High-Value Ceramic Capacitors: They Stink, and You Can't Get Them Anyway

There are widespread shortages of electronics parts at the moment, especially passives.  Quoted factory lead times are 40 weeks or thereabouts, and since the industry is capacity-limited, it isn't clear that the situation is going to get better any time soon, so everybody's starting to panic.   Given all this churn I've been spending an unconscionable amount of time lately finding suitable replacements for out-of-stock parts. 

High value ceramic caps are the worst--their capacitance drops by at least 60% and at worst 95% at rated voltage, so finding an adequate substitute involves a lot more than the package, value and voltage rating.  Most of their data sheets are useless, which is frustrating.  However, all is not lost: most makers have websites where you can look at the C(V) curves. 

Here's an alphabetical list.  Many of these links can also be used for resistors, inductors, and other component types as well.

AVX SpiCat (This one is super clunky--every time you select something it displays a throbber for 5-10 seconds.  It does give you soakage models though, which is nice if they're vaguely accurate.)

Cornell Dubilier has a lifetime vs temperature-calculator

Kemet KSIM  (About the best; slow on some browsers)

CAD for Kyocera capacitors

Murata SimSurfing (Honourable mention):

Panasonic has downloadable selection tools that don't run under WINE, so I don't know if they're any good.

Taiyo Yuden TY Compas (Honourable mention):

TDK tools

Yageo: selection tools

Samsung has some good searchable datasheets that you can get to from e.g. Digikey's product page, but a lot of their products are the pits, e.g. this one, whose capacitance falls off by over 90% at rated voltage.  Note that you want the characteristics link and not the datasheet link.

Others don't seem to, e.g. Johanson, Vishay, and most of the smaller Chinese outfits.  Sure would be nice if everybody had decent datasheets like Samsung's better ones.

BEOS outtakes: Photographic Film

From the cutting room floor at Building Electro-Optical Systems, Third Edition:

Photographic Film
 Okay, okay.  Photographic film isn't a detector of the sort we've been discussing.  Film is so out of fashion, so inconvenient.  It needs messy chemicals.  Getting it to be highly sensitive requires all sorts of 1960s alchemy such as pre-flashing and hypersensitizing in a forming gas or hot hydrogen atmosphere.  Why do we care about it at all, in these days of 4k x 4k CMOS imagers?

 There are two reasons.  Firstly, a telescope has a lot more than 4k x 4k resolvable spots.  The Palomar Schmidt has a 6.6\degrees\ square field.  At 1 arcsecond resolution, its plates were digitized at 23040 pixels square for the Digital Palomar Observatory Sky Survey (DPOSS).  That's 530 Mpel, which is a lot of imager chips, but just one photographic plate.  The plate can be digitized later on a scanning microdensitometer that also has many more than 4k x 4k resolvable spots.  Even an ordinary 35-mm camera produces images equivalent to 30 Mpel--and that's real pixels, not Marketing Megapixels (TM)  (see Section 3.9.14). The defect density in photographic film is lower than in IC imagers, too, and it makes a nice archival record that is guaranteed to represent the measurement data well. 

Photographic film has a power-law response over a huge range of signals.  The contrast exponent gamma can be anywhere from 4 down to 0.5, which compresses the dynamic range and makes bright and dim objects visible simultaneously.  Using low-contrast developers such as POTA, photographic film can record images whose dynamic range approaches 10^6:1, optical, e.g. a bomb flash and its surroundings, which is a task beyond any silicon imaging sensor whatever.

Film has two sorts of noise: grain, which is analogous to the digital nibblies from CCD pixels, and fog which is analogous to dark current.  Fog is due to a few grains being rendered developable by a few loose electron/hole pairs in the emulsion, and contributes random noise in the same way.


The second reason to talk about film is that some modern alchemy has got photographic film up to a QE of 1.0, and a multiplication gain of 2, so that a single photon can expose a grain of silver halide.  This is the quantum efficiency of the best CCDs, so there's no waste of photons any more. {J. Belloni et al., Nature V. 402, p. 865 (Dec.  1999).}  The trick is to add formate ions to the emulsion to scavenge all the excess holes without increasing the fog.  Unlike other hypersensitizing tricks, this one works at room temperature and is stable indefinitely.

Next: The Hurter-Driffield Curve

Temperature Measurement is Hard

Measuring temperature is surprisingly subtle.  There are lots of sensors out there; Digikey sells thermistor sensors interchangeable to +- 0.1 C from several vendors for about $3 in onesies.  IC sensors tout good accuracy and linearity, and come in both analogue and digital versions for way under a buck.  So what's the issue?

The issue is: temperature sensors measure the temperature of the sensor, whereas what we want is the temperature of something else: air, fluid, or some solid object we're trying to control.  So the problem is to get the sensor temperature to track the temperature we actually care about.   IC sensors are especially bad, because they have stout leads made of copper (400 W/m/K thermal conductivity) and small packages made of plastic (0.1 W/m/K).  Thus they basically measure the temperature of their leads, and are horrible at measuring air temperature, for instance.

National Semiconductor used to put out a very useful Temperature Measurements Handbook.  Since TI bought them, it seems to have disappeared from the web, so here's the  2007 edition. Not much has changed about the properties of plastic and metal since then, so it's still very current.

Decap Photo of Terabeam APD Photoreceiver

Decap picture of a Terabeam CD3109 APD/TIA module, taken with a lens glued to a cell
phone camera

Thermal Runaway Found Useful

This odd circuit  is an on-chip temperature balancer that uses thermal runaway to force N transistor arrays to all run at the same temperature.  BJT dissipation goes up at low temperature, with very high gain.  Here's its step response.