# How to make a simple voltage divider to sense DC voltage with protection of AC voltage?

#### Shamoooot

Joined Apr 3, 2023
10
Hello

Need some help with making a simple voltage divider to sense DC voltage of 0-30V range with my DAQ which has 0-10V analog input.

I was wondering if the input voltage is an AC voltage , what would be the reading on the DAQ ? will there be any risk to the DAQ ? and how can I make rectify the AC voltage without a bridge rectifier (just for protecting the DAQ and prevent AC voltage input) ?.

#### crutschow

Joined Mar 14, 2008
32,926
if the input voltage is an AC voltage , what would be the reading on the DAQ ? will there be any risk to the DAQ ?
There shouldn't be a problem.
The reading will likely fluctuate depending upon what point of the AC waveform it happens to sample, and the plus and minus voltages will be limited to about +9.1V and -0.7V by the Zener.
You may want to put a 10kΩ resistor in series with the converter input to limit any negative current.

I assume it is an ADC, not a DAC that you have(?).

#### sghioto

Joined Dec 31, 2017
4,918
I would think this is all you need to rectify the AC. This will give a DC voltage reading equal to appx the peak voltage of the positive side of the waveform so may need to adjust the value of R9 although the zener will limit the output.

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#### crutschow

Joined Mar 14, 2008
32,926
I would think this is all you need to rectify the AC.
I thought so to at first, but I think he doesn't necessarily want to read the AC, just concerned about whether it will damage the converter.

#### sghioto

Joined Dec 31, 2017
4,918
I think he doesn't necessarily want to read the AC, just concerned about whether it will damage the converter.
Understand but what would be the purpose of connecting AC to the voltage divider unless it was possibly by accident?

#### Shamoooot

Joined Apr 3, 2023
10
Thank you guys..

Yes it shouldn't happen but it might by mistake that I connect an AC voltage instead of DC. I am only interested in the DC readings. So the AC reading doesn't matter, I only need to be sure that my DAQ is safe.

#### crutschow

Joined Mar 14, 2008
32,926
Reverse your diode and capacitor to form a level shifter .... You will also need to re-adjust the voltage divider
Why do that if the TS is interested in measuring only DC voltages?

#### Beau Schwabe

Joined Nov 7, 2019
143
"Why do that if the TS is interested in measuring only DC voltages? " - He's got a Sine wave going in

With the diode capacitor arrangement in post #3 you need to provide some method of decay or discharge or else you have a peak detector

#### sghioto

Joined Dec 31, 2017
4,918
provide some method of decay or discharge or else you have a peak detector
Depends on the input impedance of the ADC. Adding a 100K resistor across the zener should suffice if needed.

#### Shamoooot

Joined Apr 3, 2023
10
I appreciate you guys, thank you all..
As a beginner I feel a bit overwhelmed, do you guys think this circuit would be good alternative:

#### BobTPH

Joined Jun 5, 2013
7,557
Are you using a commercial DAQ (data acquisition) board? If so, it probably has protection for it’s inputs. It should be in the specs.

#### Shamoooot

Joined Apr 3, 2023
10
That's pretty much all I can find for it:

#### Ian0

Joined Aug 7, 2020
8,407
I assume it is an ADC, not a DAC that you have(?).
DAQ = Data AcQuisition (I think)

#### DickCappels

Joined Aug 21, 2008
10,068
I think most of us read “DAQ” as “DAC“, but let’s not start a war over it

The purpose of writing is to communicate and for that reason I like to use the simplest terms most common abbreviations or acronyms that fit the purpose. Also, on an international site like this, it is safer to spell things out rather than to abbreviate or use slang.

Off my soapbox now.