[SOLVED] AC Voltage sensing - Adding DC bias

What is your output node?

How I'd do it:
* [GND] --> transformer --> C1 --> R1 --> [ADC]
* also: [3V3] --> R2 --> [ADC] --> R3 --> [GND]
(All three resistors are connected at the ADC node.)

You use a small transformer, maybe just 1W (short circuit proof) to step down voltage.
Mind that the 5V are for full load. Unloaded they may go up to 8V RMS, meaning about 23Vpp

How the circuit works .. and values:
R2 = R3 = 10k, they generate a voltage divider, making 3.3V/2 = 1.65V to center the ADC values.
R1 = 30k, act as the input part of a voltage divider to step down 23Vpp to 3.3Vpp. (The "GND part" of the voltage divider is R2||R3 = 5k Ohms)
C1 = 4u7, acts as 1Hz high pass filter to enable DC offset at the ADC

I recommend to add [ADC]--> 33nF --> [GND] to suppress high frequency noise and transient signals
Also: [3V3] --> (K) D1 --> [ADC] --> (K) D2 --> [GND] as overvoltage protection. Use schottky diodes like BAT85

Please check that there is enough headroom at the ADC input. (Avoid clipping). Adjust R1.

Klaus

KlausST said:

What is your output node?

Click to expand...

Oh Sorry, I forgot to mark it. Here I update it.




So from your suggestion, circuit would be like this, Am I right?


1. If I am right, The function of C1 is just for the purpose of filtering. say, If I used 1uF, It's High Pass filter with fo = 5Hz.
2. R1 decides headroom for AC and if I want to calculate Maximum Voltage that adc point in circuit can reach for 15VPk (30Vpk-pk) transformer output:

Voutadcmax = vpkmax*(R2||R3)/(R1+(R2||R3))+1.65V





Also It would be great if you can help me understand the other 2 circuits as well, as to see which components decide the headroom

There is a previous thread about using small mains transformer.......it could well be inaccurate.....the thread is from approx somewhere 2018 to 2019 on edaboard

The attached is an accurate way to measure mains voltage......using an InAmp....or you can use a diff amp if you want instead if you want cheaper.