zero crossing using resisters and opto coupler

Thread Starter

Nanda Kumar 1

Joined Aug 25, 2016
40
Hi to all .I want to detect a zero with simple circuit , resisters are connected to live and neutral and that resisters are connected to bridge diodes .I am facing problem is when I am checking the bridge out put with cro its showing the half wave rectifier out put .Bellow i attached my circuit with out optocoupler . Please suggest me any one for getting correct wave forms from the bridge rectifier .
 

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RichardO

Joined May 4, 2013
2,271
Here in the US the electrical wiring has neutral connected to safety ground. The scope has its ground connected to safety ground. The result is that when the scope is connected to your circuit the neutral is shorted to the ground side of the scope. If the bottom connection of you scope is ground then diode, D3, and the bottom 560K resistor are shorted across. I think this will result in the waveform you are seeing.
 

crutschow

Joined Mar 14, 2008
25,633
You can't proprtly view the output (and it's very dangerous to do so :eek:) of a bridge rectifier connected to mains voltage with an oscilloscope.
It's a good way to electrocute yourself and/or blow up your oscilloscope.
Please do not try that again.
Connect the opto coupler to the bridge output with the output properly biased with a voltage and a resistor, and observe the opto output waveform.
And for safety, have the circuit plugged into an outlet with a Ground Fault Interrupter (GFI).
 

AlbertHall

Joined Jun 4, 2014
10,536
That's because you are using a mains powered 'scope and so the ground lead from the scope is grounded and shorting out the negative pin of the bridge. If you leave the 'scope ground disconnected and connect the probe to each of the bridge outputs you will find two half-wave rectified waveforms, one positive going and one negative going which will indicate the bridge is working correctly.

Mod edit: moved here from duplicate post
 
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Dodgydave

Joined Jun 22, 2012
9,637
Your scope is shorting out the Negative output with the Earth lead, use an Isolation transformer and you will get a full wave signal.
 

Thread Starter

Nanda Kumar 1

Joined Aug 25, 2016
40
Your scope is shorting out the Negative output with the Earth lead, use an Isolation transformer and you will get a full wave signal.
Hello Dodgydave thankyou for your reply my requirement is without transformer i need to do..can you suggest any other options
 

Thread Starter

Nanda Kumar 1

Joined Aug 25, 2016
40
C post: 1092768 said:
Here in the US the electrical wiring has neutral connected to safety ground. The scope has its ground connected to safety ground. The result is that when the scope is connected to your circuit the neutral is shorted to the ground side of the scope. If the bottom connection of you scope is ground then diode, D3, and the bottom 560K resistor are shorted across. I think this will result in the waveform you are seeing.
hello Dodgydave.I am not providing any short in that circuit dc ground and neutral are isolate each other. suggest me some other circuit with resisters
 

Dodgydave

Joined Jun 22, 2012
9,637
Well unless you use an Isolation transformer, you cant wiew the waveform with a mains scope, unless you remove the earth, which is Dangerous! So use a battery scope.
 

JohnInTX

Joined Jun 26, 2012
4,338
You can make a simple zero crossing detector with a single opto that has back to back LED inputs like a PS2505-1 or H11AA1. Size the input resistors to saturate the photo-transistor output early in the cycle. The photo-transistor will be ON except when the input is very close to zero. With a suitable collector resistor, you will get a short, positive going spike at each zero cross. It is not an exact zero crossing detector but enough for a light dimmer.
ZVS.jpg

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