# How to amplify millivolts to drive a relay

#### ebeowulf17

Joined Aug 12, 2014
2,977
sir for comparator i also have to centre tapped ground power supply as i did for opamp first stage?
As long as you're feeding it negative input voltages, yes. However, I'm not sure you'll want to keep feeding it negative voltages. More on that later...

#### ebeowulf17

Joined Aug 12, 2014
2,977
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.
Based on the simulations above, I realized what I should've seen all along. A comparator activates when the positive input is higher than the negative input. If you have two AC sources with no DC offset, then that's guaranteed to happen half the time no matter which signal has the higher AC voltage. If signal is above threshold, the positive input goes higher on positive half cycles. If signal is below threshold, positive input doesn't go as low on negative half cycles... and not going as low is the same as being higher voltage! So, either way your comparator is on 50% of the time.

I think you need to rectify your AC signal and then compare it to a DC reference.

#### MisterBill2

Joined Jan 23, 2018
4,607
There is another problem that I am amazed nobody has commented about yet. During normal operation both sides of that circuit breaker are at a line potential of 230 volts, since the other side of the circuit is the neutral side of the line, close to ground potential. That means that all of the connections associated with any amplifier circuit are 230 volts above ground, at the mains potential. Not only will that lead to electrical noise problems, it will also make working with the monitoring circuit rather unsafe.AND if the power supply for the comparator or amplifier has a grounded center point, there will certainly be a fair amount of electrical destruction when it is connected. In addition to those problem areas there is also a problem with connecting to the circuit breaker, which is that the one side of the breaker is not easily accessible.

#### ebeowulf17

Joined Aug 12, 2014
2,977
There is another problem that I am amazed nobody has commented about yet. During normal operation both sides of that circuit breaker are at a line potential of 230 volts, since the other side of the circuit is the neutral side of the line, close to ground potential. That means that all of the connections associated with any amplifier circuit are 230 volts above ground, at the mains potential. Not only will that lead to electrical noise problems, it will also make working with the monitoring circuit rather unsafe.AND if the power supply for the comparator or amplifier has a grounded center point, there will certainly be a fair amount of electrical destruction when it is connected. In addition to those problem areas there is also a problem with connecting to the circuit breaker, which is that the one side of the breaker is not easily accessible.
Agreed on basically all counts - for some reason I can't resist trying to make sense of the part of the circuit that can be portrayed as a simple schematic and simulated, but I agree that getting this to work on a circuit breaker involves numerous challenges and safety concerns. I certainly won't be trying it!

#### ebeowulf17

Joined Aug 12, 2014
2,977
Here's an example of rectifying the signal and then comparing it to a DC threshold reference. It adds another op-amp stage, a couple diodes, and a couple resistors. In this scenario, if the AC voltage is above a certain level, the output will activate for some portion of each cycle. If it's not, it will not activate at all. This may still not be the intended circuit behavior, but I think it's getting closer.

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#### MisterBill2

Joined Jan 23, 2018
4,607
Agreed on basically all counts - for some reason I can't resist trying to make sense of the part of the circuit that can be portrayed as a simple schematic and simulated, but I agree that getting this to work on a circuit breaker involves numerous challenges and safety concerns. I certainly won't be trying it!
The worst is that the safety issue has been ignored. And I am bothered by the TS claim on their profile that they are an electrical engineer. ANY electrical engineer would understand the issues of what is being proposed. And anybody who has looked inside a circuit breaker panel would understand that even if a circuit to provide what is requested could be created,it would not work.
What could serve as a short circuit detector, but has been rejected, is a fairly high resistance sensitive relay across the breaker contacts.

Joined May 25, 2019
50
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.
Sir would you suggest some thing to achive my stated objective by ammending circuit?

Joined May 25, 2019
50
There is another problem that I am amazed nobody has commented about yet. During normal operation both sides of that circuit breaker are at a line potential of 230 volts, since the other side of the circuit is the neutral side of the line, close to ground potential. That means that all of the connections associated with any amplifier circuit are 230 volts above ground, at the mains potential. Not only will that lead to electrical noise problems, it will also make working with the monitoring circuit rather unsafe.AND if the power supply for the comparator or amplifier has a grounded center point, there will certainly be a fair amount of electrical destruction when it is connected. In addition to those problem areas there is also a problem with connecting to the circuit breaker, which is that the one side of the breaker is not easily accessible.
Sir for normal operation if we supply through dry NC contacts of circuit breaker,if CB is ON,then NC will open and cut supply to the electronics circuit.Means it will only be in circuit when breaker is tripped?
Secondly neon is working fine without electronics when there is a shortcircuit,but problem is i already highlighted when there is some low reistance load connected,neon still turns ON,this is i want to differentiiate.Any idea other these circumstances ill be appreciated sir,,As i have not too much expertise here sir..

Joined May 25, 2019
50
Here's an example of rectifying the signal and then comparing it to a DC threshold reference. It adds another op-amp stage, a couple diodes, and a couple resistors. In this scenario, if the AC voltage is above a certain level, the output will activate for some portion of each cycle. If it's not, it will not activate at all. This may still not be the intended circuit behavior, but I think it's getting closer.
Sir it looks better,but as rectified is still ac is there any way to make it more like dc to be fed to comparator?

#### MisterBill2

Joined Jan 23, 2018
4,607
Sir for normal operation if we supply through dry NC contacts of circuit breaker,if CB is ON,then NC will open and cut supply to the electronics circuit.Means it will only be in circuit when breaker is tripped?
Secondly neon is working fine without electronics when there is a shortcircuit,but problem is i already highlighted when there is some low reistance load connected,neon still turns ON,this is i want to differentiiate.Any idea other these circumstances ill be appreciated sir,,As i have not too much expertise here sir..
I have already suggested a relay with the coil across the breaker contacts.
The big problem is that some load that is not a short circuit might draw 10 amps, or even 15 amps, which is not an overload for a 16 amp breaker. But then just a bit more, to 17 amps, is an overload and would trip the breaker. And if that overload stays connected then how can a system see the difference between an overload and a short circuit? In the power distribution systems if the breaker is tripped then there is an automatic reset quite quickly, and if it is still a short circuit then the breaker is tripped again.
And one more problem is that the difference in resistance between a short circuit and a heavy load such as an iron or a large electric heater. The system with an amplifier and a comparator will not be able to detect the difference by the method you have described.

#### MisterBill2

Joined Jan 23, 2018
4,607
Sir for normal operation if we supply through dry NC contacts of circuit breaker,if CB is ON,then NC will open and cut supply to the electronics circuit.Means it will only be in circuit when breaker is tripped?
Secondly neon is working fine without electronics when there is a shortcircuit,but problem is i already highlighted when there is some low reistance load connected,neon still turns ON,this is i want to differentiiate.Any idea other these circumstances ill be appreciated sir,,As i have not too much expertise here sir..
If the wires to the load are very long and run next to each other as power wires normally do, then the neon lamp will light even when there is nothing connected, because of the capacitance between the line and the neutral wire. I have observed that when checking circuits many times. To detect the difference between a short circuit and a heavy load you need to have much more current flow in the wires and then measure and know the resistance. That was the purpose of using an incandescent lamp, that it would draw a lot more current than some amplifier circuit.
The solution would be a different type of circuit breaker that can sense the current and know if it is a short circuit or an overload, and if just an overload, then reset. hose are available but they cost several hundred dollars each.

Joined May 25, 2019
50
If the wires to the load are very long and run next to each other as power wires normally do, then the neon lamp will light even when there is nothing connected, because of the capacitance between the line and the neutral wire. I have observed that when checking circuits many times. To detect the difference between a short circuit and a heavy load you need to have much more current flow in the wires and then measure and know the resistance. That was the purpose of using an incandescent lamp, that it would draw a lot more current than some amplifier circuit.
The solution would be a different type of circuit breaker that can sense the current and know if it is a short circuit or an overload, and if just an overload, then reset. hose are available but they cost several hundred dollars each.
Is there some option to make decision upon current in the circuit?But only after breaker is tripped?When breaker trips due to short circuit current is limited by only neon reistance and current limiting resistor around 200k,but when breaker trips and there is heavy load like electric iron current through circuit is limited by neon, current limiting resistor and resistance of load(electric iron which is around 50 ohm).So there is a little difference in current flowing..Current in milliamps..Is this pssible to use this criteria to distinguish?any suggestion sir?Because if am not designing a protection system,I am working on Permanent fault Indicator,,,,,

#### MisterBill2

Joined Jan 23, 2018
4,607
Sensing the difference in current between having 200,000 ohms in series with 500 ohms and having 200,000 ohms in series with 1 ohm is quite a challenge. And if the line voltage can vary by 5 volts up or down, then it is probably not possible to do in a simple arrangement. One option would be to put a lower resistance across the circuit breaker when it is tripped that will limit the current to 15 amps if there is a short circuit. Then there will be a much greater difference in the voltage and it will be very simple to know if the fault is a permanent fault or just a momentary overload. Consider that 15 amps and one ohm gives 15 volts, while 15 amps and ten ohms gives 150 volts, a much greater difference. For that difference no amplifier will be required.