# How to find the Resistor Values

Discussion in 'Homework Help' started by Cerkit, Jan 8, 2009.

1. ### Cerkit Thread Starter Active Member

Jan 4, 2009
275
3
Anyone know how I can find the resistor values in the following circuit taking into account that the circuit is hidden in a box. It will be a buggy which will do the analysis ie. there will be circuitry which analyses the circuit.

Node 1 O--------[ Ra ]-------[ Rb ]-------O Node 2
|
|
--
| | 1K2
--
|
|
O---------------------------------O Ground

The 1K2 resistor goes across the rails inbetween Ra and Rb but the wall post shifts it to the wrong position!!

Last edited: Jan 8, 2009
2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Just fixing up your ASCII schematic - the board makes a mess of them unless you use the CODE blocks

3. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
If you measure from Node 1 to Ground, you will get Ra+1k2.
Subtract 1k2 from the above reading, and then you will know the value of Ra.
If you measure from Node 1 to Node 2, you will get Ra+Rb.
If you then subtract Ra from the above reading, you will know the value of Rb.
Verify by measuring from Node 2 to Ground, and subtract 1k2 from that result.

4. ### Cerkit Thread Starter Active Member

Jan 4, 2009
275
3
Yeah i understand up to that point. The issue is that the buggy analyser is responsible for placing a voltage onto the circuit as well as analysing it so if I set it up so that i have for example a voltage of 5V between node1 and ground then I dont know how to make the circuit take valid measurements which will lead to me obtaining values for the resistors.

By the way I only have access to those three points node1, node2 and the ground point I can not tap a reading from anywhere else.

5. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Apply a voltage (V1) between node1 and ground and measure the voltage between node 2 and ground (V2) with a high impedance voltmeter.

The relation between V1 and V2 (assuming no current flows through Rb due to high impedance voltmeter) is:

V2=(V1*1K2)/(Ra+1K2)

Thus you can find Ra. Do the same procedure with the other node to find Rb.

6. ### Cerkit Thread Starter Active Member

Jan 4, 2009
275
3
Thanks for the help.

I will need a voltmeter on the buggy for analysing other circuits as well and they will not all need a high impedance voltmeter, therefore do you recon I could multiplex the voltmeter input, whereby the ouput from a circuit under observation can optionally be put through a high impedance line before entering the voltmeter??
Also do you know any digital voltmeters to recommend me putting on the buggy?

7. ### mik3 Senior Member

Feb 4, 2008
4,846
63
The higher the input impedance of the voltmeter the better it is for all circuits. You can make a voltage follower with a JFET input op amp to achieve very high input impedance and measure the voltage on the output of the op amp.

8. ### Cerkit Thread Starter Active Member

Jan 4, 2009
275
3
What about the following circuit, how is it possible to find the resistor values??

Node1 O-----[Ra]------ONode2
| |
-- --
| | 1K | | Rb
-- --
| |
O-----------------o Gnd

The 1k and Rb are supposed to go down the left and right of Ra respectively, the posting messes up, sorry dont know how to use the CODE blocks yet!

9. ### hgmjr Moderator

Jan 28, 2005
9,030
214
This thread sounds like a class assignment. I am moving it to the "Homework" section as a result.

hgmjr

10. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Apply a voltage at node 2 (V2) and measure the voltage at node 1 (V1).

Then V1=(V2*1K)/(1K+Ra), you can find Ra

Because you know Ra apply a voltage at node 1 and measure the voltage at node 2 to find Rb.

11. ### Cerkit Thread Starter Active Member

Jan 4, 2009
275
3
Ok.But when applying voltage from either node to ground doesnt current pass through all the resistors??

12. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Yes but this is not a problem as far as you measure both V1 and V2 before making the calculation to be sure of their values. I am saying that because loading of the power supply may change its output voltage when you measure it with no load on it.

13. ### Cerkit Thread Starter Active Member

Jan 4, 2009
275
3
Nice. One more thing. What could I do to ensure that the voltage I apply does not drop significantly enough to wreck my calculations. It is because the measurement will be done by circuitry not by a human therefore there is no option of measuring what the applied voltage is once it has been placed on the load.