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EE newbie resistor placement question... — Parallax Forums

EE newbie resistor placement question...

trendtrend Posts: 112
edited 2004-12-06 18:59 in General Discussion
Hello, in the attached picture, there are 4 leds that have 4 resistors. and is powered by 5v.. Lets say the pins on the left side is a bs2 sinking all 4 pins.

My question is, why not put 1 resistor at the top (right after the 5v is applied to the line).. then we will not have to worry about the other 3 resistors.

I was wondering this because.. on my mcp23016 chip, I have to have some pullup resistors to assign an address to the chip.. and was wondering why I had to use a pullup resistor for each pin that needs to be high :/


Hopefully my wording wasn't to confusing on such a simple question

thanks for the help [noparse]:)[/noparse]

Lee
resistors.JPG
318 x 281 - 11K

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2004-12-06 15:33
    Okay, the two questions aren't entirely related, necessarily...But in regards to the LEDs...Each resistor sets the current for each LED at the given voltage, which is 5V, in this case.· If you used a common resistor for all LEDs, then the current draw would change depending on how many LEDs were on.· At the very least their brightness would change depending on how many were on.· The more LEDs turned on, the dimmer they'd be.

    As for resistors for pulling up lines on inputs...There is no way to share a resistor...Per se...All the pins that shared the resistor would effectively be tied together.· Individual resistors ensure that each line remains independent.· If your address lines need to be able to be changed, you wouldn't be able to employ this method at all.· And I wouldn't recoomend it anyway.· If you need seperate pull-ups, use them, since resistors are cheap anyway.· Is space is an issue, consider using a SIP resistor pack.



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    Chris Savage

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  • Jon WilliamsJon Williams Posts: 6,491
    edited 2004-12-06 16:28
    You must use separate pull-ups for every pin that needs to be pulled high. Why? Well, if you attach one pull-up to every pin, when one goes low they all go low, and this is probably not the behavior you want.

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    Jon Williams
    Applications Engineer, Parallax
    Dallas Office
  • trendtrend Posts: 112
    edited 2004-12-06 18:59
    Ahh, ok thanks guys! That makes since.. I don't know why I didn't think of that :/

    ·
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