Here's G0TXL's post about D109 failing:?
Here's discussion of how zener diodes fail:?
?
I would assume the cause of D109 failing is a step change to a higher input voltage.
The STM32F did not adjust the PWM_3V3 signal to the SMPS at Q107 soon enough,
resulting in a too high 3.3v rail.
?
The QMX+ assembly manual says D109 is a 3.6V 500mW SOD-123?
One such part is this from Mouser?
?
From 7 lines down on page 3 of the datasheet, we find that the MMSZ5227
zener voltage at 20 mA and a temp of 30C is between 3.42 and 3.78 Volts.
Fig 9 on page 6 of the datasheet shows little variance in zener voltage as
the current goes up, but only shows a max of 20ma, or 0.020*3.6 = 72 mW,
though device dissipation is rated at 500 mW.
I don't see anything in the datasheet showing what will happen at above 20 mA.
?
I find that a bit concerning.??
We are lucky if that 3.3v zener at D109 typically fails as a short instead as an open.
It is protecting the 100 pin STM32F which few of us would be able to replace.
When it fails, it may have several amps going through it at well over 4 Volts.
?
What firmware version do you have?
Hans reports that the latest firmware should significantly reduce SMPS failures:
?
If the latest firmware is having failures, we may want to replace the 5v switcher with
a TO220 linear regulator and power the 78M33 at IC101 from 5v for the 3.3v rail
until a better solution is found.? This increases supply current to a couple hundred mA
on receive, but that should not be an issue unless operating portable on?a small battery.??
?
Jerry, KE7ER
?
?
On Sun, Oct 20, 2024 at 04:30 AM, Steve MW0KST wrote:
there are a lot of posts re D109 but the one that prompted me to check was from Paul G0TXL
