# Comparison of two ac signals of millivolt range and driving output

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HI everybody
I am having problem in comparing two ac signals and make a decision based on the status of these inputs
1st signal is of variable amplitude ,50hertz ,ranging from 5millivolt to 100millivolt
2nd signal is also from same supply(50hertz) and is a reference signal.it is of 5-10 millivolt(will be 5 millivolt or 10 millivolt but one value)

What is that decision that you want to make? If the 1st signal is present? If its amplitude is grater than the 2nd signal? Something else?

A good quality op amp with a gain of 20 or so driving a microcontrollers a to d ought to do the job.

I am having problem in comparing two ac signals and make a decision based on the status of these inputs
You really need to describe in some detail exactly what decision you want to make? Decision base on phase relationship, amplitude, or any number of other parameters.

Ron

Actually i want to detect whether there is a short circuit across two points or there is some load resistance connected between that point.When there is short circuit between that points,voltage drop to zero.but when there is some resistve load(like electric iron having very small resistance) is connetced between that point,only millivolts are present across that point.i want to differentiate between short circuit condition and very low ohmic reisistance load connected.This all topology is based on 230Vac supply(domestic ).Picture is uploaded for reference please.But this whole operation is after tripping of circuit breaker as shown in figure

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Actually i want to detect whether there is a short circuit across two points or there is some load resistance connected between that point.W

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Decision is just to drive a relay or transistor when there is some load connected

A good quality op amp with a gain of 20 or so driving a microcontrollers a to d ought to do the job.
Can you please guide with circuit?use as opamp?can opamp compare millivolts ac input signal?which one should i use?and in comparator mode or opamp mode?

You really need to describe in some detail exactly what decision you want to make? Decision base on phase relationship, amplitude, or any number of other parameters.

Ron
Decision is based just on amplitude of one signal greater than other ,output should go high to drive a relay or transistor.But signals are millivolts ac

What is that decision that you want to make? If the 1st signal is present? If its amplitude is grater than the 2nd signal? Something else?
yes exactly if one signal(miliivolts ac) greater than other output should drive relay or transistor

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With mains power involved I dont think an op amp is the way to go, I didnt know that.

1st signal is of variable amplitude ,50hertz ,ranging from 5millivolt to 100millivolt
2nd signal is also from same supply(50hertz) and is a reference signal.it is of 5-10 millivolt(will be 5 millivolt or 10 millivolt but one value)
I am not seeing that in your drawing? Additionally doing this with low voltage DC verse AC would be much simpler.

Ron

I am not seeing that in your drawing? Additionally doing this with low voltage DC verse AC would be much simpler.

Ron

I have attached drawing.As there is little difference betwen inputs so if we use to convert dc then voltage will be dropped by diodes and so oriignal signal will not be as it is? so difficult to differentiate ?
The variable voltage ac milivots are across load (which is electric iron 1000watt@230vac)

Secondly load is 1000watts@230volts so resistance is very low, i think around 0.085ohms i guess(electric iron nichrome wire element),but as it doesnot drop full voltage as voltage are dropped by neon (60-70vac) and 230k ohm resitor(in series with neon) so only millivolts are dropped across resistive load(electric iron or some other possible resitive load at domestic level with very low resistance).so when there is short circuit across point ab (load resistance),then almost zero volts are there.so either i compare voltage across this load resistor(point a b) with zero volt ac to differentiate between load and short circuit? but how to do?any other idea please sir
In figure below i have set reference to 8mv instead of zero just to account for wire resistance drop but if i use zero volt ac reference it would also be feasible but guidance required

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With mains power involved I dont think an op amp is the way to go, I didnt know that.
i am comparing millivolts ac?any other idea how can i check differentiate whther there is short circuit across point a b or there is low resistive load (like electric iron 1kilowatt having very low resistance across point ab?)

Secondly load is 1000watts@230volts so resistance is very low, i think around 0.085ohms
Wrong.

R = V x V/W =230 x 230 / 1000 = 52.9 Ohms.

JimB

any other idea how can i check differentiate whther there is short circuit across point a b or there is low resistive load (like electric iron 1kilowatt having very low resistance across point ab?)

The voltage in your first drawing will be 230 VAC across the load assuming the supply side of 230 VAC can provide the current. As you have the first drawing, that is what it is. I have no idea as to your circuit breaker rating but if there was a short between A and B the breaker will open assuming enough supply current and the voltage across the load will drop to zero. All I see is 230 VAC or Zero VAC across the load with the circuit drawn as it is. Additionally the load current will drop to zero. I have no idea where you are getting the milli-volt signals from?

Ron

He's got a neon lamp and a 230K resistor across the circuit breaker. This provides about 0.7mA of current to the load/short.

He's got a neon lamp and a 230K resistor across the circuit breaker. This provides about 0.7mA of current to the load/short.
That's right. I didn't think about that being there.

Ron

Wrong.

R = V x V/W =230 x 230 / 1000 = 52.9 Ohms.

JimB
Sir you are right for the case when load have 230v across it.but in my case there are only milivolts dropped across load (which is very low resistance like eelctric iron) so what i can proceed is directly measure resistance of electric iron which is very small(milliohms).Now if the circuit breaker trips due to shortcircuit then ,voltage across point a b are zero.Now at this case if i compare this with reference which i can set either to zero volts or some 8mv to account for wire resistance,then volt across load (point ab ) become either equal to zero (reference) .This condition i am working to solve is not frequent means its not like that there is short circuit at one instannt so one signla become greater than reference and other instant there is no short circuit.itis not like that when there is short circuit or there is some low ohmic reisistance connected,i just need to differentiate between this situation once the circuit breaker trips due to any reason(may be due to fault in breaker etc)

The voltage in your first drawing will be 230 VAC across the load assuming the supply side of 230 VAC can provide the current. As you have the first drawing, that is what it is. I have no idea as to your circuit breaker rating but if there was a short between A and B the breaker will open assuming enough supply current and the voltage across the load will drop to zero. All I see is 230 VAC or Zero VAC across the load with the circuit drawn as it is. Additionally the load current will drop to zero. I have no idea where you are getting the milli-volt signals from?

Ron
Sir, there are only millivolts across load after the circuit breaker trips .This is actually permanent fault indicator concept.Normally we have devices almost all devices that trip either on instaneous value or if some delay invloved,once Circuit breaker tripper,we get indication about status of relay or circuit breaker dry contact but user dont know if there is still shortcircuit or there was unwanted tripping may be due to fault in breaker or may due to some temporary overcurrent (due to say heavy loading for some time)

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