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Measure a resistance with IC

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PedroPT

New Member
Hello guys,

I need to do something a little bit tricky (for me)..

I have several unknown elect. resistances that range from 1 to 400 Ohms.
I need to be able to measure them with an accuracy of 1Ohm, so that 2 Ohms is actually different than 3 Ohms even though the scale ranges from 0 to 400.

I though of Weathstone bridges but I dont think they have the necessary accuracy for the range I need. Another idea is to use the time constant of an RC filter but I am afraid that once again I'lll get an approximate value and not a accurate one.

Obviously I cannot use a multimeter.. the idea is to replace the use of a multimeter by creating a specific tool.. The tool however can use the mechanism that a multimeter uses (I honestly don't know what it is..)

Thank you in advance for all your input.
Must be easy for top notch electronics engineers such as you guys!
 

ericgibbs

Well-Known Member
Most Helpful Member
hi,
A common method is to use a constant current source to drive the resistor under test and measure the voltage across the resistor, use the PIC's adc for the conversion.
 

gaspode42

Member
You beat me to it Eric, I was just doing the math to see if that idea would work!
 

ericgibbs

Well-Known Member
Most Helpful Member
You beat me to it Eric, I was just doing the math to see if that idea would work!
hi R.
Many modern DVM use that method.:)

I am suprised to hear him say that a bridge isnt accurate enough.?
 

PedroPT

New Member
Thx,

I also thought on that one, just didnt know if I could use the 5 volt to measure 400 Ohm as the adc would have to have a resolution of 1.25 mV. that was one of the problems that I had with the withstone bridge idea.

Can you think of a way that would be independant of the voltage level? I can only think of the time costant to charge the capacitor.... :(

Anyway I wanna give that idea a go, what do you suggest for a reliable constant current source (yes, I am that rookie!)


Thanks to both of you guys!
Pedro
 

PedroPT

New Member
it may be accurate I didnt give it a go beacuse I was afraid of the accuracy of teh adc.. 1mv Accuracy I feel like any little noise could mess up the reading...
 

gaspode42

Member
Hi

Just checked the maths on this.
If you use a 10mA constant current source that would give 0.02V for 2Ω and 4V for 400Ω.
If you use 5V as the A/D reference that would give you a 0.0048V per 'digit' resoultion so the above would work perfectly.

Hope that helps :)
 

ericgibbs

Well-Known Member
Most Helpful Member
hi,
As pointed out by gaspode42 10mA gives a good result.

If 10mA is to high a current, use 1mA and ampilfy the signal 10 or so.:)
 

gaspode42

Member
You can make a CCS with a transistor, a couple of resistors and a zener.
Have a look here there may be a better way, but I am sorry I don't know.

p.s. Where did you get 1.25mV from?
 
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