resistance to voltage converter

[avinash]

i am having resisistive temperature sensor,whose resistance changes with the temperature change.how to convert this resistive change to voltage change.
thanks in advance.

 

[IanP]

voltage to resistance converter

The simplest method is to build a constant current source of, say, 100µA, 1mA, 10mA or any other value that suits you, and measure voltage generated by his current across the RTD.
Other option is to build a bridge of 3 resistors and RTD and read differential voltage across the bridge. In this case you need just regulated voltage to supply the bridge..
Regards,
IanP

 

[avinash]

resistance to voltage conversion

dear IanP
the current across the resistance could not exceed 100u.moreover i have already used constant current source of 100u for resistive sensor.but the voltage across the resistor is coming to be from 64mV to 122mV.my supply is 0 to 5V.so i want the o/p from the resistance to be in the range of 0 to 5V,so that i could utilize the supply of 5V(moreover the o/p from the resistance i.e. voltage is to fed to the ADC)

pl help

 

[chinara]

convert resistance to voltage

Use regulator, such as 7805, to convert the 50V to 5V. The you should make sure the current is not too high.

 

[seadolphine2000]

converting resistance to voltage

What about using an inverting amplifier(opamp). the input resistance will be the tempreature sensor, the gain of this amp is proportional to the temp sensor.
Here you are the schematic.

 

[IanP]

resistance voltage converter

If constant current is not a good option for you then consider a bridge configuration with an opamp of, say, 100 gain, or as required. This should give you quite wide output voltage range, maybe close to 0 to 5V.
And the beauty opf this solution is that you can supplt the bridge directly from 5Vdc.
Regards,
IanP

 

[VVV]

rtd to voltage converter

Generally, for an RTD thermometer, you need a constant current source and a precision, very low tempco resistor.

You then apply the current to the precision resistor and measure the voltage across it. Then apply the same current to the RTD and measure the voltage. By taking the ratio of the measurements, you get a very accurate temperature measurement. The reason is that the current does not need to be very stable, since the same current is used to measure both the RTD and the reference resistor, so the current value cancels out.
You obtain the RTD resistance as a proportion of the precison resistor. Therefore, the precision resistor should be chosen higher than the highest RTD resistance, typically 400Ω. Adequate precison resistors are for instance Caddock's USF series.

By the way, the current can be higher, such as 1mA, or even 5mA, but applied in short pulses, just long enough to take the measurement. If you keep the resulting RMS current below 100uA, you avoid selfheating. That is the reason the current is limited to 100uA, not because physically the RTD could not withstand it. Using higher current will dramatically improve accuracy. With 5mA you get 2V max across a 400Ω resistor, without any additonal amplification, which contributes offset errors and drift. With a 2V maximum input voltage you can then use an A/D with a 2.5V reference, for better resolution. An A/D with 16 bits minimum is resommended.

The two measurements can be done with two different channels of the same A/D. But the current has to be switched to either the RTD or precision resistor using two MOSFETs or some analog switches. Or, the RTD can be in series with the precision resistor and the current will flow through both, but one of them will have a large common mode voltage, making measurement much more difficult. It's best to keep them both ground-referenced and switch the current.

 

[bamdad]

convert voltage to resistance

A _VERY_ wise and simple idea is to use an RC network with the "R" being the resistance to measure and the "C" to be a known and relatively stable capacitance, this way, measuring the time constant of this circuit will give the resistance, and the dynamic range of this measurement can be quite wide! and DOES NOT need any ADC! and uses only one digital IO pin of the microcontroller! this is the SIMPLEST and the most ELEGANT way I've seen on resistance measurement.
another way is to use the ratiometric topology, virtually all of the DVM ICs, and all of those ADCs which have differential reference input can be used in this technique. the idea is, to create a voltage divider network between the known reference resistance which in turn feeds the reference input, and the unknown resistance, which feeds the ADCs input terminal. this way, the readout of the ADC would be Vin/Vref which is equal to Rx/Rref
.b

 

[funster]

resistance to current converter

that is very easy, use a constant current source to drive your

sensor, then the voltage betweent your sensor's two ends

is the voltage that you wanted.


best regards

i am having resisistive temperature sensor,whose resistance changes with the temperature change.how to convert this resistive change to voltage change.
thanks in advance.

 

[bamdad]

convert varying resistance to voltage

Another way to measure very tiny differences in large resistance values, is to use a bridge to amplify the difference...

 

[avinash]

resistance to voltage convertor

Generally, for an RTD thermometer, you need a constant current source and a precision, very low tempco resistor.

You then apply the current to the precision resistor and measure the voltage across it. Then apply the same current to the RTD and measure the voltage. By taking the ratio of the measurements, you get a very accurate temperature measurement. The reason is that the current does not need to be very stable, since the same current is used to measure both the RTD and the reference resistor, so the current value cancels out.
You obtain the RTD resistance as a proportion of the precison resistor. Therefore, the precision resistor should be chosen higher than the highest RTD resistance, typically 400Ω. Adequate precison resistors are for instance Caddock's USF series.

By the way, the current can be higher, such as 1mA, or even 5mA, but applied in short pulses, just long enough to take the measurement. If you keep the resulting RMS current below 100uA, you avoid selfheating. That is the reason the current is limited to 100uA, not because physically the RTD could not withstand it. Using higher current will dramatically improve accuracy. With 5mA you get 2V max across a 400Ω resistor, without any additonal amplification, which contributes offset errors and drift. With a 2V maximum input voltage you can then use an A/D with a 2.5V reference, for better resolution. An A/D with 16 bits minimum is resommended.

The two measurements can be done with two different channels of the same A/D. But the current has to be switched to either the RTD or precision resistor using two MOSFETs or some analog switches. Or, the RTD can be in series with the precision resistor and the current will flow through both, but one of them will have a large common mode voltage, making measurement much more difficult. It's best to keep them both ground-referenced and switch the current.

thanks to all
but dear VVV ,
how can i give 1mA or 5mA current with short pulses.i mean what is the way out for this.presently i am using the current from voltage reference generator and converting this bandgap voltage to 100uA current that has been fed to the RTD.

dear bamdad,
i didn't underestand the second way of measuring the resistance which you have told.pl explain it.

 

[IanP]

methods of converting resistor into voltage

Maybe you should consider 16-bit (or more) A/D conversion. After all, these devices are not that expencive ..
Even with 16-bit convertion, the resolution is something like ≈75µV/per bit (at 0-5V range), so if the change (@100µA) is between 6x and 12x mV you will still have > 800 readings..
Regards,
IanP

 

[bamdad]

how to convert resistance to voltage

consider a dual-slope AD converter with differential input and reference. (like a classic DVM, for example ICL7109) if you create a resistive voltage divider network between a known (for example 1K) resistance and the unknown resistance you want to measure, and divide a fixed voltage (for example the 5V supply) between the two, the ratio of the voltages across each resistor would be relative to their resistance as we all know, now if we feed the voltage across the known resistance to the differential reference input, and the voltage across the unknown resistance to the DVM input, the DVM will measure the RATIO of these two voltages, and thus will give us the ratio of the two resistances...

if anyone can help me put a JPEG figure on my post, I can give a more clear description.

.b

 

[VVV]

convert amps to resistance

Buld a simple current generator and use two switches to steer this current to either the RTD or the precision resistor. You can use ADG701/2 from Analog devices, for instance. They are somewhat expensive.
Otherwise, you can simply use two MOSFETs, but you have to make sure they always turn on., that is take care of their drive voltage.

 

[avinash]

rtd,dvm,circuits

Maybe you should consider 16-bit (or more) A/D conversion. After all, these devices are not that expencive ..
Even with 16-bit convertion, the resolution is something like ≈75µV/per bit (at 0-5V range), so if the change (@100µA) is between 6x and 12x mV you will still have > 800 readings..
Regards,
IanP

but dear IanP,
i need 1450 reading in the range from 60mV to 120mV.

dear bamdad ,if you upload the jpg file it will be really helpful for me.

Added after 11 minutes:

dear VVV,why two switches are used and how to use them.if you send me the schematic ,it will be helpful.
thanks.

 

[IanP]

voltage resistance converter

Try to build (or buy off-the-shelf) instrumentation amplifier similar to:
http://www.ecircuitcenter.com/Circuits/instamp1/instamp1.htm
If you set gain to, say, 10, you are "home" and any 16-bit A/D will give you more than you need..
Regards,
IanP

 

[bamdad]

resistor to voltage converter

Here goes the schematic of the ratiometric converter for ICL7109 dualslope converter in protel format

 

[avinash]

convert resistance to a voltage

dear bamdad,
i am unable to open the file ADC.sch
how these files with extension .sch are opened.

 

[IanP]

convert resistance to volts

Download any software that allows you to draw schematics such as, for example, EAGLE.
The address is here:
http://cadsoft.de/download.htm
Having this you should be able to open files with extension SCH.
Regards
IanP

 

[bamdad]

voltage to restance converter

That's Protel 99SE's schematics format, you can create a new design and import the schematics to the design. also you can use Potel DXP to open the schematics file directly. if you don't have them, please let me know, I will create the PDF...