# Zero cross detection in two wire dimming circuit

#### bobdrone

Joined Jan 15, 2021
6
230v, 50Hz, two wires dimming circuit. Zero cross and phase angle are fine, but when triac is fired, due to missing neutral, and due to mains sine wave chopped by the phase control, then zero crossed is messed and the bulb goes disco blinking. If I power the circuit from separate mains source with neutral, then dimming is just fine. Any ideas how to zero cross detect in a two wire dimming circuit when sine wave is chopped by the triac? Thanks!

#### Dodgydave

Joined Jun 22, 2012
9,951
Not sure what you're on about "two wire" , can you post your circuit diagram then we can assist you.

#### bobdrone

Joined Jan 15, 2021
6
Not sure what you're on about "two wire" , can you post your circuit diagram then we can assist you.
Two wire means the neutral is not present at the wall junction box but it is on the ceiling directly to the bulb. Then the dimmer has only Live and Load wires - 2 of them. The dimmer works in series with the bulb.

#### DickCappels

Joined Aug 21, 2008
7,432
If you post your circuit we will probably understand what you need to do to solve the problem.

#### bobdrone

Joined Jan 15, 2021
6
If you post your circuit we will probably understand what you need to do to solve the problem.
I am trying to find out how to zero cross detect without having the neutral line present in the dimmer, because when bulb is 50% dimmed by the triac in that same circuit, then the sine wave ac mains has zero voltage during the 50% of the time. I am away from pc, on the phone, hard to go now for the schematics. It is also a conceptual question.

#### bobdrone

Joined Jan 15, 2021
6
How do you want to control phase angle?
See the circuits here. They have just two wires going to the dimmer circuit.
https://www.eleccircuit.com/ac-lights-dimmer-circuit/
For example I tried with NodeMCU and with ic delay, it works both ways just fine, but when I connect the triac driver MOC3023 to the control pulse then the sine wave changes for the entire circuit (no neutral) and so the zero crossing changes to a point things mess up. As you all know how a 50% dimmed by triac bulb mains sine wave looks like, how then I could detect zero cross reliable from that..

#### DickCappels

Joined Aug 21, 2008
7,432
In the case of dimming an incandescent lamp there will be voltage across the triac when the triac is off -that's when you can see the first zero crossing, and when the second zero crossing occurs the load current through the triac drops to zero. To make use of this information you need to monitor both the voltage across the tirac and the load current through the triac.

Where do you get power for that circuitry without a neutral?

Sounds like a lot of compromises and work. Is it worth it?

#### bobdrone

Joined Jan 15, 2021
6
In the case of dimming an incandescent lamp there will be voltage across the triac when the triac is off -that's when you can see the first zero crossing, and when the second zero crossing occurs the load current through the triac drops to zero. To make use of this information you need to monitor both the voltage across the tirac and the load current through the triac.

Where do you get power for that circuitry without a neutral?

Sounds like a lot of compromises and work. Is it worth it?
Now that is something, thanks! I will need some time to digest exactly what you said and how I would be able to really do it, e.g. monitor the load current across the triac.. hmm.
I get power from a small switching power supply 5v, 3W.
It is a challenge for me, as making dimmer with neutral is like a walk in the park, also in EU most of the wiring for the lighting in apartments is made without neutral in the walls.

But I was thinking... before the triac is fired, we have normal sine wave [ZX detected fine], then after triac is On, it will go off automatically on the next [ZX], - then at that moment I was expecting to have again normal sine wave for some uSec. Why then the H11A11 cannot detect it? Maybe I need to rework and make more precise the ZCD circuit. At the moment I have:
Live---MT1 --R15k 1/2W--R15k 1/2W ---->pin1 [H11AA1] pin2 <---- R15k 1/2W---R15k 1/2W-- MT2--bulb inc 70W--Neutral
GND --- pin4 [H11AA1] pin5 --+--10k --- 5V + -> high signal ZX --> transistor switch >> 5V low pulse

Strange is that the voltage of the high pulse from H11A11 is only 500mV - not going to 5V.

#### bobdrone

Joined Jan 15, 2021
6
CH 1- only got 500mV pulse from H11A11, CH 2- normal low pulse after the transistor inverts it
ZCD time is approx 1msec - is it too much or less? I also cannot explain why only 500 mV are out.

#### Dodgydave

Joined Jun 22, 2012
9,951
Now that is something, thanks! I will need some time to digest exactly what you said and how I would be able to really do it, e.g. monitor the load current across the triac.. hmm.
I get power from a small switching power supply 5v, 3W.
It is a challenge for me, as making dimmer with neutral is like a walk in the park, also in EU most of the wiring for the lighting in apartments is made without neutral in the walls.

But I was thinking... before the triac is fired, we have normal sine wave [ZX detected fine], then after triac is On, it will go off automatically on the next [ZX], - then at that moment I was expecting to have again normal sine wave for some uSec. Why then the H11A11 cannot detect it? Maybe I need to rework and make more precise the ZCD circuit. At the moment I have:
Live---MT1 --R15k 1/2W--R15k 1/2W ---->pin1 [H11AA1] pin2 <---- R15k 1/2W---R15k 1/2W-- MT2--bulb inc 70W--Neutral
GND --- pin4 [H11AA1] pin5 --+--10k --- 5V + -> high signal ZX --> transistor switch >> 5V low pulse

Strange is that the voltage of the high pulse from H11A11 is only 500mV - not going to 5V.
Just draw the circuit diagram...