View Full Version : AC Voltage monitor

01-20-2009, 03:40 AM
I'm wanting to monitor the AC Voltage from a home outlet and eventually input the data into a PC, but first things first. I'm thinking something like AC voltage in to a bridge, then into a comparator that feeds an LED graph . . .

AC ------> comparator ----> Bar Graph

Thanks for your feedback

Daniel Mueth

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"Just plug it in and let's see what happens"

Mike Green
01-20-2009, 04:09 AM
You need to isolate the AC line from everything. Given that you want to monitor the voltage, a transformer would be a cheap and very effective solution. The voltage ratio is fixed. A simple 12.6VCT transformer would do very nicely (like this: www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=221292& (http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=221292&)). You'd want some kind of varistor or other surge suppressor across the primary and you'd want a fuse on the primary.

You could use one side of the winding for sensing and the other side for +5V regulated power for the circuit.

Carl Hayes
01-20-2009, 04:37 PM
Mike's solution is excellent -- but you will save the cost of a few parts if you build only one supply.· Use the unregulated (but rectified and filtered) voltage for measuring, and also to feed the regulator (perhaps an LM7805 -- wups -- you need 3.3v, not 5v, sorry) that powers the equipment.· No need for separately rectified and filtered DC for measuring.

And heed Mike's cautions -- you absolutely must isolate from the line, for safety.· Any transformer except an autotransformer will do that·perfectly.

· -- Carl, nn5i@arrl.net (mailto:nn5i@arrl.net)

01-20-2009, 08:44 PM
Hi Daniel,

A couple of suggestions...

Have a look at this circuit:

www.edn.com/article/CA6455602.html (http://www.edn.com/article/CA6455602.html)

The schematic is attached. Basically this uses an opto-isolator feeding an RC circuit on the output. The result is a pulse-width that is proportional to the AC voltage. Perhaps you can measure the output with a Propeller timer-counter. The circuit is completely isolated from the line.

Fairchild semiconductor has the MID400 AC line monitor chip. It comes in an 8-pin DIP package too.

www.fairchildsemi.com/pf/MI/MID400.html (http://www.fairchildsemi.com/pf/MI/MID400.html)

This part doesn't have an analog output, it is a logic output; but you can set the input threshold where the output is switched.

Enjoy... David