# Infinite Impedance Observation

Discussion in 'Homework Help' started by Cerkit, Jan 9, 2009.

1. ### Cerkit Thread Starter Senior Member

Jan 4, 2009
286
3
Hi. I am a bit unsure about what to do with the following circuit. The specifications say that I should measure the voltage that would be observed at node 2 via an infinite impedance observation when a voltage between 9.8 and 10.2 is applied at node 1.

Code ( (Unknown Language)):
1.        ------------------ONode1
2.                     |
3.                    --
4.                    Ra 560R
5.                    --
6.                     |
7.                     ---------------------ONode 2
8.                     |               |
9.                    --¬ Zener       --
10.                     Δ               Rb 4K7
11.                     |               --
12.                     |               |
13.                     ----------------O Ground
The zener breakdown voltage is between 2.5 and 7.5V. The resistors values are such that if P2 is unloaded the diode will be in breakdown with 10V at node1.
Is this as simple as checking the voltage at node2 with an applied voltage of approx 10V at node1 ???

2. ### mik3 Senior Member

Feb 4, 2008
4,846
70
If this an experiment just do what they say. And yes you can check it like that.

3. ### Cerkit Thread Starter Senior Member

Jan 4, 2009
286
3
Ok was just a bit confused about the infinite impedance observation. How am I supposed to implement that in measurement?

4. ### mik3 Senior Member

Feb 4, 2008
4,846
70
What is an infinite impedance observation?

5. ### Cerkit Thread Starter Senior Member

Jan 4, 2009
286
3
I believe it means where absolutely not current is leaking through voltmeter. I suppose I could put a very large resistor after node 2 and then take the voltage measurement. Would that work??

6. ### mik3 Senior Member

Feb 4, 2008
4,846
70
No, you can't do this because a voltage drop will occur across the large resistor and you will measure less voltage. Use a JFET input op amp as a voltage follower to buffer the node. However, I don't think you need to get into all this trouble. Good multimeters draw little current.