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<sensille> i'm rapidly running out of ideas what i'm doing wrong with my ecp5 slave spi :( even with diamond i get no reaction. according to boundary scan all pins are connected as expected
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<whitequark> azonenberg: hey, what do you know about indicator LED failure moes?
<whitequark> I see something pretty bizarre here, a LED that lights up only if I cool it down to like -20°C
<whitequark> as it warms up it seems to gradually go dimmer with higher Vf until it can no longer light up from 3.3V; this is a red LED
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<azonenberg> whitequark: innnteresting
<azonenberg> i have to ask how did you discover it worked when frozen?
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<whitequark> azonenberg: intuition
<whitequark> the LED is on a completely unremarkable $10 devboard
<whitequark> and i noticed it got kinda... dimmer, maybe? over 20 minutes i worked with it
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<whitequark> and in general had strange variability in output
<whitequark> actually, my guess was not about a LED
<whitequark> this devboard has an nRF24LU1+ chip, it's one of those things you find in wireless keyboard dongles. highly integrated
<whitequark> i probed the devboard a few times with a multimeter and got really surprising voltages on some nodes, so i was thinking that maybe it has a faulty LDO or something
<whitequark> so i inverted a can of butane and blasted it
<whitequark> suddenly, the LED shines bright. i probe it with a scope and it turns out the LDO in the RFIC is fine, it's the LED, somehow
<whitequark> this was actually pretty obnoxious lol because i thought i had a bug in my code
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<pie_[bnc]> well...googling "led doesnt work when hot" and variants only yields useless stuff, and I don't know enough about semiconductor tech, now I am also curious.
<whitequark> it doesn't seem to be obviously mechanical
<whitequark> poking it doesn't change anything
<whitequark> could be inside the epoxy
<pie_[bnc]> why would you get gradual dimming then though
<whitequark> yeah i dunno either
<pie_[bnc]> unless it was somehow arcing across a bad connection or the surface area of the connection was varying?
<whitequark> if your leds are arcing you have bigger problems
<pie_[bnc]> idk how much voltage you need to arc at miniscule distances
<pie_[bnc]> theres less stuff in the way but also less charge carriers (disclaimer: no idea what im talking about)
<pie_[bnc]> *5
<whitequark> huh
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<pie_[bnc]> random google suggests mean free path at 1atm is 60nm
<pie_[bnc]> verdict: i dunno lol
<pie_[bnc]> from the plots it looks like youd still need a decent voltae
<pie_[bnc]> *voltage
* sensille once had a LED that only worked directly on mains, plain old 5mm red led
<pie_[bnc]> theres a conclusion on page 9
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<pie_[bnc]> so according to this random site https://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html the thermal expansion of "epoxy" is ~ 60 * 10^-6 m / m / K sooo...not quite sure how to get an order of magnitude estimate for mechanical movement from that
<pie_[bnc]> ok so a led is a couple millimeters across
<whitequark> hmmm
<whitequark> this one is like 1mm
<pie_[bnc]> sure lets go with that
<pie_[bnc]> lets round 6 to 10 because i cant math
<pie_[bnc]> for 1mm -> 100 * 10^-6 (m/m/K) * 10^-3m -> 10^-7m/K
<pie_[bnc]> though youre heating / cooling the entire led, so everything will expand and contract together, meaning relative distances will be less
<pie_[bnc]> to be clear i dont think this is the real reason, im just playing with this
<sorear> I’m assuming diode physics here… electron phonon scattering stuff
<pie_[bnc]> so for a distance between 1000nm and a couple hundred, the breakdown voltage is around 40-100V still, unless i tripped somewhere
<pie_[bnc]> *and a couple hundred nm
<sorear> The I-V curve as a function of temperature would be interesting, but a lot of work
<pie_[bnc]> ah but hm. the breakdown voltage is a boolean for current, would it change the actual brightness of the led?
<azonenberg> sorear: i/v/t 3d curve would not actually be that hard to do
<pie_[bnc]> a butane canister doesnt seem ideal for a stable temp meaurement :d
<pie_[bnc]> * :D
<azonenberg> well yeah
<azonenberg> you'd want a peltier plate or something
<azonenberg> but an led has ~0 thermal mass
<azonenberg> which makes it easy
<pie_[bnc]> well, if you desolder it
<pie_[bnc]> or put the plate on top i guess
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<azonenberg> yeah i was thinking desoldered
<pie_[bnc]> dont break the guinea pig in the process by accident :D hopefully its a "stable" problem
<pie_[bnc]> its no fun if it breaks
<sorear> Can you measure junction temperature with any accuracy using electrical noise?
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<pie_[bnc]> failure analysis seems kind of fun but also, wow, like, anything could be happening?
<pie_[bnc]> like temperature dependent junction size?
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<azonenberg_work> pie_[bnc]: yeah i'm wondering about some kind of weird diffusion or electromigration from overcurrent
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