# Zero Crossing Signal Timing

I have a zero crossing circuit for dimming lamps exactly like the below

According to my calculations:

$$\ I_{R7} = \frac {V}{R} = \frac {3.3}{10k} = 0.33~mA\$$.

Considering TCMT1109 transfer ratio equal 200% (worst case), when the input current get lower than $$\0.165~mA\$$ the ZC signal should start rising.

$$\ V = {R}*{I} = {200k}*{0.165~mA} = 33~V\$$.

Adding the optocoupler and the bridge voltage drop, let's assume $$\35~V\$$ the minimum input voltage to detect the zero crossing.

The mains voltage is $$\220~Vac, V_{peak}=311,12V\$$, and the frequency is $$\60~hz\$$.

$$\ V(t) = V_{peak}*sin(wt) \$$

$$\ 35 = 311,12*sin(2*pi*60*t) \$$

$$\ t = 0,299 ~ms \$$

After calculations, I was expecting the ZC signal start rising at least at 0.299 ms, but after measure at oscilloscope I got the ZC start rising around 0.450 ms.

I know this is a very low time, and I can manage that at the microcontroller firmware. I'm just worried because I can't get the 0.450ms (50% higher than 0.299ms) at any calculations I make.

Where am I missing at calculations? I considered the lower CTR for worst case.

Where is my analysis wrong?

• Any particular reason you chose that side of the bridge to put the resistors? – Spehro Pefhany Mar 24 at 0:58
• @SpehroPefhany I chose the resistors after bridge because of my pcb layout. If you tell me is "very very much better" before the bridge I can study in my pcb if that would be possible. – BrunoAraujo Mar 24 at 18:31
• Consider behavior with a 1kV transient on the mains. – Spehro Pefhany Mar 24 at 19:27