# How to amplify millivolts to drive a relay

Joined May 25, 2019
50
Hi everybody
I am working on a project permanent fault indicator at domestic level.
Diagrams for circuits are attached please
Objective is to indicate( neon bulb will glow) existence of short circuit at any time in circuit
Circuit operation
When there is a short circuit across load(point a b in picture) ,then circuit breakaer trips.now path of current is through neon bulb(connected in parallel with breaker) .So neon bulb glows and indicate there is a fault.
But when there is no short circuit and circuit breaker accidently trips(due to fault breaker) ,now neon bulb still gets ON as path is completed through neon bulb and load resistor(small value like electric iron having milli ohm resistance).So as there is no short circuit but we are getting indication that there is fault.I have to rectify this problem.
For this purpose i am using opamp (differential opamp) to sense voltage across load ressitor(point a b) and compare it with reference say O volts,when Vab>0 (no shortcircuit), output of opamp should go high and drive a relay or transistor to break path for neaon bulb throug a dry contct shown in figure.
but i am unable to get output from opamp to drive a relay.please advice me how to deal with above problem.Thanks

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#### MrChips

Joined Oct 2, 2009
22,505
Welcome to AAC!

You have two problems.

1) The neon lamp is always on, short or no short. Hence that circuit configuration will not work for your purposes.

2) Your opamp circuit to detect a short circuit will not work.

Sorry, at this point I have no solutions to offer for each of the two problems. These are not simple problems and will require some mental effort.

Joined May 25, 2019
50
Welcome to AAC!

You have two problems.

1) The neon lamp is always on, short or no short. Hence that circuit configuration will not work for your purposes.

2) Your opamp circuit to detect a short circuit will not work.

Sorry, at this point I have no solutions to offer for each of the two problems. These are not simple problems and will require some mental effort.
@MrChips If i am able to achive short circuit detection, then output of opamp will automatically cut off circuit for neon bulb. so need to know how can i achieve this.thanks

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,373
Use breakers with status contacts built-in.

Joined May 25, 2019
50
Use breakers with status contacts built-in.
circuit breakers give status about instantaneous trip status.once tripped user is not sure whther short circuit is still there or not

Joined May 25, 2019
50
Hi everybody,i am a bit a able to achive my objetctive at first stage
the circuit i uploaded has two stages
1 amplification stage
2 comparison stage

At comparison stage my purpose is to drive relay or led through output of transistor ,ALthough output switching of comparator is ok normal.But when i want to drive led or relay ,then either led is off in both cases or on in both cases,when i change reistor values for pullpup and base of transistor.please guide me how can i set values?and how to do this?

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Joined Mar 10, 2018
4,057

#### ebeowulf17

Joined Aug 12, 2014
3,275
Hi everybody,i am a bit a able to achive my objetctive at first stage
the circuit i uploaded has two stages
1 amplification stage
2 comparison stage

At comparison stage my purpose is to drive relay or led through output of transistor ,ALthough output switching of comparator is ok normal.But when i want to drive led or relay ,then either led is off in both cases or on in both cases,when i change reistor values for pullpup and base of transistor.please guide me how can i set values?and how to do this?
You mention a pull up resistor, but I don't see one in your schematic. Since the comparator has an open collector output, you'll definitely need a pull up to get anything done. Try adding a 10k resistor from 12V supply to comparator output.

Another problem I see is that you're exceeding the common mode input voltage range of the comparator. When feed from a single supply, the comparator absolute minimum input is -0.3V, but you're feeding it an AC signal with negative excursions much greater than that.

At the moment, I also don't see how any of this accomplishes short circuit detection or output latching, but I'm setting those concerns aside for the moment to address the more immediate issues I see.

Joined May 25, 2019
50
You mention a pull up resistor, but I don't see one in your schematic. Since the comparator has an open collector output, you'll definitely need a pull up to get anything done. Try adding a 10k resistor from 12V supply to comparator output.

Another problem I see is that you're exceeding the common mode input voltage range of the comparator. When feed from a single supply, the comparator absolute minimum input is -0.3V, but you're feeding it an AC signal with negative excursions much greater than that.

At the moment, I also don't see how any of this accomplishes short circuit detection or output latching, but I'm setting those concerns aside for the moment to address the more immediate issues I see.
Sorry sir i didnot have idea about this concept of common mode input range of comparator?please can you highlight this ?Secondly i have tried pullup also but result is same either led is on or off for both cases..Thirdly input resistor you see at leftmost supply is point where there is either short or some resisitve load(50 ohms for this case).

Joined May 25, 2019
50
Sorry sir i didnot have idea about this concept of common mode input range of comparator?please can you highlight this ?Secondly i have tried pullup also but result is same either led is on or off for both cases..Thirdly input resistor you see at leftmost supply is point where there is either short or some resisitve load(50 ohms for this case).
If i feed comparator from double supply then for this case also need to take account of input commonmode of comparator?

Joined May 25, 2019
50
I have tried to use pullup 1k ,but led is on in both cases means when input of comparator is high or low than reference(1.2Vac rms) then led is ON,whereas by concept,when input of comparator (output from opamp U2 at stage 1) is higher than refercne set(1.2Vac rms),comparator is off but how led is still ON?
Secondly when input (output of opamp U2 is less than set refernce),comparator should be ON(output shorted to ground) so only in this case LED should be ON,but for my case led is ON in both case..Any body please highlight problem and propose solution please

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#### MisterBill2

Joined Jan 23, 2018
7,700
It seems that the intention is to determine if the circuit breaker tripped because of a momentary short circuit fault or an actual short circuit failure, resulting in a maintained short circuit. If that is not the intention please let us know what the intention is.
Also, is the device actually a "Ground Fault Circuit Interrupter? (GFCI?) In that case all the answers will be much different.
For the circuit shown in the drawing the solution is simple: instead of using a neon bulb, use an incandescent light bulb of at least 25 watts rating. If the fault was an intermittent one the bulb would only flash briefly, while if the fault remains the light will stay on. Any connection between an op-amp and mains power, especially 230 volts , will need a whole lot of effort to be safe. My solution is very simple to implement with complete safety. If a relay is needed to provide a contact for signaling then adding a small 230 volt relay in parallel with the light will be what you need. It should be a relay with a high resistance coil, designed to work with higher voltages. When working with mains power there is no need to use sensitive electronics.

Last edited:

Joined May 25, 2019
50
It seems that the intention is to determine if the circuit breaker tripped because of a momentary short circuit fault or an actual short circuit failure, resulting in a maintained short circuit. If that is not the intention please let us know what the intention is.
Also, is the device actually a "Ground Fault Circuit Interrupter? (GFCI?) In that case all the answers will be much different.
For the circuit shown in the drawing the solution is simple: instead of using a neon bulb, use an incandescent light bulb of at least 25 watts rating. If the fault was an intermittent one the bulb would only flash briefly, while if the fault remains the light will stay on. Any connection between an op-amp and mains power, especially 230 volts , will need a whole lot of effort to be safe. My solution is very simple to implement with complete safety.
Yes project is Permanent fault indicator.Not GFCI.when breaker trips,trip may be due to fault in breaker,overload or permanent short circuit.so if it is permanent fault then neon will ON,but if there is electric iron(low resistance 50ohms) connected,neon still gets ON,i want to distinguish between this load(electric iron or short at that the point where load is connected).I am measuring millivolts and amplify them then compare with ac refercecne volts and drive a relay that will cut out supply to neon.
Sir,Secondly your suggestion regarding incandascent bulb is valuable however it is not feasable to use incandascent bulb across each circuitbreaker as every room has its own breaker in distribution ,also pwer consumption is a concern compared to neon as well as portablity

#### ebeowulf17

Joined Aug 12, 2014
3,275
If i feed comparator from double supply then for this case also need to take account of input commonmode of comparator?
A double supply would solve the input range problem.

#### ebeowulf17

Joined Aug 12, 2014
3,275
As for the remaining questions, I'll try to take a closer look this afternoon or evening. Right now I'm at work and can't spend time on it.

#### MisterBill2

Joined Jan 23, 2018
7,700
Yes project is Permanent fault indicator.Not GFCI.when breaker trips,trip may be due to fault in breaker,overload or permanent short circuit.so if it is permanent fault then neon will ON,but if there is electric iron(low resistance 50ohms) connected,neon still gets ON,i want to distinguish between this load(electric iron or short at that the point where load is connected).I am measuring millivolts and amplify them then compare with ac refercecne volts and drive a relay that will cut out supply to neon.
Sir,Secondly your suggestion regarding incandascent bulb is valuable however it is not feasable to use incandascent bulb across each circuitbreaker as every room has its own breaker in distribution ,also pwer consumption is a concern compared to neon as well as portablity
The incandescent bulbs will not use any power except when lighted, which would only happen when a circuit breaker had tripped. Yes, they would take up a fair amount of space. But now I am wondering why such a system would be required in your case when thousands of installations do not have such indicators. What is it that causes you to need such an unusual arrangement? Or is this system for a hotel or apartment building?
AND you are indicating that the neon light connected across the circuit breaker will be on even if the load is an electric iron. So now I ask what is the trip current of the circuit breakers.
It seems that there is a lot more information than we have about just what sort of things are involved, and why a relay system is required. Certainly we do not have all of the information

Joined May 25, 2019
50
The incandescent bulbs will not use any power except when lighted, which would only happen when a circuit breaker had tripped. Yes, they would take up a fair amount of space. But now I am wondering why such a system would be required in your case when thousands of installations do not have such indicators. What is it that causes you to need such an unusual arrangement? Or is this system for a hotel or apartment building?
AND you are indicating that the neon light connected across the circuit breaker will be on even if the load is an electric iron. So now I ask what is the trip current of the circuit breakers.
It seems that there is a lot more information than we have about just what sort of things are involved, and why a relay system is required. Certainly we do not have all of the information
Sir ,trip current is according to load, and normally 16amps rated or 32amps.when Breaker trips due to any reason,i want to check if there is short circuit across points or there is load cnnected like electric iron.purpose is to inhibit user from switching ON breaker befre clearing short circuit as well as continuity of supply.ie if there was unusuall tripping of breaker and no shortcircuit we can turn on breaker to ensure cntinuity of supply by checking than neon indicator .this is objective..and i am working at single phase ac system for time being and can be extended later on..currently i have problem fr driving led or relay from comparator...

Joined May 25, 2019
50
A double supply would solve the input range problem.
sir for comparator i also have to centre tapped ground power supply as i did for opamp first stage?

Joined May 25, 2019
50
As for the remaining questions, I'll try to take a closer look this afternoon or evening. Right now I'm at work and can't spend time on it.
thankyou sir i will wait

#### ebeowulf17

Joined Aug 12, 2014
3,275
Well, I'm not sure about the details of making this circuit achieve its stated purpose, but I did try two variations of it to see what it does so far, and all is as expected:

1) With a dual supply on the comparator, the output is on half of the time:
A) If input is above a set threshold, output goes high on AC positive half-cycles.
B) If input is below threshold, output goes high on AC negative half-cycles.

2) With a single supply on the comparator, it's basically on all of the time because the negative voltage swings at the input make it behave erratically.

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