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.

enter image description here

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?

  • \$\begingroup\$ Any particular reason you chose that side of the bridge to put the resistors? \$\endgroup\$ – Spehro Pefhany Mar 24 at 0:58
  • \$\begingroup\$ @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. \$\endgroup\$ – BrunoAraujo Mar 24 at 18:31
  • \$\begingroup\$ Consider behavior with a 1kV transient on the mains. \$\endgroup\$ – Spehro Pefhany Mar 24 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)

enter image description here

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


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