Tolerance capacitor

The capacitor model is used to specify capacitor voltage dependence and tolerance. The main use of the capacitor model in this manual is to specify capacitor tolerance. See Part 9, page 505, for an example of using this model to specify tolerance. 

When delennining the aclual load current of a capacitor unil in operation, a factor of 1.15 is additiottally considered to account for the allowable tolerance in the eapacitance value of the capacitor unit (Section 26.3.1(1)) ... 

This provision will also account for any diminishing variation in C, as may be caused by ambient temperature, production tolerances or failure of a few capacitor elements or even of a few units during operation. 

The factors discussed in Section 23.5.2 give rise directly to the current drawn by the capacitor unit and indirectly add to its rating. The relevant Standards on this device recommend a continuous overload capacity of 30% to account for all such factors. A capacitor can have a tolerance of up to -t-15% in its capacitance value (Section 26.3.1(1)). All current-carrying components such as breakers, contactors, switches, fuses, cables and busbar systems associated with a capacitor unit or its banks, must therefore be rated for at least 1.3 x 1.15/,., i.e. 1.54. For circuits where higher amplitudes of harmonics are envisaged, for reasons of frequent load variations or more... 

It is important to double-check the failure criteria of specific vendors. For example, Panasonic SMD capacitors allow for a 30% fall in capacitance by the end of life. That means, for a 20% tolerance capacitor, you need to start with a nominal value 79% higher than your calculated value (also don t forget to account for the additional fall in capacitance at low temperatures). 

Therefore, all capacitors shipped are within their specified tolerance at the standard reference age of 1000 hours, after having cooled through their Curie temperature. 

There are apparently customers who soldered on ceramic capacitors in their power supplies and found the clock was just too low. They figured the capacitance was above the guaranteed upper tolerance band (a rare event with commercial ceramics ), and shipped them right back to their manufacturers. But the problem was only that as soon as the PCBs went through the soldering process, age reset (or de-aging) occurred and so capacitance rose. If only they had waited for some more time, their clocks would have been right on However, I would have preferred SMD him capacitors if stability was so important. 

The last model we will look at is a capacitor model with +80% and -20% tolerance. This model is called CAP20 80 and is included in class.lib ... 

Tolerances in PSpice are set up to have equal plus and minus ranges. With a +80% and -20% capacitor we have to fudge things a little. A capacitor with a +80% and -20% tolerance has a maximum value 1.8 times the nominal value, and has a minimum value 0.8 times the nominal value. This range is equivalent to a capacitor with a nominal value of (1.8 + 0.8)/2 = 1.3 and a 38.461538% tolerance. Note that 1.3 (1 + 0.38461538) = 1.8, and 1.3 (1 - 0.38461538) = 0.8. Thus, a capacitor with a nominal value of 1 and +80% and -20% tolerance has the same capacitance range as a capacitor with a nominal value of 1.3 and 38.461538% tolerance. The only difference is the nominal value. 

This circuit contains 5% resistors, capacitors with +80%, -20% tolerance, and a 2N3904 transistor that has been modified to include tolerance in p. The modified model is shown below. See Section 7.C for instructions on how to modify a BJT model. 

Unfortunately, the lab used in the creation of the circuits in this book closely resembles the lab of other engineering companies around the world. We use 5% tolerance resistors and 10% tolerance capacitors that are either soldered to a vector board or plugged into a solderless breadboard. This introduces various parasitics and inaccuracies in the results. In order to be more precise in showing the accuracy of SPICE simulation software, we frequently run the simulations with the stated values of the resistors or capacitors used in our lab breadboards. The measured values for each resistor and capacitor used in our breadboard configuration which may be different, are shown in Fig. 3.3. 

High-power transmitter capacitors for the frequency range 0.5-50 MHz for which the main requirement is low loss a negative temperature coefficient of permittivity is tolerable since it limits the power through the unit when its temperature increases. 

A common function of circuits is the provision of an accurate resonance state. For instance, for a resonance frequency to stay within a tolerance of 0.1% over a temperature range of 100 K a temperature coefficient of less than 10 MK 1 would be required. It might be achieved in the 10-100 kHz range by using a manganese zinc ferrite pot-core inductor (see Section 9.5.1) with a small positive temperature coefficient of inductance combined with a ceramic capacitor having an equal, but negative, temperature coefficient. This is clear from the resonance condition... 

In most applications a resonance tolerance of 0.1% would only be useful if the resonance were correspondingly sharp, e.g. with a Q in the neighbourhood of 1000 (tan S = 10-3). Thus low-TCC capacitors must also be low loss if they are to be of practical value in such applications. 

Color Significant figure Multiplier % Tolerance Capacitor Rating (V)... 

Typical labeling schemes for common capacitors, (a) and (h) are ceramic capacitors of 150 and 10 pF values, respectively. Tolerances are often indicated with letters, with lower values meaning less uncertainty, e.g.,J = 5%, K = 10%. (c) and (< are tantalum and aluminum electrolytic capacitors of values 2.2 and 22 /xF, respectively. Polarity is irrelevant for ceramic capacitors but is indicated and must be maintained for electrolytic capacitors. 

For many traditional ceramics such as structural elements (tiles, bricks, etc.), white-wares, (tableware, sanitaryware, etc.), and common refractories, the raw materials are naturally occurring minerals, and moderate levels of impurities are tolerated. More specialized technical ceramics such as electronic ceramics (substrates, electronic packages, capacitors, inductors, etc.) or high performance structural ceramics (silicon carbide, silicon nitride, etc.) demand low or controlled levels of impurities and make use of higher purity powders often made by more specialized techniques. 

---