Test and Measurement Forum

I am using a thermistor in a wheatstone bridge circuit to measure temperature. I am measruing the voltage across the bridge and then using this value to calculate the resistance of the thermistor and then the temperature.

All the fixed resistors are 10k and the thermistor is 10k (at 25C). When I measure the voltage across the bridge using a multimeter it gives one reading. However, when I use the connect the ADC-40 probem, the voltage drops by approx 0.2V. Obviously the voltage reading is incorrect and subsequently the temperature ends up being wrong.

I am guessing this problem has something to do with the input impedance of the ADC-40 probe (approx 1M?). Is there any way of using the ADC-40 probe to measure the correct voltage (and subsequently temperature) using this circuit (perahps by modifying the circuit or by taking the resistance of the probe into account in the calculation?). Any help would be most appreciated.

Many thanks,

Simon

Hi,

Thank you for your post ... unfortunately, there is not enough information provided to give you an informed opinion. To help with this further we will need to see the schematic of your circuit. Colud you please email it to:

tech@picotech.com

Once we have it we will try to provide an answer to the problem.

Best regards,

Helo have you been able to do the project?Am doing the same thing bur with platinum resistance thermomer.Can u email me any info on houz9@yahoo.com

sjhagan wrote:I am using a thermistor in a wheatstone bridge circuit to measure temperature. I am measruing the voltage across the bridge and then using this value to calculate the resistance of the thermistor and then the temperature.

All the fixed resistors are 10k and the thermistor is 10k (at 25C). When I measure the voltage across the bridge using a multimeter it gives one reading. However, when I use the connect the ADC-40 probem, the voltage drops by approx 0.2V. Obviously the voltage reading is incorrect and subsequently the temperature ends up being wrong.

I am guessing this problem has something to do with the input impedance of the ADC-40 probe (approx 1M?). Is there any way of using the ADC-40 probe to measure the correct voltage (and subsequently temperature) using this circuit (perahps by modifying the circuit or by taking the resistance of the probe into account in the calculation?). Any help would be most appreciated.

Many thanks,

Simon