs replacing ?
>> 
>> On Tue, Jan 25, 2022 at 10:41:49PM -0500, Mike Beauchamp wrote:
>>> On 1/25/22 10:31 PM, Brian Willoughby wrote:
>>>> I brought this up with designers before, and someone claimed that 
>>>> it's easy to select panel-mount 1/8" jacks that are electrically 
>>>> isolated from the panel. Of course, that works until there are 
>>>> supply issues and someone in your sourcing department decides that 
>>>> another panel-mount 1/8" jack is "equivalent" ... except that it's 
>>>> no longer isolated.
>>>> 
>>>> I prefer the DIN where the shield and ground are separate from each 
>>>> other *and* separate from the MIDI signaling. That makes it *much* 
>>>> easier to ensure against ground loops.
>>> 
>>> I've never used 3.5mm midi. .but could this problem be "solved" with 
>>> a
>>> custom cable where the shield doesn't connect at one of the ends?
>>> 
>> 
>> I have solved a similar problem in the past with a custom jack cable 
>> with the braid connected with a 470 ohm resistor at each end.
>> 
>> Makes bugger all difference to shielding (because 470 ohms or 235 ohms 
>> if both ends are grounded is peanuts compared to the infinite 
>> impedance of the braid in free space), drastically reduces ground loop 
>> currents to the point where they don't have an effect.
>> 
>> --
>> Gordonjcp
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