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Capacitor Codes

Ceramic caps may be tiny, but they have lots of non ideal qualities so calculating total tolerance can get ugly

There is a voltage dependency that is in addition to both the initial tolerance and the temperature coefficient. For some types, operating at full rated voltage reduces the capacitance to less than half of the zero voltage capacitance. Z5U types have a knee at about 10Vdc and by 50Vdc they are at 50% of their rated value! X7R types have a similar knee at about 20V .

The high K types also slowly lose capacitance over time, but can be reset back to full value by heating them above their Curie temperature (125 - 150 degC for a couple of hours). Cøg types are stable over time, but X7R lose 2% per decade and Z5U 5% per decade.

High K types also exhibit microphonics: they act as tiny microphones and piezo speakers!

Dielectric Absorption (sometimes called soakage) is best explained by example. charge a cap to rated voltage. Then short the leads for 1 second - then hook a voltmeter across the leads and watch as a voltage rises.

In absorption, some of the charge migrates away from the plates and slowly returns after discharge. Dielectric Absorption is not a desirable characteristic in many circuits - particularly A/D converters or sample and hold circuits. (Use polystyrene or Teflon).

There used to be some good books on passives that covered these issues - sadly they are all out of print today, but you might be able to get them used at the following links.

The details about the non ideal characteristics of passives is something neglected in most schools - you likely will need to learn it on the street after your SPICE simulated circuit fails to work. That these books are out of print tells you that the universities are not graduating real EEs anymore.

There is also the effect of capacitance change with voltage - sometimes used as an advantage for VCOs. There are special capacitors that are designed to have a value change with voltage used in the oscillator circuit to make a VCO - but often instead the capacitance of a diode - a 'variable capacitance' or 'varactor diode'.

If used with a PLL phase-lock-loop circuit like the old 4046 - the phase error produces an error voltage that is used to control the VCO (Voltage Controlled Oscillator ) with a varactor in the heart of it.

Imperial and metric case size codes

By special request - milli, micro, nano, pico,

1 mili Farad (or any other unit) is 1/1,000th or .001 times the unit. (10-3)

1 micro = 1/1,000,000 or 0.000 001 times the unit (10-6 )

1 nano = 1/1,000,000,000 or 0.000 000 001 times the unit (10-9 )

1 pico = 1/1,000,000,000,000 or 0.000 000 000 001 times the unit (10-12 )

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