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?

 

I was thinking 100 but 80 should do it's not critical and you could probably get away without it.

 

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.

 

So what should the resistance be?

 

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