# 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 '19 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 '19 at 18:31
• Consider behavior with a 1kV transient on the mains. – Spehro Pefhany Mar 24 '19 at 19:27

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

No you did not consider your worst case CTR.

Look at the datasheet for your opto-coupler TCMT1109

The CTR varies with I(f) Your CTR at very low I(f) will plummet to only 0.1 - 0.2 of the expected value.

I work on frequency meter and i make board for sampling, those 4 images all from my work. in this case i read signal from line with PIC18 microcontrol . You most use capacitor for charging energy and use this at . by this way you can use low power resistor and fast zero crossing detection (for good accuracy, use falling edge) also with change R1 R2 R3 use more input voltag.

I offer this Practical circuit for best answer use gap under opto-coupler 