View Full Version : AC Voltage monitor
01-20-2009, 02:40 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
"Just plug it in and let's see what happens"
01-20-2009, 03:09 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.
01-20-2009, 03:37 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, email@example.com (mailto:firstname.lastname@example.org)
01-20-2009, 07:44 PM
A couple of suggestions...
Have a look at this circuit:
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.
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.