[sdiy] MIDI and ESD protection

[Julian]

The problem is not that there are no TVS diodes... there are plenty.
But the clamping voltage is always too high with the low value resistor.

for an opamp input with 100k resistor it is easy
even a TVS with a clamping voltage of 300V would only result in a
current flow of 300/100k = 3mA
That is easily handled by the opamps internal clamping diodes (50mA on
tl072)

But for 3.3V MIDI with a 74HC14 buffer and the 10 Ohm resistor, the
buffer can clamp +/-20mA
now even wit a TVS clamping voltage of 10V (and all TVS diodes I looked
at have higher clamping voltages at the 8kV/30A ESD pulse) we still get
10V/10 Ohm = 1A for the 74HC14 to sink.

I just hoped somebody would have encountered the problem before, even if
it is not really DIY relevant ;)

I did some research in the book "System Level ESD Protection" and there
it mentions a good detail.
The datasheet of the 74HC14 only specifie4s HBM and MM ESD protection
levels. But the MM 200V ESD protection indicates the internal clamping
diodes should be able to withstand a 3.5A pulse (thats the machine model
specification), which makes the search for a TVS much easier.

Or I should just ditch the 3.3V MIDI in favour for 5V, since the 220 Ohm
resistors current limiting makers the design much easier!

julian



Am 05.02.20 um 22:57 schrieb Ullrich Peter:
> Here you can find many other alternative ESD protection diodes:
>
> https://eu.mouser.com/Circuit-Protection/ESD-Suppressors-TVS-Diodes/_/N-jy72?P=1yh8o3j
>
> Ciao
> peter
>
> http://www.ullrich.at.tt
> ________________________________________
> Von: Synth-diy <synth-diy-bounces at synth-diy.org> im Auftrag von Julian <elfenjunge at gmx.net>
> Gesendet: Mittwoch, 5. Februar 2020 22:47
> An: synth-diy at synth-diy.org
> Betreff: [sdiy] MIDI and ESD protection
>
> I wonder how I should go about protecting a 3.3V MIDI out circuit
> against 8kV contact ESD.
> When I use a TVS with 5V working voltage, most of the time I still have
> 16-20V clamping voltage over the diode for 8kV contact pulses.
> With the 3.3V setup I only have a 10Ohm resistor limiting the current to
> my UART TX pin.
> with a 74hc14 as driver/buffer I can sink +/-20mA, but with 16V at 10 ohm I
> have much more to sink.
> I can't find anything about MIDI and ESD in the official specs or other
> sources.
>
> I'm new to ESD. So do I miss something? Is the short pulse time not
> enough to fry the internal clamping diodes of a 74HC chip?
>
> Any insights are welcome!
>
> Julian
>
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