I am trying to build a main voltage monitor using a UL listed ac ac converter (step down to 10vac rms) to take my measurements. To test the output, I've hooked the converter up to a dimmer switch which causes a 6 volt drop in voltage, which should be enough to test the proto.
My first question is whether the cap is electrolytic. It looks like it should be (one side is dark, however there is no +), but I want to make sure of this bc I've read that electrolytic caps can get damaged by ac.
My second problem is that I cannot read a voltage from the divided voltage to ground on a multimeter. I've tried to read it between the neutral, the house ground, and even the arduino ground. I don't yet have the cap connected, but I don't think this should be the problem.
You must be able to read a voltage - perhaps you meant you see 0.0V between the output and
Arduino ground (you shouldn't be touching neutral, that's a mains voltage wire).
Here the divider is to Arduino ground, so Arduino ground is the reference.
Measure all the voltages on the secondary side - with DC range you should see 2.5V at
the midpoint, and on the Arduino analog pin and on both secondary pins.
On AC range you should see expected voltages across secondary and output to the Arduino input.
Note that since multimeters have a finite input resistance you will read voltages a little low
(typical DMMs are 10M ohm input so its a small effect only - old AVO's are 50kohm per volt
though, so would show significant error).
Just to note, I am not actually using an arduino at this point. I am using a throwaway 5vdc rms (6.8v actual) power supply until I get the hardware worked out.
I've tried to measure the 'arduino input' against 'arduino ground', and I get 0.0v on both ac and dc measurements. I haven't yet built the second voltage divider to raise the voltage 2.5v.
Since I can't get this reading, I've tried to read the voltage after the first divider against building ground and got 0.0v as well. I even tried dc as well just to be sure.
What is odd is that I've tried to measure the voltage after the convertor but BEFORE the voltage divider against ground and it also shows up as 0.0v.
I wonder if the capacitor is necessary here. I would like to install an electrolytic cap but don't want to blow it up if it shouldn't go with ac.
I haven't yet built the second voltage divider to raise the voltage 2.5v.
Well you won't see any DC voltage then.
Electrolytic is correct, it reduces noise on the mid-rail, although arguably you need a cap to
each rail. If the voltages are such as to reverse volt the cap, your Arduino is already toast!
Have you verified the AC output of the secondary yet?
A dimmer switch doesn't keep the output voltage as a sine wave but chops it so it is not really suitable to test that circuit. Best solution is to use a variac but a quick check would be to try the circuit with a different voltage transformer.
If you wish to build a voltage monitor (rather than a simple AC detector) you need to ditch the circuit you propose since the signal reference is entirely dependant on the performance of the electrolytic capacitor - which will change with time and temperature.
You might be better off obtaining a transformer outputting a suitable voltage (or rigging up a simple voltage divider on the AC side) to produce the level of voltage required.
Then feed the AC through a precision rectifier circuit (https://www.site.uottawa.ca/~rhabash/ELG4135L8.pdf)
Is there any way to use this with a UL wall wart that knocks the voltage down below 30v? The reason is that I want to install this in a customer's house and I don't need my own UL if I can get the voltage below 30v with a UL Listed device.
The purpose of the capacitor is to hold the voltage across resistor R4 at half of the (+5V) supply voltage.
There will be 2.5V DC across it, and no AC voltage.
Are you sure that you have got an AC to AC adapter (which are relatively uncommon), and not an AC to DC adapter, which are more widely available?
