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BEAM Pieces is a BEAM Reference Library site.

A usage guide

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).

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)


Usage guide
Photopoppers (and BEAMhttp://encyclobeamia.solarbotics.net/articles/beam.htmlbots 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.

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)
  • Solaroller -- 0.033 - 0.047 F (depends on solar cell size)
  • Symet -- 3000 uF (3 x 1000 uF)

For more information...

Ben Hitchcock also has a good writeup of capacitor use in BEAM robotics here; meanwhile, Audio Magazine published an interesting article on "Picking Capacitors," which was put on the web here. For an exhaustive (almost compulsive) set of data on capacitors, make sure you check out the CapSite.

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Page author: Eric Seale
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