zero Crossing circuit with a minimum of external components

[azadfalah]

hi friends

In a project I need to detect zero Crossing in main input
* It is better to do this with the least parts
* To measure the main voltage, the ground and Main neutral are connected in some way ( So the circuit can not be isolated )
*No part of the board is not in contact with the user's body and there is at least 5 mm of plastic cover ( Do not focus on isolation )

I have seen the same circuit before on another device that has worked without problems for years but I do not feel good about it
If I use this circuit. What are the significant risks?

Thanks a lot

 

[FvM]

Presumed you are building the circuit with safe insulation, e.g. regarding clearance/creepance distances and resistor voltage rating. Circuit function will depend on relation of circuit GND to mains. How is the circuit power supplied? Are circuit parts exposed to the operator? If the circuit is plug connected, mains hot and neutral can be interchanged.

 

[KlausST]

Hi,

* It is better to do this with the least parts

Yes, nobody looks for a circuit with as many as possible parts. Every body looks for low part count.

But often this is not possible, because there are requirements.
You are the developer, thus you need to know the requirements.
Requiremnts are:
* safety. Maybe isolate, maybe not
* timing. The more precise the timing you need, the more complicated the circuit.
* clean levels. If you can live with noisy, slow rising, slow falling, ringing bad voltage levels, then the circuit may become simple
* protection. Against ESD, against other overvoltage or wrong wiring.... Often the microcontroller manufacturer specifies, that the internal protection diodes must not be used for "normal operation" - like in your circuit above.
* reliable operation. The more reliable, the more complicated. Your circuit above is likely to work when the microcontroller is battery powered, maybe even when you use a low frequency mains transformer based power supply. But maybe it does not work with a switch mode power supply, when there is high frequency coupled form primary to secondary side.

So the circuit above may work or not. It may (over long time) kill the microcontroller or not.
It may work with the one power supply but not with the other. It may work with one microcontroller (even if same partnumber) but not with the other, it may work on low humidity but not on high humidity. It may work when the device is on a table, but not when one holds it in the hands.

"Low part count design" often is "bad design".
You are free to do what you want.

Klaus

 

[azadfalah]

Thanks for your reply friends

How is the circuit power supplied?
Are circuit parts exposed to the operator?
If the circuit is plug connected, mains hot and neutral can be interchanged.

Through a 24 volt DC power supply
Not . The parts are inside the box
i add 200Kohm Series Resistor with hot line and neutral and change PD2 pin Resistor to 2Mohm so there is no problem if the inputs change

Yes, nobody looks for a circuit with as many as possible parts. Every body looks for low part count.

But often this is not possible, because there are requirements.
You are the developer, thus you need to know the requirements.
Requiremnts are:
* safety. Maybe isolate, maybe not
* timing. The more precise the timing you need, the more complicated the circuit.
* clean levels. If you can live with noisy, slow rising, slow falling, ringing bad voltage levels, then the circuit may become simple
* protection. Against ESD, against other overvoltage or wrong wiring.... Often the microcontroller manufacturer specifies, that the internal protection diodes must not be used for "normal operation" - like in your circuit above.
* reliable operation. The more reliable, the more complicated. Your circuit above is likely to work when the microcontroller is battery powered, maybe even when you use a low frequency mains transformer based power supply. But maybe it does not work with a switch mode power supply, when there is high frequency coupled form primary to secondary side.

So the circuit above may work or not. It may (over long time) kill the microcontroller or not.
It may work with the one power supply but not with the other. It may work with one microcontroller (even if same partnumber) but not with the other, it may work on low humidity but not on high humidity. It may work when the device is on a table, but not when one holds it in the hands.

"Low part count design" often is "bad design".
You are free to do what you want.

Klaus

Thank you for your answer that's why I don't feel good about this circuit , But this circuit is seen in a device with CE standard
Is the schematic below better?

 

[KlausST]

Hi,

important question: is there any possibility that HOT and NEUTRAL may be swapped?
If yes: Ask yourself: Does this cause any safety problem? Calculate possible currents and voltages.

Your circuit has only mains connection? no other connetion to outside world? (USB, RS232, display, battery, any metal item, pushbuttons, potentiometer, LEDs, programming connector..)

Your circuit:
Please read datasheet of microcontroller about "absolute maximum input voltage" at the according pin.
I doubt a 1N4148 can satisfy this. (read 1N4148 datasheet)
In doubt I recommend to use schottky diodes instead. Like BAT54S.

I don´t recommend 1N4007 in sereis with a 4V3 zener on a 5V supply. This is just on the edge. Low supply voltage makes the node float in a limited range. Too high input voltage may cause the zener to burn. Do worst case calculations (highest power supply voltage, lowest 4007 forward voltage, lowest zener reverse voltage.. take care about tolerance, aging drift, temperature drift )

A 100R instead of the 1N4007 makes more sense...

Still any coupled noise may cause multiple ZC transitions. Be sure this causes no problems. Maybe you need to adjust software on this.

A switch mode power supply causes noise. Use a scope to get a clue about frequency and magnitude.
Don´t be surprised by more than 50V AC at secondary w.r.t. earth ground.

Klaus

 

[azadfalah]

Your circuit has only mains connection? no other connetion to outside world? (USB, RS232, display, battery, any metal item, pushbuttons, potentiometer, LEDs, programming connector..)

Hi and thank you for your answer
Yes, the LEDs are on the under of the plastic cover min 5mm plastic to LED lead
there is also a switch with plastic cover with a thickness of 10 mm

do you think this schematic is better than the previous method?

 

[KlausST]

Hi,

Now you are at 10 parts. Still suffers from problems when noise is introduced. No differential signaling, no filters, no clean switching.
There is a good chance that it will work.
But still not what I call a good solution. But I always want high quality.
No need for you do the same.

May I ask why you don't use clean differential signaling and feed the microcontrollers internal analog comparator with it?

Klaus

 

[danadakk]

Zero Crossing Detection Circuits Examples

Zero Crossing Detection Circuits Examples using op amp, optocoupler with simulation results, ZCD types, where to use and Applications

microcontrollerslab.com

How to Make a Zero Crossing Detector Circuit - Homemade Circuit Projects

Making a zero crossing detector circuit is actually very easy and it could be effectively applied for protecting sensitive electronic equipment against mains switch ON surges. A zero crossing detector […]

www.homemade-circuits.com

Zero Crossing Detectors

ESP Application Notes - Zero Crossing Detectors

sound-au.com

MOC3041 Zero Crossing Optocoupler Pinout and Examples

MOC3041 is Zero Crossing Optocoupler, tutorial contains Pinout, Examples, Applications, DataSheet, Features. MOC3042, MOC3043

microcontrollerslab.com

Zero-Crossing Detectors Circuits and Applications

How to use opto-couplers like the H11AA1 to build zero-crossing detector circuits. Includes circuit diagrams.

www.bristolwatch.com

Zero crossing detector using 4N35 optocoupler

Hi, I am making a zero crossing detector using 4N35 optocoupler. I want to use it in the Electronic load controller (ELC) to dump excess power to the heater.

www.avrfreaks.net

Some possibilities....


Regards, Dana.