Thursday, February 7, 2013

Resistors and capacitors

Some notes on specifying passive components. Resistors and capacitors come in a wide variety of options.

For example, resistors types include thick film (resistive ink on ceramic), metal film (nichrome), carbon film, carbon composition, and wirewound. Important specifications and parameters include the power rating (constant power and pulse handling), tolerance, temperature coefficient, and voltage coefficient (nonlinearity). Notable parasitics are the series inductance, the parallel capacitance, thermocouple effects, and excess noise. See references [1] and [2].

Let's look at an example of specifying a resistor part: Panasonic thick-film chip resistors (ERJ series)
  • E24 and E96 series values
  • Sizes: everything from 01005 to 2512
  • Power: 1/32 W to 1 W (based on size)
  • Tolerance 0.5% or 1%
  • Temperature coefficient: 50 ppm/C to 300 ppm/C
  • Operating temperature range: −55C to +125C or +155C
Decoding the part number:


Maximum power rating is a function of package size (and type, tolerance, and temperature).  The power in a resistor is P = V2/R. Thus, the maximum voltage that can be applied across a resistor is a function of its power rating V = sqrt(PR).  For a 01005 resistor (0.031 watts) with value 100 Ω, this limit is only 1.7 volts. For a 2512 resistor (1 watt) with value 1 MΩ, this limit is 1000 volts.  However, the package may have a much lower maximum voltage rating (in this case, the limiting element voltage (LEV) is 200 volts).  The rated continuous working voltage (RCWV) is the lower of the two numbers sqrt(PR) and LEV.


The maximum power rating has to be derated above 70C.


Most Unnecessary Disclaimer Award: "These products generate Joule heat when energized. Carefully position these products so that their heat will not affect the other components."

Capacitors can be friend or foe. There are many different varieties of capacitors, and they all have their own strengths and weaknesses. See references [1] and [2].  There are a large number of properties that we can use to compare capacitors, including value, voltage, size, self time constant, temperature coefficient, dielectric absorption [3], effective series resistance and, of course, cost.

The choice of dielectric has a big effect on the quality and properties of the capacitor.
  • Bypass types: aluminum electrolytic, tantalum electrolytic, high- K ceramic
  • Filter types: NPO ceramic, polycarbonate, polyester
  • Sample-and-hold types: teflon, mica, polystyrene, polypropylene
In App Note 47 [4], Jim Williams has some warnings about bypass capacitor types (see pages 25 and 26). Figure 60 is downright scary.


Be aware of the effects of voltage and temperature.  Some types of capacitors undergo huge variations over the range of operating voltage and temperature (these plots are from the LT1763 datasheet). Note the huge variation of Y5V types.



Example 1: AVX tantalum capacitors (TPS series, low ESR)
  • Values: 0.15 μF to 1500 μF (E6 series, 1.0 1.5 2.2 3.3 4.7 6.8)
  • Sizes: 0805 to 2924 (see table of custom case codes)
  • Voltage: 2.5 V to 50 V (derate by 2/3 above 85C)
  • Tolerance 10% or 20%
  • Effective series resistance (ESR): 0.03 Ω to 9 Ω (see chart)
  • Operating temperature range: −55C to +125C
Datasheet tour:
  • Table of case sizes
  • Decoding the part number
  • Rated voltage and surge voltage (and derating)
  • Table of capacitance, voltage, and ESR
  • DCL, DF, ESR, RMS current, and RMS voltage
Example 2: Murata chip monolithic ceramic capacitors (datasheet dated Oct. 1, 2012).  Start with decoding the part number, for example GRM188R71E153KA01D:
  • GRM tin-plated layer capacitor
  • 18 size 1.6x0.8mm (0603)
  • 8 thickness 0.8mm
  • R7 temperature X7R type −55C to +125C, change ±15%
  • 1E rated voltage 25V
  • 153 capacitance 15,000 pF (E6 series)
  • K tolerance ±10%
  • A01 internal specification code
  • D paper tape
Datasheet tour:
  • Decoding the part number, pages 15–17
  • Actual capacitor listed on page 70
  • Temperature characteristics, page 119
  • Soldering and mounting, page 129

References:

[1] James Bryant, Walt Jung, and Walt Kester, "Passive components," in Op Amp Applications, Walter G. Jung, Ed. Norwood, Mass.: Analog Devices, 2002, ch. 7-1, pp. 7.1–7.24.

[2] Robert A. Pease, Troubleshooting Analog Circuits. Boston: Butterworth-Heinemann, 1991.

[3] Robert A. Pease, "Understand capacitor soakage to optimize analog systems," EDN, p. 125, Oct. 13, 1982.

[4] Jim Williams, "High speed amplifier techniques: A designer’s companion for wideband circuitry," Linear Technology Corp., Milpitas, Calif., Application Note 47, Aug. 1991.

1 comment:

  1. Nicely done especially the decoding the part number section. Where were you when I was in school?

    For homework you could assign a problem of decoding a lemo connector part number. I've been out of school 25+ years and still have to triple check my results when specifying push/pull connectors :)

    See pg 2-8 here http://www.lemo.co.uk/images/catalogue_downloads/LEMO_Unipole_Multipole.pdf

    ReplyDelete