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Need help to understand how circuit functions__please!

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mischa

New Member
Hey there guys. I need some help to understand the circuit attached and how it functions. This circuit is a zero-crossing detector and its output is connected to a microcontroller pin (the pin is an input). It takes a 230V ac signal and I assume somehow gives a 5V or a 0v signal when there is a zero crossing or no zero crossing. Can someone please explain how this works and what the output of the circuit will be. Thank you so very much. I am really confused and need some help. The zener diodes used are 5.1V diodes.

zero-crossing detector circuit!!!.jpg

mischa
 

Sceadwian

Banned
That seems like an awfully complicated circuit for a zero crossing detector to a micro controller. All that's needed to allow a micro controller to sense zero crossing from mains is a resistor and two clamp diodes, perhaps a small bypass capacitor for transients.
 
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mischa

New Member
Hey sceadwian. How would you go about the zero crossing detector? Please help me. I have been working on this one problem for weeks and i feel like i am banging my head against a brick wall. I am using the pic16f877 and ccs ide and compiler. Please help:(
 

mneary

New Member
The circuit doesn't work. The 1k base resistor on Q3 will only develop about 27 millivolts when Q1 or Q2 tries to go high.
 

kchriste

New Member
Forum Supporter
Use an optoisolator if the PIC needs to be isolated from the AC line which it should be in all but the most rare situations.
 

mbarazeen

Member
it wont work, since Q3 is fed via D4 & D5 and both would be complimentarily biasing the transistor, thus Q3 alway would remain "on".

look for some other zero crossing circuits
 

Gayan Soyza

Active Member
I'm with Sceadwian

I was detecting ZX using 1 Mohm approach over the years.

I'm feeding hot wire to the PIC input via the 1Mohm directly.With the help of the inbuilt clamp diodes PIC sees the logic levels nicely.For resistive loads it worked fine for me.For transients & glitches I was fine tuning the code.

See this thread.
https://www.electro-tech-online.com/threads/phase-controlled-circuit.40863/
 
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mischa

New Member
Hey gayan soyza. i am assuming you would use the rbo pin as an external interrupt. I would ideally like to use this but i really have no idea how to go about coding it. Can you help me. I am using the pic16f877 and the ccs compiler and ide
 

Rajppd

New Member
Hey there guys. I need some help to understand the circuit attached and how it functions. This circuit is a zero-crossing detector and its output is connected to a microcontroller pin (the pin is an input). It takes a 230V ac signal and I assume somehow gives a 5V or a 0v signal when there is a zero crossing or no zero crossing. Can someone please explain how this works and what the output of the circuit will be. Thank you so very much. I am really confused and need some help. The zener diodes used are 5.1V diodes.

View attachment 35187

mischa
I would suggest, you can try to probe with an oscilloscope.
 

MrAl

Well-Known Member
Most Helpful Member
Hi,


That circuit, if it was working with the right values chosen, works like this...
The functions of the three transistors from left to right are:
1. sense/input buffer
2. two phase generator
3. output buffer

It is kinda complicated and that might be because they are trying to use only
transistors rather than ic chips to generate a two phase zero cross signal.

Many zero cross applications can use a single phase zero cross detector
so you should probably consider that first. The deciding factor is just
what kind of circuit you intend to use your zero cross detector with.

For example, you might be able to get away with two diodes and one
resistor (a total of three parts) or two diodes, two resistors, and one
transistor. It all depends on your application circuit so perhaps you should
let us know what you are going to use this for.
 

mischa

New Member
Hey gayan soza. I basically want to output a 120khz pwm output everytime there is a zero crossing. This will eventually control a motor
 

mneary

New Member
120khz is going to be really difficult. If the input is 60hz you need a 2000x frequency multiplier. Probably best to set a 4046 PLL on the 60hz before the zero cross detector.
 
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