# Voltage divider circuit to measure 440v AC supply using ATMEGA8

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##### Newbie level 3
I am trying to measure 440V AC with atmega8. I have used transformer 440V-5V and bridge ractifire. Now I am trying to use voltage divider to convert it to 2.56V as my refrence volatge is 2.56V but I am unable to do so please help to design hardware for it.

#### betwixt

##### Super Moderator
Staff member
If that's 440V down to 5V RMS the peak will be (5 * 1.414) - (2 * Vf) or about 5.8V DC assuming you have added a reservoir capacitor. So to drop it to 2.56 you need a resistive divider with a series to parallel ratio of 1:1.264

You could use a series resistor of 13K and a parallel resistor across the ADC input of 10K. It would be wise to add protection across the ADC input in the voltage went higher than 440V.

Brian.

#### asdf44

A step down transformer would also be a nice way to do it and would have the added benefit of isolation.

But a divider is ok too. Just know that it's going to have to be implemented with many resistors in series. Resistors have voltage ratings as well as current ratings. 0805 is often 50-100V for example.

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### logicon

Points: 2

#### logicon

##### Member level 1
If you are working with phase angle, don't forget to invert the secondary waveform if you use a transformer. Hope this help.

#### KlausST

##### Super Moderator
Staff member
Hi,

You could use a rectifier, then a capacitor, then a voltage divider.
With this configuration you will more see the "peak" value of the AC.

You could also use a rectifier, then a voltage divider, then a capacitor.
With this configuration you will more seee the "average" of the signal.

I´d prefer the "average" configuration, because voltage spikes on the AC side are more suppressed and you get a value - from the meaning - closer to RMS.

****

Especially with low power transformers you will see a much higher output voltage (unladed) than expected. (x 1.5 .... x2)
Therfore you need to adjust for this.
Either you need a new calculation for the voltage divider, or you put a reasonable load to the transformer.Mind the dissipated heat. Maybe only useful for very low power transformers like 0.5...1W.
**
Here a tiny and precise circuit with tau (63%) of about 0.25s and a ripple of less than 1LSB for a 10 bit ADC

**
Good luck

Klaus

#### mohsenkit.ir

##### Newbie level 1
Hi
My english language is not good and i'm already sorry for it.
is there any way to measure ac voltage
i mean instead using transformer can i devide ac voltage from 220v to about 4v by resistor voltage devider and rectify it and measure by atmega8
and then i can control the over and under voltage between 180v to 240v

thanks

#### KlausST

##### Super Moderator
Staff member
Hi,

You are not the first with this problem...
* you have to deicde what "AC voltage" you want to measure (RMS, peak...). How often you need a new value and what precision you need.
* do you have pure sine or is it distorted somehow?
* do you need to know the DC component of your signal?
* did you do a search in the internet?
* did you do a search within this form. At bottom of this page you find "similar threads"

*****
A resistive divider is possible.
If you want to calculate the RMS from the ADC values then you need an extra capacitor and an extra resistor. I recommend to use two diodes to protect the ADC input.

WARNING: without a transformer you dont have isolation. You have to take care about the high voltage. All the circuitry must be treated as if it is a mains voltage application.

Klaus

mohsenkit.ir

### mohsenkit.ir

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