0
\$\begingroup\$

I am trying to measure the resistance of a nichrome wire that has a PWM (1000Hz and different duty cycles between 0 and 100%) voltage applied to it from a 5.0V regulator. The setup I have in mind looks like this:

5V PWM ---- Wire (~20 Ohm) ----X---- 1W Resistor (0.5 Ohm) --- Ground

I then measure the voltage at point labeled X with respect to ground, and amplify it using an instrumentation amplifier (TI INA122). As the nichrome wire is heating, I want to obtain its temperature by measuring its change in resistance which would correspond to change in measured voltage given by my amplifier. However, I expect noise so I want to filter it out. I don't know to go about doing this since the signal is PWM. If it was straight up DC then I'd use a low pass filter, but now I am stuck.

Here is a picture of what I'm trying to accomplish. Basically smooth out the noise like shown. enter image description here

Thank you in advance!

\$\endgroup\$

1 Answer 1

Reset to default
0
\$\begingroup\$

You can still use a low pass filter but you might need to use a higher order filter in order to reduce the switching noise of the PWM. I'd place a low pass filter between 1 ohm resistor and input to the amplifier just to kill-off the bulk of the PWM noise and then after the amplifier maybe add another low pass filter. I take it that this will then feed some micro controller - if so the MCU can perform digital filtering to reduce noise even further.

\$\endgroup\$
2
  • \$\begingroup\$ This will make the output be a function of the duty cycle. I want the output to be a function of the temperature only. \$\endgroup\$
    – Nic
    Apr 28, 2015 at 20:30
  • \$\begingroup\$ You could gate the positive part of the waveform using an analog switch. The output of the switch would feed a capacitor. Then use an opamp filter afterwards. Sorry for originally misinterpreting your question. Btw this technique is called sample and hold. \$\endgroup\$
    – Andy aka
    Apr 29, 2015 at 11:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.