Need circuit idea for resistance measurment

[udhay_cit]

I need to design a circuit of a TAP POSITION INDICATOR for transformers. For every tap positions the resistance steps will increase or decrease. The two wire will run for 5 meter distance approximately. The step resistance may vary 10Ω, 100Ω or 1kΩ depend on the transformer, but these three are standard. The maximum possible number of positions will be 39.

I don't have idea how to design circuit for this. If i use constant current source to measure 39kΩ (1KΩ * 39 position) resistance, i need to use 128µA constant current to get 5V. I think its not good to send 128µA current for 5 meter distance. Please help me to solve this problem....

Regards
Udhay

 

[barry]

If you're cable is properly shielded, and you add some filtering, I don't see why you can't use 128uA.

 

[crutschow]

To minimize the effects of noise pickup you could use either 10Ω or 100Ω steps, using 12.8mA or 1.28mA constant current for measurement which is still acceptable. 4mA-20mA current loops are commonly used in industrial applications for remote measurements hundreds of feet away.

 

[ftsolutions]

I'd also use either 1.28mA or 12.8mA, or some other convenient, similar magnitude current level for a production test environment. Probe contacts and connectors wear out with repeated use, and oxidation or contamination can more significantly affect your measurements when you are only sourcing 128uA. You also want to be aware of the potential for trying to take measurements in an electrically noisy environment, so using a larger magnitude source signal to improve your noise margins is attractive.