Capacitor
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[edit] Introduction
A passive component can store energy in the electric field between a pair of conductors (called "plates"). The process of storing energy in the capacitor is known as "charging", and involves electric charges of equal magnitude, but opposite polarity, building up on each plate.
In BEAM circuits, capacitors are often used instead of a battery, as it charges and discharges much more efficiently (batteries, though, can store much more charge).
In a basic capacitor the two conductors are separated by an insulator, or dielectric. The dielectric can be made of paper, plastic, mica, ceramic, glass, a vacuum or nearly any other nonconductive material. In some capacitors (called electrolytics) their dielectric is made up of a thin layer of oxide formed on an aluminum or tantalum foil conductor.
Capacitor electron storing ability (called capacitance) is measured in Farads. One Farad is actually a huge amount of charge (6,280,000,000,000,000,000 electrons to be exact), so we usually rate capacitors in microfarads (uF = 0.000,001F) and picofarads (pF = 0.000,000,000,001F ).
Capacitors are also graded by their breakdown (i.e., smoke) voltage. Capacitors rated for lower voltages are generally smaller in size and weight; you don't want to use too low a voltage rating, though, unless you enjoy replacing burnt-out capacitors in your creation.
For BEAMbots, you'll need to know about 2 main types of capacitors:
[edit] Non-polarized fixed capacitor
A non-polarized ("non polar") capacitor is a type of capacitor that has no implicit polarity -- it can be connected either way in a circuit. Ceramic, mica and some electrolytic capacitors are non-polarized. You'll also sometimes hear people call them "bipolar" capacitors.
[edit] Polarized fixed capacitor
A polarized ("polar") capacitor is a type of capacitor that have implicit polarity -- it can only be connected one way in a circuit. The positive lead is shown on the schematic (and often on the capacitor) with a little "+" symbol. The negative lead is generally not shown on the schematic, but may be marked on the capacitor with a bar or "-" symbol. Polarized capacitors are generally electrolytics.
Note that you really need to pay attention to correctly hooking a polarized capacitor up (both with respect to polarity, as well as not pushing a capacitor past its rated voltage). If you "push" a polarized capacitor hard enough, it is possible to begin "electrolyzing" the moist electrolyte. Modern electrolytic capacitors usually have a pressure relief vent to prevent catastrophic failure of the aluminum can (but don't bet your eyesight on this).
[edit] Abbreviations
Capacitor values are given as a combination of numbers and letters, where the `N' stands for `nF' or nano-farad. In the same fashion, a `P' would mean `pF', or pico-farad. While the range of letters one might use is longer, you'll mostly encounter:
- p (pico) 10 to the -12
- n (nano) 10 to the -9
- uF (micro) 10 to the -6
- mF (milli) 10 to the -3
Note that on any real capacitor, you are unlikely to read something as straightforward as `100N' or even `100nF'. For reasons obscure to me, it might be `100nS', or, somewhat less obscure, `104' (meaning 10 with 4 zeroes behind). Or, again for reasons beyond my understanding, you may unable to read anything at all because the lettering is incredibly tiny and in low-contrast colors...
[edit] Capacitator Usage in BEAMbots
As a rule, BEAM robots tend to use capacitors in two, very different roles -- small ones in Nv neurons and solar engines (filter capacitors), and bigger ones as energy storage devices (storage capacitors).
[edit] Filter capacitors
The following 4 sizes of filter caps seem to be most often used in BEAM bots (you'll want to stock up on these):
- 0.1 uF -- normal Nv timing usage
- 0.22 uF -- Nv nets with longer delays
- 0.47 uF -- across motor power leads (power stabilization)
- 4.7 uF -- storage cap internal resistance mitigation (i.e., attached in parallel to main storage cap)
[edit] Usage guide
Photopoppers (and BEAM bots with motors, in general) require the use of storage capacitors with internal resistance of less than half an ohm. As a general rule, a decent cap for driving a [[[motor]] is longer than it is wide by a factor of 2 or more. (eg. 10 mm diameter, 20 mm long). This is due to the way that they're constructed. Long and skinny generally means low internal resistance.
You can, of course, also put a small-but-efficient capacitor] (namely, a filter cap -- maybe 4.7 uF) in parallel to your storage cap in order to reduce its effective internal resistance.
[edit] Choosing the right size storage capacitor
First you need to think about what your application is. Do you want a motor to spin for a long time or just a a few turns? If you want it to spin for a long time then you're going to need a capacitor with higher capacitance. If you want a motor to spin for only a quick burst then a smaller capacitor is what you want.
Nothing comes for free, of course, so the bigger the capacitor you have, the longer it will take to charge up and move. So a 2200uF (0.0022 F) capacitor might take only a second to charge but a 1F capacitor could take 8 minutes.
Here's a few sizes commonly used
- Dual SE photopopper -- 3300 uF (0.0033 F)
- Solarrollers -- 0.033 - 0.047 F (depends on solar cell size)
- Symet -- 3000 uF (3 x 1000 uF)
[edit] Types of Capacitors
- Ceramic capacitor
- Electrolytic capacitor
- Monolithic ceramic capacitor
- Super capacitor
- Tantalum capacitor
[edit] External References
- Capacitors available from Solarbotics.com
- Wikipedia Article: Capacitors
- Wikipedia Article: Capacitor (component)
- Wikipedia Article: Types of capacitors
- Wikipedia Article: Capacitor plague
- Capacitor Identification
- DIFFERENT TYPES OF CAPACITORS
- How to Read Capacitors
- Capacitor Code Guide
- Capacitor Colour Codes
- Capacitor uF - nF - pF Conversion Chart
- Capacitors: A Field Guide to Types and Habitats
- Capacitor Linearity
