Steve said:
What is the typical contact resistance ?

I need to test the CR from low level say 20 milliOhm to 2 Ohm
(since i would test many type of Reed switches).

I think this is the info you have asked.

Richard said:
in your case, two different currents, then you have to have two current sources
i have decided to use only one test current = 10mA

Richard said:
Now you can measure the voltage in the resistor, or the voltage across the relay, and either one will allow you to compute the contact resistance.

can you give me little info on how the measure of drop at the resistor(in the emitter that leads to the reed switch) could be used in computing the CR of the switch? i doubt,with that how could i arrive at the voltage being dropped accross the switch!

Terry said:
Kai posted a current limiting circuit for a lamp driver in your lamp thread. You asked how it worked. I gave you a simplified exlpanation . Did you read it?
yes i went through it. thought of getting clarified with few issues! Let me go through it again and i would let you know using that thread.

Terry said:
That circuit with the correct emitter resistor would work here.

Yes.
Richard has also described the same method in his post.

Terry said:
If you want long term accuracy and stability 'you must know what you need to achieve before you start designing'.

Definitely.
I should frankly accept that i was not having a clear idea when i started this thread - only gave some info that i got from people over here to describe what i need to do. i say this since i dont have any other electronics engineer to whom i could consult before coming here.
So hopefully to know myself what i could achieve with this work i was given, i am discussing here to get relevent details regarding the CR measurement and would think what should be done that would be more efficient as of i could do that.

Terry said:
As yet you have not supplied even rudementary specifications to allow us to guide you towards a design that has a chance of meeting the test's required accuracy specification.

Specifications:
To test CR of various types of Reed switches (from 20 mOhm to 2 Ohm)
Test current = 10mA
The CR would be measured with 25% overdrive.
If any other info is required kindly let me know for it.

Description of my Project:
I think its better to describe about my project.
The CR measurement setup would be a micro(AT89S52) based one with LCD display.
The voltage sensed accross the Reed Switch would be amplified and given to micro through an ADC. After performing required math will get displayed in LCD. For switch closure (AT would also be read) a DAC would be used with the micro to generate the ramp.
Actually the setup i have described has been used in an Addon Computer card here(ISA card). Now i think i could start doing these things with a micro and a suitable display with at most precision.

Neil said:
Do you need and op amp that depends on the DVM and the contact resistance.

If iam right - DVM stands for Digital Volt Meter , i prefer using an LCD as i have described above.

Thanking you all,
Suresh.