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Capacitor — Parallax Forums

Capacitor

LightfootLightfoot Posts: 228
edited 2006-03-26 19:15 in General Discussion
I am powering an SX28 chip from 3 AA batteries. Hooked to the RB and RC ports are 7 LED's. They are colored and timed in accordence with a traffic light. They draw about 20ma each. The resistor across them is 470 ohms. What value cap should I use across the battery poles? Should I have one elsewhere besides there?

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Well well, I'm seeing things, three of them.

-Stanley Blystone

Comments

  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-03-26 11:11
    Why do you want capacitors? Do you want to regulate voltage or filter out transients?

    The rule of thumb is to ALWAYS provide a 0.1uf cap as close as possible to the SX-28 for the later.
    Reguarding the former, I suspect you MAY need none as you are not using AC power source or a a voltage regulator.

    Of course, if you add motors or relays, things may significantly change. Induction brings transient noise into the circuit as it collaspes quicklly when power is switched off.

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    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

    ······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-03-26 15:01
    Three of Them said...
    ...are 7 LED's. They are colored and timed in accordence with a traffic light. They draw about 20ma each. The resistor across them is 470 ohms.
    · Whoa! Whoa!· Resistor across them?· No, no, no.
    · The resistor should be in series with the LED (output pin, resistor, LED, ground.)· If you have 20mA through/from an output, it's because the only thing limiting current is the LED and the output's limit (which isn't infinite.)
    · The SX has nominal 5V outputs, so with a 470 ohm resistor the nominal current through each LED should be around 6mA.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-03-26 15:34
    Obviously, more is better. I completely missed that.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

    ······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
  • LightfootLightfoot Posts: 228
    edited 2006-03-26 17:56
    You misread me, I mean in series.

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    Well well, I'm seeing things, three of them.

    -Stanley Blystone
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2006-03-26 19:15
    Keep in mind, Moe, that LEDs are rated for a max current but the resistor controls it. In your case, a power supply of 4.5v and 470 series limiters means an LED current of about 6 mA, assuming about 1.5v forward voltage for the LEDs.

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    Jon Williams
    Applications Engineer, Parallax
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