Nuts & Volts - September 2005

■ CAPACITOR FUNDAMENTALS

Rolled up

Aluminum Foil

Dielectric film or porous electrolyte carrier

Wire lead

OR

Radial Leads

Axial Leads

HOW BIG

Is a FARAD?

One farad (1 F) is a large value as capacitors go, but what does that mean? Well, if a 1 F capacitor is charged up to 12 V, it contains enough energy to accelerate a 165-gram Frisbee™ to 67 miles per hour! That’s a pretty stiff toss! Similarly, to get that 1 F of capacitance, two plates separated by air and only 1/10 mm apart would have to be 11,300,000 square meters in area! That’s 4. 36 square miles!

R_Pexcept in very low-power and high-impedance circuits.

FIGURE 3. Roll capacitors are made from strips of metal and dielectric materials rolled into a cylinder.

be ignored, but as the frequency increases, so does its reactance. In fact, at a sufficiently high frequency, L_Sand C form a series-resonant circuit. This is the self-resonant frequency, f_Oof the capacitor. Above f_O, the capacitor acts more like a small inductor! R_S(also called ESR for Equivalent Series Resistance) acts just like a separate resistor, dissipating a little energy as heat when current flows. R_Scan be as high as several tens of ohms, but is generally only important when the capacitor current is high. R_Pprovides a leakage path for current across the dielectric and is typically in the megohms. You can ignore

There are two common ways of efficiently making capacitors: the Roll and the Stack. Figure 3 shows a typical roll capacitor — two strips of very thin aluminum foil separated by a dielectric. After leads are attached to the foil strips, the sandwich is rolled up and either placed in a metal can or coated with plastic. Radial leads both stick out of one end. Axial leads stick out from both ends along the roll’s axis. Because of the long rolled strips, the roll capacitor’s L_Sis high.

Temperature Coefficient

The materials that make up a capacitor expand and contract with temperature, meaning that the capacitance changes with temperature, as well. In many applications, such as IC bypassing or DC blocking, a change of a few percent is of no concern. However, in a timing circuit or an oscillator, it’s a big deal!

A capacitor’s change in value with temperature is called its temperature coefficient or tempco and is denoted by an industry standard code. (The nominal value is specified at 25°C.) The three most common tempcos are:

Z5U = +22%/-56% change over - 10°C to + 85°C X7R = ±15% change over - 55°C to +125°C NP0 = 0± 30 ppm/°C over - 55°C to + 125°C

As you can see, the Z5U rating allows quite a bit of change, but these are inexpensive capacitors used for garden variety applications or where temperature won’t be changing much. The NP0 (that’s a zero, not a capital O) capacitors are only used where holding a steady value is important. You can find a nice display of temperature coefficients at www.niccomp.com/ Products/TC_Ceramics.pdf