# VOLTAGE SENSOR

Discussion in 'The Projects Forum' started by bear95, Jul 5, 2015.

1. ### bear95 Thread Starter New Member

Jul 5, 2015
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Hi guys.
i am creating a voltage sensor with the rectifying bridge and an optocoupler.
Could you guys give me suggestions in order to go about the right design procedure for the rectifying bridge with a capacitor filter with 200v 50hz input?

2. ### ErnieM AAC Fanatic!

Apr 24, 2011
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What special quality of this voltage are you sensing?

Is there some accuracy spec or any other parameters you can share about your secret project?

3. ### DickCappels Moderator

Aug 21, 2008
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Just curious, 200VAC/50 Hz is an unusual combination. 220/230/240, have seen that. What is the source of the 200 VAC?

If I may make a suggestion, put as little on the 200 volt side as possible. For example, if you get away with moving the capacitor to the output of the optocoupler you can use a lower voltage (and safer) capacitor.

Something like this:

The diode across the LED might have to be external if your coupler doesn't already have it. It can be something small like a 1N914. You might want to make the collector resistor larger and put a capacitor across the collector/emitter so that get steady voltage present/not present signal.

4. ### bear95 Thread Starter New Member

Jul 5, 2015
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I want to sense the voltage at the outlet(indian). I want to make it as accurate as possible.

5. ### bear95 Thread Starter New Member

Jul 5, 2015
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i am sorry 230v not 220. I am using a rectifying bridge to convert the Ac to Dc (since i havent found an opto coupler which takes direct ac input) and the i read somewhere that the DC output is ~.070 AC. Is that right?

6. ### Alec_t AAC Fanatic!

Sep 17, 2013
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Why not use a low-voltage transformer and measure the output from that? Much safer. The output voltage will be a fixed fraction of the mains voltage.

7. ### MikeML AAC Fanatic!

Oct 2, 2009
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I use an old ac-output wall-wart transformer for this purpose. These are a dime-a-dozen at garage sales around here. I have even broken off the plastic case and used just the transformer to get rid of the rectifier in a DC output wall-wart.

Lightly loaded by just a measuring circuit, these make an ideal line-voltage measuring method. If you are going into a micro ADC, you can put an opamp precision full-wave rectifier between the low-voltage secondary and the ADC input.

Jan 15, 2015
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No, the AC is a sinusoidal waveform. Mains line voltage is normally specified as the RMS (0.707 * E peak value). So the RMS value is equal to the DC value. However, if a cap is added following the full wave rectifier then the DC level will be 1.414 * RMS value or in your case about 325 VDC.

My understanding is you just want to "sense" and not measure the mains voltage, the mains voltage is there or it isn't, Yes or No. I would just do as suggested and use a plain old simple wall wart adapter as was suggested. Safe, simple and inexpensive.

Ron

9. ### AnalogKid Distinguished Member

Aug 1, 2013
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What is it that you want to "sense"? Presence/absence, peak value, RMS value, frequency, current - ??? Except for current, all of these can be "sensed" through a transformer with more safety and accuracy than with an optocoupler. And BTW there are AC input optocouplers that have two back-to-back LEDs internally. Like all opto's, these cannot be connected directly to an AC mains without current limiting and transient protection, but they can eliminate the need for a bridge rectifier in some applications.

ak

10. ### mcgyvr AAC Fanatic!

Oct 15, 2009
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Did "transformerless" discussions become allowed now with automotive topics?

11. ### bear95 Thread Starter New Member

Jul 5, 2015
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i would like to use an ardiuno and the measure the voltage. Apologies, i should have mentioned that i was connecting the optocoupler to the ardiuno.

12. ### bear95 Thread Starter New Member

Jul 5, 2015
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Thats good to know. Could you suggest some optocouplers which could be used?

13. ### bear95 Thread Starter New Member

Jul 5, 2015
6
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i would like to sense the value of voltage a device derives from the outlet and read it through the arduino.
It would be like connecting this voltage sensor and the device plug to the outlet and measure the voltage consumed but the device
.

Jan 15, 2015
963
232
Yeah, that sort of changes things quite a bit. I would use a transformer to reduce the 230 VAC to a low DC level like 5.0 VDC. Just a matter of a full wave rectifier, a linear 230 VAC to 5 VDC would likely do fine. I would then do a simple test to make sure the wall wart was linear using a variac to drive the wall wart. In theory if 230 VAC provides 5 VDC then 115 VAC for example should provide around 2.5 VDC. The DC from the wall wart goes to your Arduino AI (Analog Input). Then do the math in the code.
There are also commercially available meters available to do what you want so it's a buy or build option.

Ron

15. ### MikeML AAC Fanatic!

Oct 2, 2009
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Reading voltage can be done much easier with a voltmeter. Measuring "power" going into an appliance requires measuring ac current...

16. ### AnalogKid Distinguished Member

Aug 1, 2013
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1,251
While an optocoupler can be used for isolated voltage measurement, it isn't easy because an optocoupler's transfer characteristic is not linear. Better to use the smallest AC transformer you can find. When followed by a bridge resistor and a catch diode as in the left schematic, there is no non-linear voltage loss through a series rectifier or bridge, but you have to read the voltage multiple times to capture its peak value. The alternative is in the right schematic, a diode and capacitor to give you a DC voltage that is proportional to the AC RMS input. It takes more math to calculate the relationship, but since the output is DC you can read it anytime.

ak

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Last edited: Jul 8, 2015