# Would this circuit not fry the op-amp?

Found a circuit and thought it kind of strange that noting was done to reduce the line voltage other than running it through a couple of resistors before feeding it to the op-amp.

Once you know the voltage you can calculate the current that can pass through four 1.2 Meg resistors in series.

230V AC

thought it kind of strange that noting was done to reduce the line voltage other than running it through a couple of resistors

Four 1.2 Megohm resistors, to be somewhat more accurate.

And the current flow through those resistors is...?

8.6* e^-5. Very little. Thank you for clarifying!

Re: Would this circuit not fry the op-amp?

That's a good question but it's not the ONLY question... Another question is, will it fry YOU, or the Arduino, or computer if connected to USB?

It's DANGEROUS to have ANY direct connection to the power line unless the circuit is entirely enclosed and insulated with no other connections coming out. For example, a digital multimeter has all of the electronics inside a plastic case, except the probes so it's reasonably safe. If a multimeter has a USB port, the port is optically isolated from the measurement circuitry. And if the multimeter is AC powered, of course the AC power is transformer isolated.

So there are products built that way, but during design, development, testing, and debugging, you'll need access to the circuitry so you have to be careful.

The general problem with voltage dividers across the AC line is that if the hot & neutral are reversed (anywhere in the wiring or an extension cord, etc.) you can get a low voltage output that's referenced to the hot wire but it still has lethal voltage relative to neutral and earth ground. So the op-amp sees low voltage as long as there is no earth ground, but if you touch the op-amp circuit and earth ground you could be killed. Or bad things can happen to your computer if USB is connected, etc.

RippoZero:
Found a circuit and thought it kind of strange that noting was done to reduce the line voltage other than running it through a couple of resistors before feeding it to the op-amp.

Interesting circuit.

I see a number voltage dividers there: this includes C1, C2 and C3 (a capacitor is like a resistor to AC - the reactance of the first two is about 1.5MΩ at 50 Hz, C3 is about 32kΩ). It's a bit complex of course as you're mixing AC and DC circuits in one.

The AC circuit (from V1): current flows through R1 - R2 - C2 - C1 - R5 - R4. So you have a voltage divider here, dividing it in three parts: R1+ R2, C2 + C1 and R5 + R4.

Now parallel to C1 is the (much lower resistance) path R6-R8-R9-R10 (about 44k total), and parallel to C2 the path R3-R7-C3 (about 56k total) for the AC to flow through.

So the voltage divider on the mains comes to 2M4 - 100k - 2M4, the 100k part between the positive and negative junction of the comparator, and giving 2.5V RMS (so 3.5V peak value).

The DC paths are of course quite different, as the capacitors don't pass DC.

Not sure of the function of C1 and C2.

I've seen this kind of thing before with multiple resistors used in series when you could just have one. The reason is that fault voltages can arc right over the resistor. Adding more resistors gives a longer distance and effectively higher resistance to the arc.

This is a common and relatively safe zero cross detector circuit, used in lamp dimmers, clocks, power line health monitors, etc.

Of course the high voltage components will be mounted on a PCB in isolated sections, and the PCB will be securely enclosed in a commercial product.