Zero crossing detector inconsistency

Question

I designed the following zero crossing detector using an FOD819S optocoupler (U5):

The source of the optocoupler is connected to a MB6F full-bridge rectifier (D2) which is supplied by a 9V transformer connected to 220V AC/50Hz. The 3.3V regulator (U8) supplies power to an ESP32 microcontroller and a segment display (not shown in schematic).

I probed the circuit in T1 (in blue, relative to GND) and it works as expected most of the time. For reference, I connected another probe to T2 (in yellow) before the full-bridge rectifier.

However, when cutting and re-applying AC to the transformer, it sometimes shows some odd behavior:

Or it simply stays at 0V:

Another issue is that the detected frequency is ~10% above 100Hz (not stable) which is also what I detect from a microcontroller using an interrupt on falling edge. It might be a consequence of the slow rise time.

What is wrong with this circuit? Should I add a constant load before the regulator?

Solution

Here is the signal when using the first circuit from Dave Tweed (with a pull-down resistor):

Answer 1

If you want to monitor the output of the bridge rectifier, put your optoisolator in parallel with the power supply, not in series with it. Note that this requires an extra blocking diode (D6) in order to prevent the power supply input capacitor from driving the optoisolator LED.

^{simulate this circuit – Schematic created using CircuitLab}

Better still, put the optoisolator upstream of the bridge rectifier. Then every rising and falling edge at its output represents a zero crossing of the AC voltage.

^{simulate this circuit}

Answer 2

First, I wouldn't call that a "zero crossing detector" per se, although in normal operation it does seem to be enough to measure line frequency.

In startup, the current into C1 will be much higher, and for a longer period of time. That's part of the reason you're seeing shorter high-going pulses out of the thing. I suspect that the rest of the reason is that in that period of time you're pushing way more than the 1.5mA that the thing is rated for through the diode, and saturating the transistor beyond what the manufacturer rates the device for.

With your circuit as given, I would be concerned about exceeding the LED's maximum-rated 50mA during startup. If you're absolutely married to using that circuit, you may want to consider a resistor in series with the coupler.

As for the higher-than-expected measured frequency -- you are probably correct that you are misreading the output. If your microcontroller has a way to implement filtering on its input pins (a few do) then do so. If not, then you need a Schmitt trigger on that signal before it goes into the microprocessor.