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resistance to voltage converter

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adil.sid10

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Hello i have having a water level indicator installed on a machine i want the voltage output from it lets say 0-10v dc the instruments changes its resistance in Mega ohms as the level changes kindly help me out as soon as possible i would be very thankful to you.

Regards,

Adil.
 
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Re: resistance to vlotage converter

Linear resistance to voltage conversion simply needs a current source. If you don't mind a non-linear output you can simply put a resistor to a power supply in series with the sense resistor, but you will not get a full range of output voltage.

Keith
 

Re: resistance to vlotage converter

thnx for your response but keith i need to have output in between 0-10 v and i have a 24v dc source available there but no current source so can i use that 24dc voltage source instead of current and the sensor minimunm resistance is 245k ohm and maximum isa 35 M ohm. Kindly tell me the solution i will be very thankful to you.

Regards,

Adil.
 

Re: resistance to vlotage converter

No such a specific circuit exists on the internet or in literature and need to be designed.
 

Re: resistance to vlotage converter

As stated, you need to design a current source. There a quite a few posts about this such as https://www.edaboard.com/threads/190301/

The example there actually sinks current rather than sources it, but the same principles can be used to generate a source.

You can make a simple current source with a transistor, zener diode and a couple of resistors but it won't be very accurate. With 24V available and a 0-10V output range then you may be able to do better than 5% accuracy with a simple one. An opamp based circuit can be very accurate.

One issue you need to watch out for is leakage currents. Your resistance is very high so transistor leakage could affect accuracy. A JFET based source may therefore be better than a bipolar transistor based on.

Keith.
 

Re: resistance to vlotage converter

It would be interesting to know what is the indicator resistance at 10% water level. My idea is that near the very low water level the indicator becomes nonlinear or at certain low level is opencircuited. Maybe the usefull range of indicator doe not need to be up tu 35Mohms and the indicator can be placed over the opamp as a feedback resistor. If indicator is linear then max resistance at 10% level should be arround 2.5Mohms.
So adil.sid10 give us that data.
 

I think you are right to wonder about the linearity. My experience with water detection with AC voltages is that it rapidly reaches a lower limit of resistance once the probe touches the water and does not give a progressive level. Hopefully Adil has some measurements to support the system design.

Keith.
 

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