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# Need some ideas for the design of digital impedance meter

#### saurabh17g

Mar 8, 2010
72
hello friends,
I need some ideas to design a digital impedance meter. It should have a display showing the impedance of the medium to which it is connected.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Firstly you'll need to determine how you're going to measure the impedance.

Since impedance has a real and an imaginary part, what will you want to measure? Just the real part (resistance), or just the imaginary part (reactance), or the magnitude and phase angle?

Given that impedance (well the imaginary part) is generally dependant on frequency, you'll need to decide what frequency or frequencies you'll want to use.

Then you'll need to decide what sort of display you want.

Or do you want to measure inductance and/or capacitance?

#### prakhyata

Mar 16, 2010
1
i wanted to know the chip
'ST' V60LBL
SEAGATE
CCZZOOLTQ20
9E927062
AA063
TWN 99 940 58

what these no. show

#### saurabh17g

Mar 8, 2010
72
I want to measure the net impedance of the medium. Is there any direct way to accomplish this?
if there is no direct way, then,
I feel that i have to measure resistance(real part) and reactance(imaginary part)
then , i feel i have to do (real^2+imag^2)^0.5
and display it on LCD.
any ideas as to how to go about it? measurement of real and imaginary part separately?
For the time being , i do not want to measure inductance and capacitance separately.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
What are you trying to measure the impedance of?

#### saurabh17g

Mar 8, 2010
72
Impedance of the medium. It is unknown. It can be resistance, or inductance+cap or any combo of inductance+cap+Res. I might be water, wood , metal as well.
Just a general meter which when connected to a medium display's its inductance.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
OK, then what frequency do you wish to measure the impedance at?

#### saurabh17g

Mar 8, 2010
72
Lets take it as 50Hz

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
In that case you need a source of a 50Hz signal.

You could use a bridge arrangement with a calibrated potentiometer and read off the impedance from this once the bridge is balanced. I'm pretty sure (although not certain) that if you use an analogue meter to measure the bridge balance, the imbalance caused by phase shift should be averaged out.

#### saurabh17g

Mar 8, 2010
72
@thanks

so, i will synthesize the 50Hz signal using IC 8038 and use transistor at the sinewave output terminal as a current booster if required.
Or i might use XR2206 or AD9833, lets c upon the future requirments

So, i will connect the above generated source to the Vex of the below bridge circuit.
http://zone.ni.com/cms/images/devzone/tut/a/baf96ce2224.gif

(Now u can assume that Vex is sinewave source instead of DC source as shown in the diagram.)
where, R4 in the diagram is the caliberated potentiometer.
'caliberated pot'- is the one which has markings or resistance or something else?
Now, where should i connect the medium(the impedance to be measured) in place of R1/R2/R3 and how will i measure it?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
By calibrated, I mean it is a pot with a dial where the resistance of the pot is marked on a panel so you can read it off from the pointer of whatever knob you're using.

Your unknown impedance replaces R1

An analogue (AC) voltmeter is placed at Vo

Resistors R2 and R3 need to be chosen appropriately.

R4 is adjusted until the meter reads 0

The impedance is then such that R1/R2 = R4/R3 R1 is the unknown, R4 is either measured, or read from your dial pointer.

#### saurabh17g

Mar 8, 2010
72
Ok.
Further steps:

Lets say i select R1 = unknown impedance and R2 = 100Ohm.
I will make R2 = R3 so that R1(unknown) = R4(potentiometer)

I connect ADC at Vo. And I have to adjust R4 so that the reading of ADC(read by microcontroller) will be 0.

The question is, how wil i adjust the resistance R4. I would not be able to do it manually because that R4 is in that device. I just have two probes which are connected to the medium.

Last edited:

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Generally, yes, manually. By turning the big knob with one hand and watching the meter with the other.

Or perhaps watching it with your eyes. Just don't try to turn the knob with your eyes.

(I'm poking fun at me, not you. Yes I really wrote that you should watch the meter with your hand -- I should wake up before posting in the future)

The reading at Vo will never be zero unless the imaginary part of the impedance is zero (i.e. it is a pure resistance). It will, however be symmetrical about zero when the bridge is balanced (or as balanced as it can be with a resistance on one side and a reactance on the other.

Sorry, I actually said an AC voltmeter. That was a mistake. you need to measure the average voltage, so that would be done with a DC range. A digital meter would not work, and it would be hard to do it with a microcontroller also.

#### saurabh17g

Mar 8, 2010
72
so the output of ADC will be an average value or a varying entity(since the input is AC).
If its varying entity, we have to collect the samples and determine the maximum count. And calculate the RMS or average value. I hope +128(0 to 128) will corrospond to max value and -127(128 to 255) will correspond to min value. So how to adjust R4 so that the reading is made constant at 0.

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