# 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