Voltage detectors how work?
- Thread starter flash295
- Start date
- Status
Re: Voltage detectors how work?
Looks like you almost have it Rattus, but I see one problem. The capacatiance between the device and the Line (C1) is a stray capacitance, right? Same for the (C2). So C1 is equally split to both sides of R1. And C2 is also equally split to both sides of R1. So it looks like the voltage will always be zero across R1. I don't see any way to make the capacitance non-symetrical to get a voltage across R.
I guess I should have taken the time to read the link lauraj gave.
Steve
Looks like you almost have it Rattus, but I see one problem. The capacatiance between the device and the Line (C1) is a stray capacitance, right? Same for the (C2). So C1 is equally split to both sides of R1. And C2 is also equally split to both sides of R1. So it looks like the voltage will always be zero across R1. I don't see any way to make the capacitance non-symetrical to get a voltage across R.
I guess I should have taken the time to read the link lauraj gave.
Steve
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: Voltage detectors how work?
Now we know how the voltage detector works.
So can we get it to detect accurately an overvoltage condition above 120V L-G, so as to offer overvoltage protection on the main or branch breakers in a residence at risk for a loose/disconnected service neutral? Take the accurate Hall effect gizmo and shove it in a breaker case (like AFCI or GFCI).
I smell patent infringement .....
[ January 11, 2006, 03:33 PM: Message edited by: ron ]
Now we know how the voltage detector works.
So can we get it to detect accurately an overvoltage condition above 120V L-G, so as to offer overvoltage protection on the main or branch breakers in a residence at risk for a loose/disconnected service neutral? Take the accurate Hall effect gizmo and shove it in a breaker case (like AFCI or GFCI).
I smell patent infringement .....
[ January 11, 2006, 03:33 PM: Message edited by: ron ]
macmikeman
Senior Member
- Location
- Planet macmikeman
Re: Voltage detectors how work?
Did you guys go to private school ? I was just wondering cause I also know how volt detectors work. They work when you press down on the clip while you hold them near a hot wire......
Did you guys go to private school ? I was just wondering cause I also know how volt detectors work. They work when you press down on the clip while you hold them near a hot wire......
lauraj
Senior Member
- Location
- Portland, Oregon
Re: Voltage detectors how work?
Did you know that a voltage detector will not detect voltage present on romex if it is wet?
Did you know that a voltage detector will not detect voltage present on romex if it is wet?
Re: Voltage detectors how work?
Again, I am sure it is much more complicated than this, but this is the basic idea.
The caps are not necessarily equally split, and even if they were, there is still current through R1. And, it doesn't take much voltage to drive the opamp output to the rails.
Again, I am sure it is much more complicated than this, but this is the basic idea.
physis
Senior Member
Re: Voltage detectors how work?
Without (still) having given it much thought, I tend to think that Rattus is correct Steve. But really I'm just going with what feels intuitive at the moment.
I will look at it more closely when the mood strikes me.
Lauraj; I didn't know about the wet MC either. I think I might grab a spray bottle.
And neither of mine have a button to hold down. That would suck actually.
Without (still) having given it much thought, I tend to think that Rattus is correct Steve. But really I'm just going with what feels intuitive at the moment.
I will look at it more closely when the mood strikes me.
Lauraj; I didn't know about the wet MC either. I think I might grab a spray bottle.
And neither of mine have a button to hold down. That would suck actually.
lauraj
Senior Member
- Location
- Portland, Oregon
Re: Voltage detectors how work?
Physis, that was NM, not MC. We do a demonstration here, where we've got a piece of romex with a cord cap on it, that has been soaked in water, so the paper inside is nice and wet. Plug it in to an outlet and the tick tracer will not pick it up. Now can anyone explain why that happens?
Physis, that was NM, not MC. We do a demonstration here, where we've got a piece of romex with a cord cap on it, that has been soaked in water, so the paper inside is nice and wet. Plug it in to an outlet and the tick tracer will not pick it up. Now can anyone explain why that happens?
Re: Voltage detectors how work?
But I think I know what is missing in the above diagram. (Maybe it was obvious to everyone else.) If you tie one side of the resistor to a pointed probe, both "C's" on that side increase. But they have to increase by different ratio's to get a voltage difference.
If I get time, I'll do the math to show this creates a voltage difference.
Steve
Steve
[ January 13, 2006, 06:31 PM: Message edited by: steve66 ]
I wasn't saying Rattus was incorrect, just that a small piece was missing. If you think about it, the resistor is in the middle of a bridge of capacitors. Think of a wheatstone bridge. You have two capacitors on the top with equal values "C1", and two capacitors on the bottom with value "C2". With a resistor across the middle, the voltage has to be zero.
But I think I know what is missing in the above diagram. (Maybe it was obvious to everyone else.) If you tie one side of the resistor to a pointed probe, both "C's" on that side increase. But they have to increase by different ratio's to get a voltage difference.
If I get time, I'll do the math to show this creates a voltage difference.
Steve
Steve
[ January 13, 2006, 06:31 PM: Message edited by: steve66 ]
physis
Senior Member
Re: Voltage detectors how work?
lauraj, MC was a typo, I meant NM, oops.
Wet paper? Not an unimportant detail.
Steve, I've given it more thought and have decided that C2 (the one furthest from the probe end) is unnecessary. Doesn't do anything other than change the R and C values, doesn't even do a lot of that, I think. So lose C2.
I come up with:
C1--R1--GND
OP amp inputs across R1 with near open loop negative gain. (I think that's how I remember it should be done) The point is to go near infinite gain.
I can't find a need for C2.
For the electrical engineers, keep in mind that these things are powered by two dry cells or about 3 volts more or less optimally. That means they probably work on around 2 volts just fine.
For the devices I used to use that's a pretty tight supply range.
I still think Rattus is pretty close here. Even though he might be destroying the planet with his volgar and wasteful use of unnecessary capacitors.
lauraj, MC was a typo, I meant NM, oops.
Wet paper? Not an unimportant detail.
Steve, I've given it more thought and have decided that C2 (the one furthest from the probe end) is unnecessary. Doesn't do anything other than change the R and C values, doesn't even do a lot of that, I think. So lose C2.
I come up with:
C1--R1--GND
OP amp inputs across R1 with near open loop negative gain. (I think that's how I remember it should be done) The point is to go near infinite gain.
I can't find a need for C2.
For the electrical engineers, keep in mind that these things are powered by two dry cells or about 3 volts more or less optimally. That means they probably work on around 2 volts just fine.
For the devices I used to use that's a pretty tight supply range.
I still think Rattus is pretty close here. Even though he might be destroying the planet with his volgar and wasteful use of unnecessary capacitors.
physis
Senior Member
Re: Voltage detectors how work?
The paper is conductive?
Aren't we looking at electric fields here? Where'd you get current from?
It would certainly change the capacitance.
I'll have to give this some thought too though. An interesting precept.
Edit: Error A
[ January 16, 2006, 12:58 AM: Message edited by: physis ]
That's not so easy as Rattus says I don't think.
The paper is conductive?
Aren't we looking at electric fields here? Where'd you get current from?
It would certainly change the capacitance.
I'll have to give this some thought too though. An interesting precept.
Edit: Error A
[ January 16, 2006, 12:58 AM: Message edited by: physis ]
Re: Voltage detectors how work?
Sam,
The Romex has been soaked in tap water! Even a film of water around the outer sheath would be conductive.
As for C2, you can say it is unnecessary, but it is there nonetheless, so you had better include it.
Furthermore, without C2, you would have an open circuit. Then there would be no current and no voltage change to sense. Gotta have it.
It is quite possible that some sort of bridge arrangement is used, and perhaps some unity gain amps to raise impedance. Also, some manner of autozero and self calibration circuitry might be required.
Anyway, this is the way I would start such a design; then I would let science take its course.
Sam,
The Romex has been soaked in tap water! Even a film of water around the outer sheath would be conductive.
As for C2, you can say it is unnecessary, but it is there nonetheless, so you had better include it.
Furthermore, without C2, you would have an open circuit. Then there would be no current and no voltage change to sense. Gotta have it.
It is quite possible that some sort of bridge arrangement is used, and perhaps some unity gain amps to raise impedance. Also, some manner of autozero and self calibration circuitry might be required.
Anyway, this is the way I would start such a design; then I would let science take its course.
Re: Voltage detectors how work?
Without C2, there would be no voltage across the opamp inputs.
This thing is basically a three element voltage divider, and we are sensing the voltage across the center element, R1.
Larger values of C2 would allow more current, hence more voltage across R1. We should really say Z1 since we do not know exactly what it is.
All this is in my head. I don't want to blow $15 prying mine apart.
Sam, decoupling caps are used on supply buses, not on opamp inputs. The whole thing "floats"; there is no ground reference as such.Originally posted by physis:
Wait a second, C2 would decouple the positive OP amp input from Ground. That makes sense.
Did you take one apart Rattus?
Without C2, there would be no voltage across the opamp inputs.
This thing is basically a three element voltage divider, and we are sensing the voltage across the center element, R1.
Larger values of C2 would allow more current, hence more voltage across R1. We should really say Z1 since we do not know exactly what it is.
All this is in my head. I don't want to blow $15 prying mine apart.
Re: Voltage detectors how work?
I think you are both still missing the boat
There are 4 capacitors. None are electrical parts soldered inside the detector. They are all "stray" capacitors. You can't get rid of any one of them even if you wanted too.
They form a bridge, with the resistor across the middle. The voltage is developed because one side of the bridge is closer to the voltage, and the other side is closer to ground.
But one could probably use math to reduce the bridge circuit to an equivalent circuit (like Rattus's voltage divider).
By the way, does our tester work on DC also, or is is limited to AC? I don't see anything that prevents it from working on DC.
Steve
I think you are both still missing the boat
There are 4 capacitors. None are electrical parts soldered inside the detector. They are all "stray" capacitors. You can't get rid of any one of them even if you wanted too.
They form a bridge, with the resistor across the middle. The voltage is developed because one side of the bridge is closer to the voltage, and the other side is closer to ground.
But one could probably use math to reduce the bridge circuit to an equivalent circuit (like Rattus's voltage divider).
By the way, does our tester work on DC also, or is is limited to AC? I don't see anything that prevents it from working on DC.
Steve
Re: Voltage detectors how work?
Steve,
Yes, you can look at this thing as a bridge, but not in the ordinary sense because the bridge is unbalanced, and this fact makes the thing work.
If we periodically force the opamp input to zero, then the AC signal will produce a signal across R1 when the zeroing function is complete.
It is likely that R1 does not exist, merely stray capacitance instead.
No, this scheme will not work with DC, although such a device might be possible.
Steve,
Yes, you can look at this thing as a bridge, but not in the ordinary sense because the bridge is unbalanced, and this fact makes the thing work.
If we periodically force the opamp input to zero, then the AC signal will produce a signal across R1 when the zeroing function is complete.
It is likely that R1 does not exist, merely stray capacitance instead.
No, this scheme will not work with DC, although such a device might be possible.
- Status