The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Month: December 2012

Peltier Module PWM Frequency

The tech reviewer for my Circuit Cellar columns on the MOSFET tester commented that the 32 kHz PWM frequency I used for the Peltier module temperature controller was much too high:

Peltier Noise – VDS – PWM Shutdown

He thought something around 1 Hz would be more appropriate.

Turns out we were both off by a bit. That reference suggests a PWM frequency in the 300-to-3000 Hz range. The lower limit avoids thermal cycling effects (the module’s thermal time constant is much slower) and, I presume, the higher limit avoids major losses from un-snubbed transients (they still occur, but with a very low duty cycle).

Peltier Turn-Off Transient

The Peltier PWM drive comes from PWM 10, which uses Timer 1. The VDS and ID setpoints come from PWM 11 and PWM 3, respectively, which use Timer 2. So I can just not tweak the Timer 1 PWM frequency, take the default 488 Hz, and it’s all good. That ever-popular post has the frequency-changing details.

I’d still use a snubber:

Peltier Drain – 82 ohm 3.9 nF snubber

2012-12-03
Kindle Fire Security: Burn Them. Burn Them All.
My Kindle Fire automagically updates itself whenever Amazon decides it should. Sometimes an update produces a notice that an app (why don’t we call them “programs” these days?) needs more permissions, but the process generally goes unremarked.

This one wasn’t subtle at all:

Kindle Fire – File Expert Trojan warning

I had just fired up File Expert, which immediately dimmed the screen and presented a dialog box with only two unpalatable choices. Here’s a closeup:

Kindle Fire – File Expert Trojan warning – detail

Well, what would you do?

Needless to say, I didn’t press the Download Now button; it probably wouldn’t have worked anyway, because I turned off the Allow Installation of Applications from Unknown Sources option a long time ago. Pressing Exit bails out of the ~~program~~ app and returns to the Home screen.

Some questions immediately spring to mind:
- If the app has been compromised, exactly how did it regain control and complain about the situation?
- If this is truly a compromised app, why wouldn’t the Trojan just download malware without asking?
- How did this pass the ahem QC and auditing that allegedly justifies having a sole-source Amazon App Store? After all, I can load random crap from the Interweb onto a PC all by myself.
- How does one validate the origin of those random security questions that regularly appear on various computer screens? Why wouldn’t malware just pop up a random dialog box asking for the password, any password, and gleefully use whatever you type?
This appears to be a false positive, as explained there. I assume that any malware worth its salt would also kill off any built-in integrity checking, but what do I know? It’s gone missing from the storefront, probably cast forth into the outer darkness away from the light of Kindle Fires…
2012-12-02
Please Use the Water Fountain for Drinking Purposes Only

Please Use the Water Fountain for Drinking Purposes Only

OK, I need some help on this one…

I understand the English wording and suppose that the Hebrew version says roughly the same thing. What I flat-out don’t understand is why such signs appear over the water fountains in the hallways outside the toilet rooms (which have, FWIW, the expected, perfectly serviceable, sinks with hot & cold running water and soap dispensers).

We were northbound on I-90 at the Clifton Park rest stop. Given the location, I’d pick French as the second language and maybe Spanish would be reasonable, but Hebrew?

Is there some mysterious ritual involving water fountain misuse that happens only in upstate New York?

Obviously, I don’t get out nearly enough…

[Update: I will ruthlessly squash ethic & religious jokes, snide remarks, and off-point speculation. Selah.]

2012-12-01