Detecting 240 volts between 120 volt outlets

Diodes in series with each probe. Instead of measuring the voltage difference you'll need to measure the phase difference of the DC pulses from each leg.
That strikes me as difficult unless your test set is provided with a separate reference ground.
Hmmm... that's a real judgement call as to what the readings mean. You might need an embedded microprocessor with a small hard disk to accomplish that one! :-)
Kick the doctor if he whacks your knee with a rubber mallot?

Would limiting the current that could flow to a very tiny amount be sufficient?
(Folks routinely use thier body to "ground" one side of a "neon tester" when searching for "hots.")
I just don't see how two neon testers in series could present any kind of safety issue.

Phil, somebody already makes this. I think they just have a relay on each phase to neutral so it won't connect to the "common" side until it sees line voltage.

|>I would like to design a circuit which would have TWO normal USA (NEMA 5-15P) |>plugs, and when plugged in to two outlets, would detect not only if these |>are wired corrctly (e.g. like an ordinary circuit tester), but also detect |>if there is 240 volts between them. |> |>THE BIG SAFETY ISSUE: there must be NO VOLTAGE on the 2nd plug's prongs |>when the 1st is plugged in, regardless of which is plugged in first. | | Diodes in series with each probe. Instead of measuring the | voltage difference you'll need to measure the phase difference | of the DC pulses from each leg. | |>I have not yet started to design this. I've just recognized a need to have |>such a device, but have never seen such a device. |> |>Additionally, I'd like it to be able to detect 240 volts between outlets |>even if either or both are wired wrong. | | That strikes me as difficult unless your test set is provided | with a separate reference ground.
Why would ground be needed. Suppose you have 3 terminals on your left and 3 more terminals on your right. You have a volt meter in hand. You connect probes to all 9 combinations of comparing the terminals on the left with the terminals on the right. Did any show 240 volts? Did you need ground to do that?
|>And then later, I'd like to add on a capability to detect if this circuit |>is wired with a shared neutral, or wire with separate neutrals back to its |>panel source. In this case it would require an ohmmeter sensitive to the |>subtle difference in low resistance of the lengths of wire that could be |>involved. | | Hmmm... that's a real judgement call as to what the readings | mean. You might need an embedded microprocessor with a small | hard disk to accomplish that one! :-)
It might have to depend on the wire size.

| |> THE BIG SAFETY ISSUE: there must be NO VOLTAGE on the 2nd plug's prongs |> when the 1st is plugged in, regardless of which is plugged in first. | | Would limiting the current that could flow to a very tiny amount be | sufficient?
Perhaps. How about 1 milliamp?
| (Folks routinely use thier body to "ground" one side of a "neon tester" when | searching for "hots.") | | I just don't see how two neon testers in series could present any kind of | safety issue.
How much current flows in these?

| |>I would like to design a circuit which would have TWO normal USA (NEMA 5-15P) |>plugs, and when plugged in to two outlets, would detect not only if these |>are wired corrctly (e.g. like an ordinary circuit tester), but also detect |>if there is 240 volts between them. |> | | Phil, somebody already makes this. I think they just have a relay on | each phase to neutral so it won't connect to the "common" side until | it sees line voltage.
That's actually a good idea.

The "if either or both are wired wrong" is the key to my comment. You have to have at least one known in order to determine if the others make sense.

I picked up a time domain reflectometer in an MoD (military surplus) auction, some years ago - for 9GBP, IIRC.
It has proven itself absolutely invaluable for all sorts of fault finding and "what the heck have we got here!" scenarios.
For the UK ring main, it shows where every socket on the ring is, in distance from the measuring point. It also shows where spurs have been taken off (usually under the floorboards) and where cables have been joined. It shows earth continuity a treat - although it is *not* an adequate test for earth impedance. It is great for lighting circuits, too.
In the OP's situation - it would instatly show how long the neutral wire was, before it joined something.
I don't know how much one costs new these days - but did notice 3 going in an auction only a week or so ago, so they are still available for not a lot, in this part of the World, at least.

Don't know off hand. I suspect it is a fraction of a ma. The voltage drop across the bulbs is on the order of 65 volts so it just would not take much current to make a light bright enough to see under ordinary conditions.
If you live in the US, you can often get a cheap "neon tester" for about $1.00. You can "sacrifice" it and measure the resistance of the built in device. Or you can get a neon tube and experiment yourself. Select a value that will give a 1/2 ma to start.
In you application, the "220 volt" sensing section would have twice the resistance of the "110 volt" sensing section.

I see someone has beat me to the relay connecting to the rest of the circuit when it sees 120V on its coil (x2).
Also, what possible use is such a device?
snipped-for-privacy@ipal.net writes:
Means a minimum 120K resistor in series with each lead. You could easily use larger resistors connected to somewhat sensitive electronics if necessary.
Neon bulbs attached to 120V typically have a 470K series resistor in one leg. They usually need 90+ volts across the bulb to fire and 60V to stay on.

From: Phil
I would like to design a circuit which would have TWO normal USA (NEMA 5-15P) plugs, and when plugged in to two outlets, would detect not only if these are wired corrctly (e.g. like an ordinary circuit tester), but also detect if there is 240 volts between them. Phil what are you up to now ??? }:-) You are trying to make life easier by complicaing it more :-) Why 2 PLUGS ?
what you need is a crew of high speed nanobots (Electronic Troubleshooters).
Sue seems to have it almost down to a T.
You'd insert them into an outlet (via a coupler) having programed them to report on every outlet in the branch and to identify any seperate branch circuit outlet on a different phase/neutral in each room/wall/etc.,and they'd tell you if any strands where missed in the wiring connection, faulty contacts, and any pertinent adverse wiring/device conditions like broken insulation, brownout, demished conductance etc.., All through an lcd color monitor with a digital interface linking down to every individual nanobot, voice command optional };-)
=AEoy
email me when you're ready to design it

prongs when the 1st is plugged in, regardless of which is plugged in first. <
let's see TWO PLUGS end to end with some type of monitoring/reporting box in between that would withhold the voltage at the end plug until it is connected ??? test procedure imagery unclear and awkward .... but Hmmmmmm.
Semiconductors come to mind but I'd rather wait until we train some nanobots };-)
=AEoy

LOL, no need, they used to be seen everywhere under the=20 different name of "apprentices".. I think I was a nanny bot..
The TDR is pretty useful for many, many things - but can't=20 make a brew. ;)
--=20
Sue
I'd better not say what I use my Polyskop and LCR bridges for...

From: Phil I would like to design a circuit which would have TWO normal USA (NEMA 5-15P) plugs, and when plugged in to two outlets, would detect not only if these are wired corrctly (e.g. like an ordinary circuit tester), but also detect if there is 240 volts between them. Phil what are you up to now ??? }:-) You are trying to make life easier by complicaing it more :-) Why 2 PLUGS ? what you need is a crew of high speed nanobots (Electronic Troubleshooters). Sue seems to have it almost down to a T. You'd insert them into an outlet (via a coupler) having programed them to report on every outlet in the branch and to identify any seperate branch circuit outlet on a different phase/neutral in each room/wall/etc.,and they'd tell you if any strands where missed in the wiring connection, faulty contacts, and any pertinent adverse wiring/device conditions like broken insulation, brownout, demished conductance etc.., All through an lcd color monitor with a digital interface linking down to every individual nanobot, voice command optional };-) =C2=AEoy email me when you're ready to design it LOL, no need, they used to be seen everywhere under the different name of "apprentices".. I think I was a nanny bot.. The TDR is pretty useful for many, many things - but can't make a brew. ;)

|>|>|> |>|>Additionally, I'd like it to be able to detect 240 volts between outlets |>|>even if either or both are wired wrong. |>| |>| That strikes me as difficult unless your test set is provided |>| with a separate reference ground. |> |>Why would ground be needed. Suppose you have 3 terminals on your left |>and 3 more terminals on your right. You have a volt meter in hand. |>You connect probes to all 9 combinations of comparing the terminals on |>the left with the terminals on the right. Did any show 240 volts? |>Did you need ground to do that? | | The "if either or both are wired wrong" is the key to my | comment. You have to have at least one known in order to | determine if the others make sense.
If you have something that activates only with 240 volts, and you have nine of them connected in every combination, one of them should active if there is any 240 volts there.

| --> Phil what are you up to now ??? }:-) You are trying to make life | easier by complicaing it more :-) Why 2 PLUGS ?
It's a lot easier to test the relationship between 2 outlets with 2 plugs instead of just 1 plug.

| I picked up a time domain reflectometer in an MoD (military | surplus) auction, some years ago - for 9GBP, IIRC. | | It has proven itself absolutely invaluable for all sorts of | fault finding and "what the heck have we got here!" scenarios. | | For the UK ring main, it shows where every socket on the | ring is, in distance from the measuring point. It also shows | where spurs have been taken off (usually under the | floorboards) and where cables have been joined. It shows | earth continuity a treat - although it is *not* an adequate | test for earth impedance. It is great for lighting circuits, | too. | | In the OP's situation - it would instatly show how long the | neutral wire was, before it joined something. | | I don't know how much one costs new these days - but did | notice 3 going in an auction only a week or so ago, so they | are still available for not a lot, in this part of the | World, at least.
Interesting idea. I'm not sure it would tell me a whole lot with respect to the relationship between 2 outlets, but it could tell some interesting things. Now if I could figure out a way to make one that would show me a 3-D plot of all the wiring :-)