Resistance Question - can anyone do this?

Discussion in 'Homework Help' started by lostlego, Aug 15, 2006.

  1. lostlego

    Thread Starter New Member

    Aug 15, 2006
    2
    0
    hey guys,

    I've got this question which i am having trouble doing, i was wondering if anyone out there knows a valid resistance for this question?

    DeviceA........................................................................DeviceB
    ........................R=?
    3.3v o-----------/\/\/\------o-------------o---------------o ~3.3v
    ........................................|..................|
    ........................................|..................|
    ..............................3.6v ----- .............___+10uF
    ........................zener diode/.\...............___
    ........................................|..................|
    ........................................|..................|
    0v o------------------------o-------------o---------------o 0v

    Ok so this is the circuit and i was wondering what the Resistance of R would be? Device A can supply either 5v or 3.3v however Device B only works on 3.3v so the zener diode is there just as precaution if Device A accidently operates on 5v. Now Device B be has a current consumption which should not exceed 10mA.

    I estimated R to be 80 ohms, but it is just a guess. What do you guys think?
     
  2. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    I was thinking 100 but 80 should do it's not critical and you could probably get away without it.
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Device B impedence is 3.3v / 0.01a = 330ohms.

    If R is too large, then Device B might not be able to operate. R does need to be large enough keep zener current within safe operating limits. I suggest keeping current below 100ma when Device A is at 5v.
     
  4. lostlego

    Thread Starter New Member

    Aug 15, 2006
    2
    0
    So what should the resistance be?
     
  5. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    You need two knowns to find the third unknown, in this case the key is the 10mA limitation:

    1.4V = 5 - 3.6V

    140 ohms = 1.4V / 10mA
     
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