Viewing posts for the category SED

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.

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.

Sine Wave Generation with TANH Wave Shaper

Sine wave generation is a perennial problem.


Direct-digital synthesis (DDS) uses a bunch of counters, lookup tables, and DACs,
but that's a relatively heavyweight solution that doesn't fit all problems.
BJT differential pairs naturally have a hyperbolic tangent (tanh) characteristic, which can be used to round off a triangle wave into a very passable sine.  I'm not old enough to have invented this technique, but here are a couple of illustrations of how it works: TANH Sine Wave Shaper (PDF) and TANH Sine Wave Shaper (Mathcad).