120Vac signal conditioning for 5Vdc input data logger

Thread Starter


Joined Oct 21, 2020
Hello All,

I have an 120VAC shaded pole motor being driven by a triac to control its speed. The sine wave is chopped to deliver different voltage levels for speed control. I want to log the motor voltage to analyze what the motor control voltage is over time. My logger only has DC voltage inputs. I built a simple step down circuit using a transformer, bridge,cap and voltage divider to get a 0-5Vdc signal from a varying 120Vac input, which works OK with a sine wave but not with the chopped triac motor driver signal. Any thought on how to do this would be great.

Thanks in advance!


Joined Feb 16, 2022
Hi Mike,
Your circuit is not suitable for distorted sine wave. Otput voltage is proportional to the peak voltage, not to average voltage.

Modify the circuit : Transformer- bridge-voltage divider (R1-R2). Lower divider resistor (R2) shunted with capacitor. Select the values of R1 and R2:
The time constant must be large enough:
T>=5*0.01 sec= 0.05 sec
where 0.01 sec is the period ( if mains frequency is 50Hz)

T=C*R2, so C=T/R2

schmitt trigger

Joined Jul 12, 2010
The problem with measuring the chopped sinewave is that it is so distorted that the average responding measurements will be meaningless. Been there, done that.

You will require to perform an RMS-to-DC conversion prior to the ADC.
There are dedicated ICs for that purpose. Unfortunately they are not cheap, but the least expensive and simplest to use is the LTC1966.