need help for Ohm Meter problems

[Amin Moinez]

i'm new to pic18f4580 microcontroller and currently doing digital ohm meter problem. by any chance, can u guys help me?? i'm totally clueless about the program that needed to do

 

[ArticCynda]

Welcome to the forum Amin!

It's not exactly clear what you're trying to build, so you'll have to elaborate on that before we can give you any useful advice. Where exactly are you stuck?

 

[paulfjujo]

hello,


What is the range of your measurement
Ohms, milliohms, Kohms,Mohms ?
check if you can apply "Ohm law"
U=RI
Constant current * R => Volt to measure with ADC...
or you can use Time mesure with a monostbale ciruit LM555 ... mesure R*C constant with
C value constant (and known)..
example : **broken link removed**
..but reverse use :
measure R instead of C .


Voltage value involved for the R measure ..like insulation measure ?

 

[rmrps]

first form the circuit using some op amps and measure voltage output of op amp using pic controller keep one resistance as unknown value and remaining resistor as known value by using op amp equation you will find the unknown resistance value easily

 

[Amin Moinez]

I have build the circuit but don't know whether it is correct or not. Second problem is how i need to connect my constant current source and what type circuit to be used. lastly about my c language program

 

[Amin Moinez]

thanks but if you can provide example it will be more helpful. Thank you very much. I'm totally new in pic18f4580 but have basic using c language

 

[Amin Moinez]

Welcome to the forum Amin!

It's not exactly clear what you're trying to build, so you'll have to elaborate on that before we can give you any useful advice. Where exactly are you stuck?

I have build the circuit but don't know whether it is correct or not. Second problem is how i need to connect my constant current source and what type circuit to be used. lastly about my c language program

 

[Amin Moinez]

first form the circuit using some op amps and measure voltage output of op amp using pic controller keep one resistance as unknown value and remaining resistor as known value by using op amp equation you will find the unknown resistance value easily

thanks but if you can provide example it will be more helpful. Thank you very much. I'm totally new in pic18f4580 but have basic using c language

 

[paulfjujo]

hello


Give some answer if you want further help...

What is the range of your measurement
Ohms, milliohms, Kohms,Mohms ?

for example , if you have a current source of 1mA..maximum measure in ohm, will be 5000 ohms
with an ADC 10 bits 5V
some MCU like 18F46K22 have current source possibility, adjust by software ....


i gave you another way
RC Time measure ...

 

[Amin Moinez]

hello


Give some answer if you want further help...




for example , if you have a current source of 1mA..maximum measure in ohm, will be 5000 ohms
with an ADC 10 bits 5V
some MCU like 18F46K22 have current source possibility, adjust by software ....


i gave you another way
RC Time measure ...

for current source minimum at least 1mA is enough. The measured ohm ranging from 100 - 10k ohm. I'm doing pic18f4580 microcontroller and 10bits ADC 16x2. For pic I only using pic18f4580. If you have an example it'll be more helpful.

 

[paulfjujo]

hello

suppose you have a current source of 500µA (0,5mA)
with 100 ohms , you will get 50mV across it
with 10K ohms you will get 5V acrross it
and this signal goes into the ADC input
5,00V gives 1023 points
so you have a poor resolution to measure a resistor value with a factor de 100
100 to 10 000 ..

for me, the RC measure is a better solution...
you can use an ICM7555 monostable with C=1µF
The PIC arme the monostable and start a CCP capture with a timer..
to catch the duration time of the ouput pulse of monostable.
100 & 1µF => 200µS
1K & 1µF => 2000 µS
10k & 1µF => 20 000 µS
you have a good resolution

and you can add AUTOCALIBRATION to avoid drift of C=1µF