How to find unknown capacitance?
Tuesday, January 26th, 2010 at
12:15 pm
I have recently gotten in the habit of taking off electronic components from motherboards, circuit boards, etc. and I want to know what the capacitance is of the capacitors I take off.
Any help?
Tagged with: capacitance • capacitors • Circuit Boards • electronic components • habit
Filed under: Electronic Circuit Capacitor
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http://en.wikipedia.org/wiki/Capacitors
[edit] Capacitors
Main article: Capacitor
The capacitance of the majority of capacitors used in electronic circuits is several orders of magnitude smaller than the farad. The most common subunits of capacitance in use today are the millifarad (mF), microfarad (µF), the nanofarad (nF) and the picofarad (pF).
The capacitance can be calculated if the geometry of the conductors and the dielectric properties of the insulator between the conductors are known. For example, the capacitance of a parallel-plate capacitor constructed of two parallel plates both of area A separated by a distance d is approximately equal to the following, in SI units:
where
C is the capacitance, in farads;
A is the area of overlap of the two plates, in square metres;
εr is the relative static permittivity (sometimes called the dielectric constant) of the material between the plates (for a vacuum, εr = 1);
ε0 is the electric constant (ε0 ≈ 8.854×10−12 F m−1); and
d is the separation between the plates, in metres.
Capacitance is proportional to the area of overlap and inversely proportional to the separation between conducting sheets. The closer the sheets are to each other, the greater the capacitance. The equation is a good approximation if d is small compared to the other dimensions of the plates so the field in the capacitor over most of its area is uniform, and the so-called fringing field around the periphery provides a small contribution.
Capacitors have either the actual value or a code that represent that value. If there are 2 digits with no letters the capacitor value is in picofarads, pf. A 3 digit code like 101 means the capacitor is 10 x 10^1 or 100 pf, the 3rd digit is the multiplier, 121 would be 12 x 10^1 or 120 pf, 102 would be 10 x 10^2 which is 1,000 pf. 103 would be 10 x10^3 or 10,000 pf which is 0.01 uf.
You can use this formula:
Capacitance (F) = Charge (C)
————–
Voltage (V)