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Resistor sizes ? — Parallax Forums

Resistor sizes ?

Areal PersonAreal Person Posts: 197
edited 2006-12-01 19:48 in Propeller 1
·What would be the effect of replacing the 240 ohm resistor on the VGA
DAC (on a custom demo board setup) with a 220 or a 270 ohm?
·
I’m new at electronics, so please excuse my dumb questions.
·
I’ll be needing to wait 3-4 days to hook up my VGA as I don’t have the correct size resistors.
·
Is there any other solution ?
·
Thanks,
-Areal

Comments

  • PaulPaul Posts: 263
    edited 2006-12-01 17:54
    I paralleled two 470 ohms together to get 235 ohms. Seems to work ok.

    I think the purpose of the resistor is to current limit between the 3.3V chip and the 5V monitor circuit. I would go with the higher value 270 first to see if it works.

    --Paul
  • Mike GreenMike Green Posts: 23,101
    edited 2006-12-01 18:16
    The 240 ohm resistors are used both for protection (of the Propeller chip) and as part of a "ratiometric" digital to analog conversion circuit along with the 470 ohm resistors (approximately twice the value). As a voltage divider, these give a 1/3 and 2/3 voltage depending on the High/Low settings of the Propeller I/O pins. With both Low, you get 0/3 and with both High, you get 3/3.
  • Areal PersonAreal Person Posts: 197
    edited 2006-12-01 19:03
    Ua.. Ok, I guess that means it's ok to use a 270 in place of a 240 ohm.
    And I'm gussing the RCA TV connection is the same way also ?
  • Mike GreenMike Green Posts: 23,101
    edited 2006-12-01 19:48
    Yeah. There's only about a 10% error. Who'll notice?. The 470 ohm resistors are only approximately 470 ohms anyway.

    The TV connection works the same way except that there's another resistor to make a 3 bit converter (with 8 levels).
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