LM358 as a differential amplifer gain not coming out right

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Thread Starter

amad1

Joined May 25, 2019
50
@amad1

I am attempting to get a clear understanding of what you are trying to do.

Please review these items (1.-6.) and indicate (i.e. write in an AAC post) ANY changes or corrections you believe necessary.
1. There is a source of 240VAC/50Hz having the capability to deliver large current and power.
2. Connected to the source is a "convenience outlet" (e.g. a wall socket), the current through which is limited by a circuit breaker. This circuit breaker is known to open its contacts due to either (a) current that exceeds the breaker rating or (b) a fault within the breaker itself. I suggest that an additional fault condition--indistinguishable from an open breaker--can exist due to defective or failed wiring anywhere in the path through which the 240VAC drives current. Wires do break; wires do get damaged; screws holding wires to sockets, switches, etc do become loose--all allowing loss of contact.
3. Various appliances may be connected via the convenience outlet. During normal operation these appliances may consume a range of power (and current) from quite small (0W) to rather large (1500W).
4. Connected appliances can fail. After failing they may continue to draw current ranging from 0.0A (e.g. the appliance power switch fails: a common failure mode) to the maximum current allowed by the circuit breaker. Appliances may contain temperature sensors that turn off the appliance at certain temperatures; this may appear as a temporary failure.
5. There is a resistor in series with a neon bulb, that for brevity I will refer to as the "neon circuit." The neon circuit is connected in parallel with the circuit breaker. When driven by a 240VAC source the neon circuit will pass a current in the range of 2-10mA (depending on resistor value and specific neon bulb used).
6. It is expected that the neon circuit will light whenever the circuit breaker is "open" and some load is connected at the convenience outlet. Such load includes any appliance that is switched "on" and was (a) working normally prior to the opening of the breaker or (b) had failed before or after the breaker opened. Such load also includes any appliance that has within it an automatic switch (e.g. temperature limit) to reduces the load current to 0A.
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An observation: In many countries the connection of the neon circuit would be prohibited by regulations because it allows (a small) current (but potentially large voltage) to flow to the load even when the breaker is open. That is, a person should be entitled to assume that no dangerous voltage is available at the convenience outlet if the breaker is open. I do not know whether such regulation is in force at your location, wherever that may be.
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Your goal is to set up circuitry such that a distinct visual indication (i.e. a combination or pattern of lights) is shown whenever a LOAD IS CONNECTED (and switched "on" if that feature is available) at the convenience outlet AND one of the following conditions exists. The lights must uniquely identify which condition exists (i.e. by viewing the lights you can tell which condition exists).

I believe that the following conditions could exist:
a. The load is operating normally. The circuit breaker is closed.
b. The load was operating normally but the circuit breaker has opened due to an internal fault within the breaker, or within the wiring from the 240VAC source to the load, or within the wiring from the load returning to the 240VAC source.
c. The load has failed (as defined at 4. above) but the circuit breaker remains closed.
d. The load has failed (as defined at 4. above) and the circuit breaker, in response to the failure, has opened.

Please review the possible fault causes discussed in paragraph 4. above. Indicate which of these causes you wish to consider for your design.

Please review the operating conditions listed above (a.-d.). Indicate which of these conditions your design must identify.

Thank you for clarifying your goal.
Respected sir..many thanks for your valuable time and detailed explaination that I was missing...
A) Yes all the things you highlighted are my goal...That include points 1-6 as well as points a-d...However my main focus is on short circuit fault..means when short circuit or temporary overlaod or breaker failure occur ,breaker trips..then user donot have any clue whether there is still short circuit exisiting or there ws temporary overcurrent.If he has no idea abou it..there are likely two conditions.
a) If there was temporary overlaod or breaker abnrmality(temporary) and he dont switch On breaker,then unwanted discontinuiy of supply.
b) If there is short circuit existing and user turns breaker On ,then there wil again tripping and breaker may burst or life deteriorates.I want to distinguish whether there is load connected or short circuit,,,beacuse here for the case Neonn is always On after breaker tripping.For normal operation,circuit breaker bypasses neon circuit, so neon is off.
B) Secondly your concern about connecting neon in parallel with branch circuit breaker is of vital importance and agreeable ..however I have tested different loads by plugging in socket outlet and tripping breaker ..they include :
1) electric iron
2) pedestal fan(inductive load)
3) Charger(cap load can say if i am not wrong)
If breaker trips there are some millivolts across outlet for electric iron(resistive load) and inductive load ..however ther are 10-20 volts for laptop charger connected in outlet..
Lastly I want to work with least effort and components(requirement)
Thanks for your response
Regards
 

TeeKay6

Joined Apr 20, 2019
572
Respected sir..many thanks for your valuable time and detailed explaination that I was missing...
A) Yes all the things you highlighted are my goal...That include points 1-6 as well as points a-d...However my main focus is on short circuit fault..means when short circuit or temporary overlaod or breaker failure occur ,breaker trips..then user donot have any clue whether there is still short circuit exisiting or there ws temporary overcurrent.If he has no idea abou it..there are likely two conditions.
a) If there was temporary overlaod or breaker abnrmality(temporary) and he dont switch On breaker,then unwanted discontinuiy of supply.
b) If there is short circuit existing and user turns breaker On ,then there wil again tripping and breaker may burst or life deteriorates.I want to distinguish whether there is load connected or short circuit,,,beacuse here for the case Neonn is always On after breaker tripping.For normal operation,circuit breaker bypasses neon circuit, so neon is off.
B) Secondly your concern about connecting neon in parallel with branch circuit breaker is of vital importance and agreeable ..however I have tested different loads by plugging in socket outlet and tripping breaker ..they include :
1) electric iron
2) pedestal fan(inductive load)
3) Charger(cap load can say if i am not wrong)
If breaker trips there are some millivolts across outlet for electric iron(resistive load) and inductive load ..however ther are 10-20 volts for laptop charger connected in outlet..
Lastly I want to work with least effort and components(requirement)
Thanks for your response
Regards
@amad1
I do not believe it is possible to support all the conditions that I stated...without the use of an intelligent robot. I will ponder your response above and try to offer a recommended sub-set of conditions to test for...within a couple of days (max).
 

Thread Starter

amad1

Joined May 25, 2019
50
@amad1
I do not believe it is possible to support all the conditions that I stated...without the use of an intelligent robot. I will ponder your response above and try to offer a recommended sub-set of conditions to test for...within a couple of days (max).
Thankyou very much sir for your consideration..
Regards
 

Thread Starter

amad1

Joined May 25, 2019
50
@amad1
I do not believe it is possible to support all the conditions that I stated...without the use of an intelligent robot. I will ponder your response above and try to offer a recommended sub-set of conditions to test for...within a couple of days (max).
Sir..I am waiting for your valuable response

Regards
 

Alec_t

Joined Sep 17, 2013
11,878
I want to distinguish whether there is load connected or short circuit,,,beacuse here for the case Neonn is always On after breaker tripping.
If the breaker trips, where will the neon get its power? A neon needs around 80V to strike.
 

Thread Starter

amad1

Joined May 25, 2019
50
If the breaker trips, where will the neon get its power? A neon needs around 80V to strike.
sir thankyou very much for your valuable response.neon is connected in parallel with breaker.when breaker closes.neon shorted via breaker.when breaker trips neon circuit cmpletes through load connected or short across load...
Regards
 

TeeKay6

Joined Apr 20, 2019
572
Sir..I am waiting for your valuable response

Regards
@amad1
Please find attached/uploaded a schematic file (.jpg) and a "How it works" (.txt file).

In addition to those documents I offer the following comments:
1 You must translate the schematic into the MultiSim format that you use.
2. You have been cautioned about the danger and regulatory risk of bypassing the circuit breaker.
3. I myself do not believe that your approach is viable for the detection of appliance failure. There are failure modes that a single voltage measurement cannot reliably distinguish between a failed appliance and a merely weakly powered appliance (via Neon current).
4. I suggest that a better initial approach to your project would be to use an ordinary VOM capable of measuring AC volts to verify your ability to distinguish good from failed appliances via a voltage measurement. Such a VOM is low-cost, reliable, and immediately available compared to the cost, delay, and complexity of creating your own design as a "proof of concept".
5. Note that no portion of the circuit can be allowed to contact earth/ground or a user.
6View attachment 182670 . Again, working with the AC line is dangerous. Use care.
 
Last edited:

ci139

Joined Jul 11, 2016
1,696
the thing has it's input and output midpoints different
e.g. if you set ±0V in the unity gain configuration to it's input
the output will be not at ±0V you need to adjust level possible by various means and precision
https://www.analog.com/media/en/training-seminars/tutorials/MT-037.pdf
also it is PNP input pair , the common mode is more to the neg. supply side, the output can't reach the top rail too good e.g. it has non-balanced outp unlike ??? TL3472
 
Last edited:

TeeKay6

Joined Apr 20, 2019
572
the thing has it's input and output midpoints different
e.g. if you set ±0V in the unity gain configuration to it's input
the output will be not at ±0V you need to adjust level possible by various means and precision
https://www.analog.com/media/en/training-seminars/tutorials/MT-037.pdf
also it is PNP input pair , the common mode is more to the neg. supply side, the output can't reach the top rail too good e.g. it has non-balanced outp unlike ??? TL3472
@ci139
Are your comments intended to apply to the schematic shown in post #48? As I understand your comments, none of them apply to that schematic. The LM339 is a comparator, not an opamp; it cannot be configured for "unity gain". Agreed, the LM339 input common mode range is centered below the midpoint of the power supplies; thus the references set by (R3 & R8) and (R6 & R7) are set at about 1/3 of the power supply. In this circuit there is never a need for the LM339 output to go higher than the base-emitter voltage of Q1, about 0.75V.
 

Thread Starter

amad1

Joined May 25, 2019
50
@ci139
Are your comments intended to apply to the schematic shown in post #48? As I understand your comments, none of them apply to that schematic. The LM339 is a comparator, not an opamp; it cannot be configured for "unity gain". Agreed, the LM339 input common mode range is centered below the midpoint of the power supplies; thus the references set by (R3 & R8) and (R6 & R7) are set at about 1/3 of the power supply. In this circuit there is never a need for the LM339 output to go higher than the base-emitter voltage of Q1, about 0.75V.
@amad1
Please find attached/uploaded a schematic file (.jpg) and a "How it works" (.txt file).

In addition to those documents I offer the following comments:
1 You must translate the schematic into the MultiSim format that you use.
2. You have been cautioned about the danger and regulatory risk of bypassing the circuit breaker.
3. I myself do not believe that your approach is viable for the detection of appliance failure. There are failure modes that a single voltage measurement cannot reliably distinguish between a failed appliance and a merely weakly powered appliance (via Neon current).
4. I suggest that a better initial approach to your project would be to use an ordinary VOM capable of measuring AC volts to verify your ability to distinguish good from failed appliances via a voltage measurement. Such a VOM is low-cost, reliable, and immediately available compared to the cost, delay, and complexity of creating your own design as a "proof of concept".
5. Note that no portion of the circuit can be allowed to contact earth/ground or a user.
6View attachment 182670 . Again, working with the AC line is dangerous. Use care.
Thankyou very much sir for your vaulabe work and comments in detail.Refering to circuit attached and working explained..I have following queries...
1). When the LM339 output is low (Vcmp < Vth), then the LM339 output transistor will be on, pulling R12 close to 0V and thus keeping Q1 off. When the LM339 output is off, current through R12 will flow into the base of Q1, turning Q1 on.
Here comparator non inverting input is less than inverting(threshold) then output should be high so transistor should turn ON and led should be ON? but in working its written converse of it..Am i doing mistake at some way,if so please correct me?
2) Secondly normally circuit breaker will be ON so full voltage will be applied to R1 and thus divide according to Resistor values and fed to comparator..I mean to ask if circuit breaker is ON then this circuit will still work wihtout being destroyed?
3) There is no need to ground/ earth any signal ,ac/dc?
4) When circuit breaker is tripped then there are only millivolts across load so these when fed to R2 ,then are they still reach comparator?

Regards
 

TeeKay6

Joined Apr 20, 2019
572
Thankyou very much sir for your vaulabe work and comments in detail.Refering to circuit attached and working explained..I have following queries...
1). When the LM339 output is low (Vcmp < Vth), then the LM339 output transistor will be on, pulling R12 close to 0V and thus keeping Q1 off. When the LM339 output is off, current through R12 will flow into the base of Q1, turning Q1 on.
Here comparator non inverting input is less than inverting(threshold) then output should be high so transistor should turn ON and led should be ON? but in working its written converse of it..Am i doing mistake at some way,if so please correct me?
2) Secondly normally circuit breaker will be ON so full voltage will be applied to R1 and thus divide according to Resistor values and fed to comparator..I mean to ask if circuit breaker is ON then this circuit will still work wihtout being destroyed?
3) There is no need to ground/ earth any signal ,ac/dc?
4) When circuit breaker is tripped then there are only millivolts across load so these when fed to R2 ,then are they still reach comparator?

Regards
@amad1
1) You are quite correct about the LM339 logic; I had several circuit versions and failed to notice I had not changed that. I have uploaded a new, corrected schematic.
2) With the breaker closed, 240VAC will be applied across R1/LOAD. Current will be limited by R2 (that is why it is a high value) to a safe level. The goal is that with the breaker closed, Vcmp will remain within the input common mode voltage range of the LM339. With the values shown, that is true.
3. For safety and per (USA) regulations, neither the "line" nor "neutral" mains connections can connect to earth ground, nor can either "line" or "neutral" be accessible to touch by a user. This is the reason you ought not bypass the breaker. I had to show a "ground" symbol in order for LTspice to work; that symbol refers to circuit common, not earth.
4. Indeed, your requirement that the input range from 0 to 240VAC, while using only simple circuitry, is a very stringent demand. How small a LOAD voltage you can reliably detect will depend on many factors including, but not limited to, the component tolerances, the stability of the current through the neon bulb, the input offset voltage and input noise of the LM339, the noise picked up at the LM339 input from other circuit elements and other nearby circuits, etc. You quoted a level of "10-20VAC for the laptop charger" in an earlier post. I have repeatedly stated that I do not believe a single voltage measurement (at any one level) is sufficient to discriminate good from failed appliances. It is up to you to find such a magic level. I do suggest that making Vth variable (with a trimpot) will considerably ease "calibration" (i.e. the setting of the threshold level); the values I show are merely a starting point.

It would be possible to create a design with R2 having a much lower value (and higher wattage to withstand 240VAC); this would enable greater sensitivity to small voltages but would require additional components to restrain Vcmp within the allowed common mode voltage range when the breaker was closed (or the load not connected). At this time it seems clear that you do not (yet) know that "magic" voltage that distinguishes a good from a failed appliance. Therefore I opted to not complicate the design unless that is made necessary by whatever single voltage you select.

I continue to believe that the correct design approach would be to use an ordinary AC voltmeter to find the "magic" voltage. Then, knowing that voltage, design circuitry to light an LED when appropriate.

View attachment 182757
 
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Thread Starter

amad1

Joined May 25, 2019
50
@amad1
1) You are quite correct about the LM339 logic; I had several circuit versions and failed to notice I had not changed that. I have uploaded a new, corrected schematic.
2) With the breaker closed, 240VAC will be applied across R1/LOAD. Current will be limited by R2 (that is why it is a high value) to a safe level. The goal is that with the breaker closed, Vcmp will remain within the input common mode voltage range of the LM339. With the values shown, that is true.
3. For safety and per (USA) regulations, neither the "line" nor "neutral" mains connections can connect to earth ground, nor can either "line" or "neutral" be accessible to touch by a user. This is the reason you ought not bypass the breaker. I had to show a "ground" symbol in order for LTspice to work; that symbol refers to circuit common, not earth.
4. Indeed, your requirement that the input range from 0 to 240VAC, while using only simple circuitry, is a very stringent demand. How small a LOAD voltage you can reliably detect will depend on many factors including, but not limited to, the component tolerances, the stability of the current through the neon bulb, the input offset voltage and input noise of the LM339, the noise picked up at the LM339 input from other circuit elements and other nearby circuits, etc. You quoted a level of "10-20VAC for the laptop charger" in an earlier post. I have repeatedly stated that I do not believe a single voltage measurement (at any one level) is sufficient to discriminate good from failed appliances. It is up to you to find such a magic level. I do suggest that making Vth variable (with a trimpot) will considerably ease "calibration" (i.e. the setting of the threshold level); the values I show are merely a starting point.

It would be possible to create a design with R2 having a much lower value (and higher wattage to withstand 240VAC); this would enable greater sensitivity to small voltages but would require additional components to restrain Vcmp within the allowed common mode voltage range when the breaker was closed (or the load not connected). At this time it seems clear that you do not (yet) know that "magic" voltage that distinguishes a good from a failed appliance. Therefore I opted to not complicate the design unless that is made necessary by whatever single voltage you select.

I continue to believe that the correct design approach would be to use an ordinary AC voltmeter to find the "magic" voltage. Then, knowing that voltage, design circuitry to light an LED when appropriate.

View attachment 182757
Thankyou verymuch sir for your valuale feedback..I have simulated circuit after drawing it in Multisim...However response is as under.
1) When breaker is closed(ie 240volt ac applied across load),then led is turned Off....Its fine
2) Howvever when there is high wattage load connected and breaker is tripped(i manually turned off),then there is no disntinguishing response?
3) if if put a short across load(and tripp breaker) still i didnot get any output?

Note..Referring to defined voltage for disntinghuing..i did experiment on Neon with breaker tripped and i connected different load across load point(socket outlet)..I got 50miilvolt-10Vac voltage range when a load is connected and almost zero volt ac when short...so the previous circuit (last time with two comparators) works fine with millivolt range but how about 10 Voltac range?it will damaged?I still can furthur reduce these 10volt ac to 6 volt ac if the solution can work for these volts?

Regards
 

TeeKay6

Joined Apr 20, 2019
572
Thankyou verymuch sir for your valuale feedback..I have simulated circuit after drawing it in Multisim...However response is as under.
1) When breaker is closed(ie 240volt ac applied across load),then led is turned Off....Its fine
2) Howvever when there is high wattage load connected and breaker is tripped(i manually turned off),then there is no disntinguishing response?
3) if if put a short across load(and tripp breaker) still i didnot get any output?

Note..Referring to defined voltage for disntinghuing..i did experiment on Neon with breaker tripped and i connected different load across load point(socket outlet)..I got 50miilvolt-10Vac voltage range when a load is connected and almost zero volt ac when short...so the previous circuit (last time with two comparators) works fine with millivolt range but how about 10 Voltac range?it will damaged?I still can furthur reduce these 10volt ac to 6 volt ac if the solution can work for these volts?

Regards
I believe that what you are describing is that a threshold has not been set by adjusting R3 or R8 (hence my recommendation that R3-R8 include a trimpot). To set a threshold: Connect in place of R1 an AC voltage of the desired threshold (e.g. 10VAC) plus a small margin (e.g. 11V instead of 10VAC). Then adjust the R8/R3 ratio until the LED flickers or just barely remains off. Check threshold by applying varying input (LOAD) voltages both below and above the threshold. As I noted, the transition from LED on to off (or off to on) is not very sharp (because only a small portion of the positive peak of the LOAD voltage is being compared to a DC reference (set by R3-R8). You can see in Multisim how Vth and Vcmp look at the threshold. For the LED to be off, no portion of the Vcmp waveform can be greater than Vth. As Vcmp increases, a point will be reached when the most positive point of the Vcmp waveform exceeds Vth and thus begins to turn the LED on. The change from LED on to off (or off to on) occurs over a small range of voltage, but is nevertheless gradual.

The most recent circuit I have provided will not easily detect millivolt changes due to the need to protect against the LOAD voltage being as high as 240VAC. That is, the values shown will protect against any load voltage from 0VAC to 240VAC (and higher). If you know what is the highest threshold you wish to use (e.g. 10VAC), then we can modify the circuitry to be more sensitive to low voltage and we can prevent a voltage much greater (e.g. 12VAC) than the threshold and protect the circuitry. However, we cannot make such a change without knowing what "magic" threshold voltage is required. Until you specify that "magic" voltage (good=above threshold; bad=below threshold), I must keep the input range at 0-240VAC. You need to be aware that a neon bulb is not a reliable current source and various bulbs may allow different amounts of current flow.
 

Thread Starter

amad1

Joined May 25, 2019
50
I believe that what you are describing is that a threshold has not been set by adjusting R3 or R8 (hence my recommendation that R3-R8 include a trimpot). To set a threshold: Connect in place of R1 an AC voltage of the desired threshold (e.g. 10VAC) plus a small margin (e.g. 11V instead of 10VAC). Then adjust the R8/R3 ratio until the LED flickers or just barely remains off. Check threshold by applying varying input (LOAD) voltages both below and above the threshold. As I noted, the transition from LED on to off (or off to on) is not very sharp (because only a small portion of the positive peak of the LOAD voltage is being compared to a DC reference (set by R3-R8). You can see in Multisim how Vth and Vcmp look at the threshold. For the LED to be off, no portion of the Vcmp waveform can be greater than Vth. As Vcmp increases, a point will be reached when the most positive point of the Vcmp waveform exceeds Vth and thus begins to turn the LED on. The change from LED on to off (or off to on) occurs over a small range of voltage, but is nevertheless gradual.

The most recent circuit I have provided will not easily detect millivolt changes due to the need to protect against the LOAD voltage being as high as 240VAC. That is, the values shown will protect against any load voltage from 0VAC to 240VAC (and higher). If you know what is the highest threshold you wish to use (e.g. 10VAC), then we can modify the circuitry to be more sensitive to low voltage and we can prevent a voltage much greater (e.g. 12VAC) than the threshold and protect the circuitry. However, we cannot make such a change without knowing what "magic" threshold voltage is required. Until you specify that "magic" voltage (good=above threshold; bad=below threshold), I must keep the input range at 0-240VAC. You need to be aware that a neon bulb is not a reliable current source and various bulbs may allow different amounts of current flow.
Thankyou very much sir for your valuable discussion..
1)Now from circuits it is clear that if i apply 240 Vac directly to circuit..then such circuit cannot detect millivolts ...
2) For simplicity we can assume here that this circuit is only active when breaker trips...
3)so now maximum Ac voltage applied to circuit are 18 Vac rms...and munimum are 60 millvolts...(depending on different loads)
4) Now can we work or ammend circuit that can translate volatges between 60mV to 18voltac as load connected(by output indication OFF) and below 60mvolt as indicatiom ON?...these values are still not v accurate but are realistic...
5) Circuit you posted(previously wth twoLM339 comparators) in mutisim ...I am thnking to test that in hardware too...but I am not sure that circuit will also work wth 18Volt ac?or you can amend that?
6)Or you recommend the latest one and you suggest improvement in it for these max and min volts?
Best Regards
 

TeeKay6

Joined Apr 20, 2019
572
Thankyou very much sir for your valuable discussion..
1)Now from circuits it is clear that if i apply 240 Vac directly to circuit..then such circuit cannot detect millivolts ...
2) For simplicity we can assume here that this circuit is only active when breaker trips...
3)so now maximum Ac voltage applied to circuit are 18 Vac rms...and munimum are 60 millvolts...(depending on different loads)
4) Now can we work or ammend circuit that can translate volatges between 60mV to 18voltac as load connected(by output indication OFF) and below 60mvolt as indicatiom ON?...these values are still not v accurate but are realistic...
5) Circuit you posted(previously wth twoLM339 comparators) in mutisim ...I am thnking to test that in hardware too...but I am not sure that circuit will also work wth 18Volt ac?or you can amend that?
6)Or you recommend the latest one and you suggest improvement in it for these max and min volts?
Best Regards
@amad1
The original circuit will not tolerate an input of 240VAC! (Nor could any of the circuits proposed by other commenters. You did not introduce the idea of 240VAC until after many comments had already been posted.

My latest circuit can tolerate inputs up to 240VAC (as any circuit must if the breaker is closed). It is possible that the latest circuit can be modified...if I can understand what it is that you want. As the circuit currently exists (and I will not design a totally different approach for you), there is a single threshold voltage. When the Load voltage is below that threshold, the LED will light; when the load voltage is above that threshold, the LED will be off. As I have noted, the LED on/off transition is somewhat gradual, but per my understanding of what you are trying to do, that gradual transition is irrelevant. The threshold can easily be set to 18VAC. When set for that threshold, all load voltages less than 18VAC will be treated equally, whether the actual load voltage is 1mV, 237mV, 2V, or 17V--the LED will be lighted. For all load voltages above 18VAC--whether 20VAC, 47VAC, 124VAC, or 240VAC--the LED will be off. If you decide on a specific threshold, e.g. 18VAC, then I may be able to make modifications--if necessary--that will sharpen the gradual on/off LED behavior somewhat. In exchange for that sharper on/off behavior, the circuit will still tolerate an input up to 240VAC but the threshold will not be adjustable over the full 0-240vAC range as it is now. If your threshold is 18VAC, then you really don't care what exact voltage greater than 18VAC is causing the LED to turn off (i.e. the load voltage could be 19VAC or 228VAC; the LED would remain off). Because of the low neon bulb current, I do need to think about the modifications before I recommend them. Note that (in my schematic) R2, R7, R6, R3, R8, and +5VDC must be stable (e.g. 1% tolerance metal film resistors with 100ppm or less temperature coefficients) if you want the threshold to be accurate & stable.

Do you understand that it makes no sense to talk about "detecting millivolts" when the threshold is set for many volts? The only thing that matters is whether the load voltage is above or below the threshold; how much above or below is irrelevant (and you have no way to indicate the actual voltage with only a single LED display).
 

Thread Starter

amad1

Joined May 25, 2019
50
@amad1
The original circuit will not tolerate an input of 240VAC! (Nor could any of the circuits proposed by other commenters. You did not introduce the idea of 240VAC until after many comments had already been posted.

My latest circuit can tolerate inputs up to 240VAC (as any circuit must if the breaker is closed). It is possible that the latest circuit can be modified...if I can understand what it is that you want. As the circuit currently exists (and I will not design a totally different approach for you), there is a single threshold voltage. When the Load voltage is below that threshold, the LED will light; when the load voltage is above that threshold, the LED will be off. As I have noted, the LED on/off transition is somewhat gradual, but per my understanding of what you are trying to do, that gradual transition is irrelevant. The threshold can easily be set to 18VAC. When set for that threshold, all load voltages less than 18VAC will be treated equally, whether the actual load voltage is 1mV, 237mV, 2V, or 17V--the LED will be lighted. For all load voltages above 18VAC--whether 20VAC, 47VAC, 124VAC, or 240VAC--the LED will be off. If you decide on a specific threshold, e.g. 18VAC, then I may be able to make modifications--if necessary--that will sharpen the gradual on/off LED behavior somewhat. In exchange for that sharper on/off behavior, the circuit will still tolerate an input up to 240VAC but the threshold will not be adjustable over the full 0-240vAC range as it is now. If your threshold is 18VAC, then you really don't care what exact voltage greater than 18VAC is causing the LED to turn off (i.e. the load voltage could be 19VAC or 228VAC; the LED would remain off). Because of the low neon bulb current, I do need to think about the modifications before I recommend them. Note that (in my schematic) R2, R7, R6, R3, R8, and +5VDC must be stable (e.g. 1% tolerance metal film resistors with 100ppm or less temperature coefficients) if you want the threshold to be accurate & stable.

Do you understand that it makes no sense to talk about "detecting millivolts" when the threshold is set for many volts? The only thing that matters is whether the load voltage is above or below the threshold; how much above or below is irrelevant (and you have no way to indicate the actual voltage with only a single LED display).
@amad1
The original circuit will not tolerate an input of 240VAC! (Nor could any of the circuits proposed by other commenters. You did not introduce the idea of 240VAC until after many comments had already been posted.

My latest circuit can tolerate inputs up to 240VAC (as any circuit must if the breaker is closed). It is possible that the latest circuit can be modified...if I can understand what it is that you want. As the circuit currently exists (and I will not design a totally different approach for you), there is a single threshold voltage. When the Load voltage is below that threshold, the LED will light; when the load voltage is above that threshold, the LED will be off. As I have noted, the LED on/off transition is somewhat gradual, but per my understanding of what you are trying to do, that gradual transition is irrelevant. The threshold can easily be set to 18VAC. When set for that threshold, all load voltages less than 18VAC will be treated equally, whether the actual load voltage is 1mV, 237mV, 2V, or 17V--the LED will be lighted. For all load voltages above 18VAC--whether 20VAC, 47VAC, 124VAC, or 240VAC--the LED will be off. If you decide on a specific threshold, e.g. 18VAC, then I may be able to make modifications--if necessary--that will sharpen the gradual on/off LED behavior somewhat. In exchange for that sharper on/off behavior, the circuit will still tolerate an input up to 240VAC but the threshold will not be adjustable over the full 0-240vAC range as it is now. If your threshold is 18VAC, then you really don't care what exact voltage greater than 18VAC is causing the LED to turn off (i.e. the load voltage could be 19VAC or 228VAC; the LED would remain off). Because of the low neon bulb current, I do need to think about the modifications before I recommend them. Note that (in my schematic) R2, R7, R6, R3, R8, and +5VDC must be stable (e.g. 1% tolerance metal film resistors with 100ppm or less temperature coefficients) if you want the threshold to be accurate & stable.

Do you understand that it makes no sense to talk about "detecting millivolts" when the threshold is set for many volts? The only thing that matters is whether the load voltage is above or below the threshold; how much above or below is irrelevant (and you have no way to indicate the actual voltage with only a single LED display).
Sir thankyou for your valuable response once again.
1) Yes in short I didnot quote early about 240Vac, but now as things are bit complicated,so still i want to exclude this 240Vac .I will make some other arrangement for interlocking.So our circuit will energize only when breaker is tripped.when breaker is ON,this circuit will not get ac supply( voltage)
2) Secondly for this case ,threshold is around about 40millivolt or say 60millivolt ac.If voltage is below this ,led should ON.if Voltage is above this then led should OFF.
3) Thirdly maximum voltage that will be applied(depending on connected load) are almost 18VAC,so this is not threshold.I am just mentioning to clarify that our circuit will be subjected to maximum 18Vac.I still can reduce these 18Vac to around 10 Voltac or a bit lower,if required for which cicruit can be easily amended...
4) Lastly if i reduce these 18vac to lower value,then offcourse milivolt threshold will reduce to 30millivolt ac or like that..

Regards
 

TeeKay6

Joined Apr 20, 2019
572
Sir thankyou for your valuable response once again.
1) Yes in short I didnot quote early about 240Vac, but now as things are bit complicated,so still i want to exclude this 240Vac .I will make some other arrangement for interlocking.So our circuit will energize only when breaker is tripped.when breaker is ON,this circuit will not get ac supply( voltage)
2) Secondly for this case ,threshold is around about 40millivolt or say 60millivolt ac.If voltage is below this ,led should ON.if Voltage is above this then led should OFF.
3) Thirdly maximum voltage that will be applied(depending on connected load) are almost 18VAC,so this is not threshold.I am just mentioning to clarify that our circuit will be subjected to maximum 18Vac.I still can reduce these 18Vac to around 10 Voltac or a bit lower,if required for which cicruit can be easily amended...
4) Lastly if i reduce these 18vac to lower value,then offcourse milivolt threshold will reduce to 30millivolt ac or like that..

Regards
@amad1
OK, let's try again. I don't care how you do it, but you will guarantee that the voltage across the LOAD will not exceed 18VAC; if the voltage does become greater than 18-25VAC, then the circuit may self-destruct (it won't explode or burn in flames but it may permanently fail and require repair). (I assume) that when the breaker is open, the test current will still be supplied by the neon bulb and resistor? You wish to be able to set a threshold of 40-60mV (AC). This is certainly doable via simple modifications to the existing circuit. Like you, I have other things to do in life, so it may be 1-3 days before I post the revised schematic. I need your confirmation of the above statements before I begin the modification...
 

Thread Starter

amad1

Joined May 25, 2019
50
@amad1
OK, let's try again. I don't care how you do it, but you will guarantee that the voltage across the LOAD will not exceed 18VAC; if the voltage does become greater than 18-25VAC, then the circuit may self-destruct (it won't explode or burn in flames but it may permanently fail and require repair). (I assume) that when the breaker is open, the test current will still be supplied by the neon bulb and resistor? You wish to be able to set a threshold of 40-60mV (AC). This is certainly doable via simple modifications to the existing circuit. Like you, I have other things to do in life, so it may be 1-3 days before I post the revised schematic. I need your confirmation of the above statements before I begin the modification...
Yes sir I confirm requirements for my above mentioned post...Threshold is 40 -60 millivolt ac. and maximum voltage applied will be 18 V ac.However if circuit is a bit flexible for threshold setting to a some greater value for future it would be an added point.(max upto which it can be easily done ),but its just an added point..not requirement at this stage.
For current stage My requirements are final as i stated in my last post.
Lastly yes circuit current is within safe limits and supplied by neon and resistor..
Best Regards sir
 
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