# design of a circuit for a polarity tester for me to use at work as an electrician

#### AshPant

Joined Sep 12, 2019
9
basically I have been trying to learn a little electronics to make myself a tester for work but am finding out that the circuit I require is more complicated than I thought and that Im very good at wiring and not so good and designing electronics.

what Im trying to do is make a polarity tester for work. at the moment I use a resistor block that I made and a modified multimeter.

I need a tester that I can just plug in and it just displays a pass fail by way of green or red LEDs I can build it but I need an electronic design.

so I have a resistor block with 10 ohms for Active conductor 20 ohms for neutral conductor earth is on the other side of the 2 resistors but I can make a new resistor block if other resistance values work better.

so the tester that I plug in to power sockets needs to have:
3 wire input ACTIVE, NEUTRAL and EARTH (linked to the resistor block)
1 green LED if the resistance equals 30 ohms between ACTIVE - NEUTRAL or a red LED if the resistance is more than 2 ohms different
1 green LED if the resistance equals 10 ohms between ACTIVE - EARTH or a red LED if the resistance is more than 2 ohms different
1 green LED if the resistance equals 20 ohms between NEUTRAL - EARTH or a red LED if the resistance is more than 2 ohms different
a 9V battery supply
and for safety it needs to either trip something out or cope with 240v between ACTIVE - NEUTRAL and ACTIVE - EARTH (maybe light up a danger indicator lamp)

would something like this be very complicated ? or is there a simple solution?

Thanks guys and girls

#### jpanhalt

Joined Jan 18, 2008
11,087
Are referring to an outlet tester such as one might use on a 3-prong wall socket to see if there is a ground and whether the wiring is correct?

If so, what socket(s) do you need to test? Why not buy one that is certified in your country for use?

#### ci139

Joined Jul 11, 2016
1,898
note :: of the not too long (10-s of meters) parallel running "live" and "from one end grounded" cable-pair the not live can show 100+V at the other End and enough current to induce spark - how to tell to your apparatus that it's not the cable right next - i don't know

... i guess it goes putting some "favorable" frequency through capacitive decoupling and somehow extracting your pure resistance part of the returned "live" or not impedances of the 3 conductor transmission line ??? . . . never done this

most close (that can be found as a reference from the web) is a shielded twisted pair -- https://electronics.stackexchange.c...air-balanced-transmission-line-implementation (that hopefully contains keywords and references to dig on) → → https://en.wikipedia.org/wiki/Twin-lead#Characteristic_impedance ≈ Z=Z₀/(π√ε,)arch(D/d) assuming D=2.5mm d=1mm ε,=? 1.2 ? → Z ≈ 170Ω for twin lead . . . ← has a nerby result ← Automotive Power Line Communication
(pg. 70, 71)

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#### AshPant

Joined Sep 12, 2019
9
all the polarity testers on the market test once the circuit is live. I need one that tests before i liven up.

The functiin that lights up when put on to 240v is just incace someone mistakenly plugs it in to a live socket.

All the resistance testing functions happen when there is no 240v ( dead)

#### jpanhalt

Joined Jan 18, 2008
11,087
Why type/system of outlet are you testing? That is, is it L1 + Common (grounded) + Ground (safety) or L1 + L2 + ground or L1 + L2 + Grounded + Ground(safety)?

#### ci139

Joined Jul 11, 2016
1,898
needs to have:
you cannot sense voltage (fast) but only current at such distance

e.g. you can't put 12V on 30 ohm and reliably get 4V from divider (unless you sample a relatively long time with some sort of integrator)

so you commute the pairs of wires.. . . (manual switches - actually may have contact resistance 5Ω and greater !!!) . . ..for 3 tests each of which lits the led (you need window comparator(s) for that or a functionally alternative circuit)

going "automated".. . . (electrical commutation - requires modding your setup - for differential measurement - otherwise you get line noise and NO VALID DATA) . . ..and storing results for simultaneous display will add cost and complexity also reduces reliability ... somewhat

it could be easier (for example) to set some ticker at the other end that - say - connects wires momentarily at the set rate and get it at the metering side -- the point is to set up a (dynamic) "marker" that can be uniquely determined - and verified by prolonged measurement (e.g. the longer you measure the more certain it becomes) EXAMPLE
a fix → & a prinipial (partial design of) what you are asking (drop .txt to falstad menu/file/import ..text) ← requires (±9V) bipolar supply for OpAmp to work and voltage to be sufficient for steady constant current generation

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Joined Jul 18, 2013
28,026
If measuring outlets etc that have no power and have earth ground, why not just test the resistance from N to GND pins Using an ordinary VOM.
In N.A. the N is connected to earth ground conductor at the panel, so the resistance should be very low.
Max.

#### jpanhalt

Joined Jan 18, 2008
11,087
On re-reading the question, sure sounds like homework. If not, where did the TS get those requirements for the LED's without also finding a circuit?

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,088
Aside (live circuits - 120V):

Ideal Suretest

At work, we had a really nice outlet tester, which I did find recently. It does the standard polarity thing and it also had a ground integrity test.

Sometimes, I like the go/no go testing and sometimes more.

For Ethernet cables there is a simple LED tester that checks the pairs and whether it's cross or straight by lighting red or green. It could be used to check the ends of a cable. You can wiggle things and see if the lights go out.

I have both the Ideal Suretest and Suretrace (old model) and a modified 3 Neon lamp tester. I mounted a bubble level on the neon tester. Really helps.

I had an issue where the 430 count Pass and Seymore outlets in our building could possibly be defective. Warranty had passd. No one wanted to replace or test all of them either. The ground connection was friction based - brass to plated steel. Wen you took an outlet apart, you could see evidence of sparks. Turns out that with the light based ground integrity tester, you could use the tester and a blank plug (IEC power cord) and wiggle the plug on the power cord. If you could get the light to go out, the outlet was bad. You had to use both outlets of the duplex for the problem to occur. You would end up with 60 VAC between grounds.

Management took the approach of all ivory colored outlets that were replaced turned to brown colored. All outlets located in a laboratory got blanket replaced. hall receptacles got a green or red colored dot if they did not share a lab circuit. The floor polishers, could knock out a circuit due to overload. Office outlets did not get checked unless you had a computer.

#### AshPant

Joined Sep 12, 2019
9
Wow thanks. I have one of those ethanet testers. Maybe i could somehow mod that.
I shows green led for a pass and red for fail. If i somhow made it check A-N as a pair then switch to A-E then N-E. It could just flash each one in sequence. Then all i need is a safety to protect the circuit if someone accedently plugs it in to 240V

Only problem is i wont be able to tell if for example A and N are reversed......or will I

Might have to do some feild testing