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[moved] Problem with full bridge rectifier!

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electronicsIUST

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Hello, I design below circuit (first figure) for measuring unbalance in three phase system! but there is a problem that I can't understand! when input to bridge (V(N0-V(N)) is like second figure I have a waveform like third figure in the output of bridge (V(1)-V(2))! It is half wave rather than full wave and also the amplitude is about 10 times to that of input (the probe in figure 3 is 10x) I think the waveform that I see in oscope is wrong. Do anyone know what is wrong!!?!
12.png13.png14.png
 

Hi,

the problem is the voltage relationship.

The input is floating related to your GND (symbol) somehow.

Is it simulation only, or real world?
What about safety isolation. Are you aware that the output may carry dangerous voltages and touching anything of your circuit may be dangerous?

Klaus
 
Hi,

the problem is the voltage relationship.

The input is floating related to your GND (symbol) somehow.

Is it simulation only, or real world?
What about safety isolation. Are you aware that the output may carry dangerous voltages and touching anything of your circuit may be dangerous?

Klaus
thanks for ur reply,
I actually use this circuit because of isolation that exist between ground and null! These are real waveforms that I have obtained by oscilloscope!! Isn't there isolation in this circuit?!
 

Hi,

There is no isolation. Where do you see it?

On the other side the two circuits on either side of the rectifier have no true relationship.
* There are times where no diode is conductive..
* and any induced current between your circuit_GND and the mains side will cause a voltage drop in your R5 (which is awfully high impedance)
--> and there will be induced currents! either by stray capacitance or by inductance.
Every microampere of induced current will cause 1V of voltage in your R5.

If we assume there are only clean 50/60 Hz signals without overtones (which is not realistic) then a capacitance of just 14pF causes a current of 1uA.
14pF... two wires in close proximity (some mm) with a length of a couple of 10cm will have 14pF to each other.

****
Your circuit is very sensitive to any introduced errors.
Therefore I recommend to redesgin the circuit.

What are your specifications?
Are you interested in AC signals only? --> then I recommend to use transformers. Three identical. Primary in star connection. All secondary in a string. Isolated, low impedance, immune against induced errors.

If you are interested in very low signals you should consider a precision rectifier.
Or feed the AC to the ADC. and process the signals with a microcontroller.

Klaus
 
Hi,

There is no isolation. Where do you see it?

On the other side the two circuits on either side of the rectifier have no true relationship.
* There are times where no diode is conductive..
* and any induced current between your circuit_GND and the mains side will cause a voltage drop in your R5 (which is awfully high impedance)
--> and there will be induced currents! either by stray capacitance or by inductance.
Every microampere of induced current will cause 1V of voltage in your R5.

If we assume there are only clean 50/60 Hz signals without overtones (which is not realistic) then a capacitance of just 14pF causes a current of 1uA.
14pF... two wires in close proximity (some mm) with a length of a couple of 10cm will have 14pF to each other.

****
Your circuit is very sensitive to any introduced errors.
Therefore I recommend to redesgin the circuit.

What are your specifications?
Are you interested in AC signals only? --> then I recommend to use transformers. Three identical. Primary in star connection. All secondary in a string. Isolated, low impedance, immune against induced errors.

If you are interested in very low signals you should consider a precision rectifier.
Or feed the AC to the ADC. and process the signals with a microcontroller.

Klaus

I saw this circuit was used for measuring floating voltages! When I give a phase and null to input of bridge it works correctly! I want to measure unbalance in three phase and so I should measure voltage difference between null and point N0 and in normal condition that all three phase are almost equal it should be zero! another way is to measure each one of phases. I don't want to use transformer because of taking space! Do you have any comment without transformer!?! thanks a lot.
 

Hi,

* Your assumtion that the circuit is isolating...
* Not answering my question if you are aware of the danger..
* not giving electrical specifications...
* the use of a scope on a non_isolated circuit...
... makes me think you are not experienced enough for that dangerous circuits. Therefore I don´t recommed you an non isolated circuit.

--> don´t hurt or kill yourself or others.

Klaus
 

Hi,

* Your assumtion that the circuit is isolating...
* Not answering my question if you are aware of the danger..
* not giving electrical specifications...
* the use of a scope on a non_isolated circuit...
... makes me think you are not experienced enough for that dangerous circuits. Therefore I don´t recommed you an non isolated circuit.

--> don´t hurt or kill yourself or others.

Klaus

Hi, thank you!
I know that just after 1Meg resistor (R10 and R11) is almost safe because of voltage drop and I am aware of danger in working with three phase non-isolated system and I carefully work! My three phase system is 220 V/50Hz/Y configuration.
Was it wrong to use scope in non-isolate circuit?!
 

Hi,

Was it wrong to use scope in non-isolate circuit?!
It depends.
It depends on your scope , (If measurement is referenced to earth GND or not. If isolated then are you within the allowed floating voltage range)
and it depends on your 220V: If the star point connected with earth gnd

***
Safety is not only using a high value resistor.
It additionally needs proper resistor type selection, creepage distances, clearance, tests with high voltage...


***
Circuit:
"You saw this circuit" if it is not modified, then the source is not very reliable.
If you modified it yourself you probably didn´t take care about common mode voltages of OPAMP...

Klaus
 

Connect a high value resistor//capacitor between N node and GND.You generators have no GND reference and therefore they are floating.Since the consumed currents are very low, you can connect a -for instance- 100K Ohm//1uF (resistor//capacitor) circuit that will connect the N node to GND in AC signals.
 

Hi,


It depends.
It depends on your scope , (If measurement is referenced to earth GND or not. If isolated then are you within the allowed floating voltage range)
and it depends on your 220V: If the star point connected with earth gnd

***
Safety is not only using a high value resistor.
It additionally needs proper resistor type selection, creepage distances, clearance, tests with high voltage...


***
Circuit:
"You saw this circuit" if it is not modified, then the source is not very reliable.
If you modified it yourself you probably didn´t take care about common mode voltages of OPAMP...

Klaus

thanks,
But this circuit is simply a differential amplifier!! I just added the bridge and R5. The simulation results is completely different from the results I have obtained.
 

Connect a high value resistor//capacitor between N node and GND.You generators have no GND reference and therefore they are floating.Since the consumed currents are very low, you can connect a -for instance- 100K Ohm//1uF (resistor//capacitor) circuit that will connect the N node to GND in AC signals.

But why when the input to bridge is one phase and null everything is ok and input and output of bridge are displayed correctly!
 

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