arduinooperational-amplifiervoltage dividervoltmeter
I'm a Electrical Engineering student and I want to sense and sample the voltage signal coming from a wall socket (110V – 220V). I came up with the following circuit using a voltage divider with high impedance and a diferential amplifier. I would like to avoid using a transformer because of weight constraints.
Is there a better way to solve this problem?
Should I use a capacitor divider instead?
Let's call the top resistor of a voltage divider R1 and the bottom R2. The output impedance is R1 // R2, and the gain is R2/(R1+R2). Set these two equations equal to what you want. Now you have two equations and two unknowns. Solve accordingly.
Note that R1 encompasses the 50 Ω of the source, so the actual top resistor (what you add directly in series with the signal generator) will be 50 Ω less than what you solve for R1. At your very low gain, this will make little difference though.
You can make a voltage divider with any number of resistors, but you must remember that any current drawn from an output of a voltage divider must come through the resistors in that divider, so will affect the output voltage.
simulate this circuit – Schematic created using CircuitLab
In Fig. 1, we have a simple voltage divider. Since the resistors are equal value, the output voltage will be half the input voltage (10V in gives 5V out).
However, in Fig 2 we've added a load (R5) intended to draw 5 mA from the voltage divider. However, that load is in parallel with R4, so we effectively have a 1K resistor (R3) in series with a 500 ohm resistor (R4 and R5 in parallel), so we only get 3.33 volts for our load, rather than the desired 5 volts.
When designing a voltage divider you must always make allowance for any currrent drawn from a tap on the divider, usually by ensuring that the current drawn by the load is much less than the current through the divider chain.
Best Answer
Regular transformers don't have to be heavy.
Figure 1. Miniature transformers.
Hammond Manufacturing, for example, make 0.5 VA transformers smaller than a 25 mm / 1" cube. This provides isolation from mains (which your circuit does not).
simulate this circuit – Schematic created using CircuitLab
Figure 2. Analog interface.
To read AC with your micro you'll need to bias the transformer to mid-DC supply as shown. R1, R2 and C2 provide this function.
R3 and R4 provide a potential divider to attenuate the transformer signal into the range suitable for your ADC.