Live/Neutral of Home-Built Transformer

My apology for this post. I don't really know how a test pen works, just some idea "some" people use it to check for "live" wire. Of course, it works if the test pen is working.

I dropped out of 1st year engineering university long ago and just have some basic knowledge of electric circuits, etc. I do suspect test pen won't work on any of the 9V and 6V terminals; not proper way to test any live.

Thanks for your suggestion about fuse for my home transformer.

While the <...test pen...> does have its uses no Electrician would rely on one to confirm either live or dead.

Other test instruments with a higher reliability are commonly used - for obvious safety reasons.

Why not purchase light bulbs capable of operating at either 9VAC or 240VAC?

-OR-

Why not purchase transformer 240VAC to 6V in the first place?

It sort of seems the tail is wagging the dog here.

I need my home built transformer for some experiments. Lighting up the 6V bulb is just to check the transformer is working.

Well, okay.

Monitor your experiment carefully, and give the advice from PWSlack in post #1 thorough consideration. Your transformer may act very much as an induction heater, resulting in an undesired unscheduled thermal event in your experimental work envelope.

Have fun, but be safe and don’t burn your house down.

Thanks for your advice about induction heating.

<...the 6V bulb is just to check the transformer is working...>

A suitable test instrument would do this without risk to the <...transformer...> or the wiring and will have loads of other applications. A suitable basic hobby multimeter, for example, may be purchased ready-to-rock for less than, say, £20GBP retail nowadays.

Yes, I am aware of it. Just test the open circuit voltage of the secondary with a multimeter; low voltages, only 3 to 6 volts. I would add first some simple LED lights for quick confirmation.

I just read an article from the internet that tried to explain how a neon bulb test pen works. It seems the author is wrong!

He said when we insert the tip to a source (live) and our finger touch the end cap, there is current flowing (suggesting through our body). He said we must be in contact with the "ground". I remember somewhere someone mentioned the test pen would still light up even if we are standing on a rubber mat or floor. What is correct.

OK. I viewed a better explanation from a youtube channel. It says our rubber shoes is an dielectric and forms a capacitor(I don't understand this part too well). So with our shoes(our body) as a "capacitor", then there is a complete circuit. So the 240VAC live indeed would cause a current flow - just very weak.

Because of "capacitance", the test pen would work only on 240VAC and not 240V DC.

Ferrite cores may not be useful for 50/60 Hz signals unless they have a lot of turns. Your mains transformer is laminated steel and is useful but has even more turns. You could just use a series resistor to drop the 9V to 6V, but the value would depend on the light bulb current. You need a multimeter rather than a test pen to check the voltage. There are inexpensive ones for about $20. The secondary is isolated from the primary, so either terminal could be called "live" with respect to the other terminal. That is why they normally have the same color wire. Good luck.

I have a new cheap multimeter; works OK from China. I have not considered using the test pen as I won't be testing the 240VAC live. The only test I may be doing would be with a multimeter for 6 - 12VAC thereabout.

I do have some idea that adding a resistor could drop secondary 9VAC to 6VAC. But my experiment must involve my home-built transformer, about 9VAC/6VAC, the 9VAC supplied by mains step-down transformer.

Indeed, I am not sure of a soft-iron core or ferrite. It is easy for me to get ferrite core online, but a little difficult for soft-iron rod. Though my transformer would not be operating for longer then for me to do some voltage measurements(say at most one minute), I still don't like soft iron because of eddy current heating. My primary winding is over a 16mm UPVC pipe.

I have ordered a ferrite rod 12mmx100mm. My enameled copper wire is AWG 19. I am not sure if prim 150 turns, secondary 100 turns enough for me with ferrite rod. I am not concerned with efficiency at all. My 6VAC output need only to light up my E10 6V bulb for just 10 seconds would do.

I could change to a soft iron core. If the ferrite rod works, it is enough for me.

I believe the "test pen" device you refer to is more of a safety or line tracing device intended to indicate "grid voltage present" or "grid voltage not present". If you are trying to do home experiments then it is not a very useful tool.

A multimeter would be a much better instrument for you to use. I don't know where you are, but Amazon, Ebay, Harbor Freight and many other vendors offer low cost meters. A good multimeter could be rather expensive, but the very low cost meters are good enough to be very useful for home experiments.

Good luck and keep experimenting. Always think about safety.

I agree with others that the test pen is not likely to work and anyway would be insufficient info to validate the transformer.

You need a multimeter to measure the real AC voltage and current at the secondary. This will allow you to measure the open circuit voltage and the short circuit current using a resistor in series with the primary.

If you still use the iron rod, open core design, my guess is that you will be disappointed with the low values obtained at the secondary due to the large leakage inductance of your design. The secondary voltage will collapse with the slightest load.

As for the protections, the power and voltage involved are so small that they are unlikely to be needed. Maybe a small fuse on one of the primary lines to protect the source from the overload this homemade transformer is likely to present due to the very low magnetizing inductance (not enough primary turns).

Your design needs to be upgraded to the complete laws of physics. Presently, you are only using the basic principles that are too simplified to produce a workable unit. You will learn a lot though, and this will motivate you to learn the rest.

That a process similar to what I did many years ago.

Good luck and have fun! There are many wonders ahead of you!

I have a new cheap multimeter, but I think OK. I am not using a test pen. My main's step-down transformer is the isolated type, not auto.

My present condition is that I could only use the simplest of design. I have a ferrite rod inside my UPVC pipe. The primary over the pipe and the secondary over the primary. It would be very difficult at present for me to build using a closed square laminated steel core. I have to do the copper windings myself.

Sure, I am a little worried that I don't have enough wire turns - 150/100. The inductance of the primary may be too small and may draw too large a current from my mains. I intent to have a 3A fuse in my primary.

Hope my 6V torchlight bulb could light up.

Thanks.

The UPVC pipe may be a problem. You would get more coupling if you wind directly on the ferrite rod. The mains transformer will probably take a short circuit for a few seconds without a problem, but the fuse is a good idea. A better solution is to use a POT-CORE instead of a ferrite rod. That will give more coupling yet and may have room for more windings if it is a big one. But try what you have and go from there. Keep trying and learning!

https://www.ebay.com/itm/263853214031?var=0&mkevt=1&mkcid=1&mkrid=711-53200-19255-0&campid=5338590836&toolid=10044&customid=6204e89d9fb912b4f9858ef04c64dbc5

My experiment happen to need my current design: single rod core, primary over core, secondary over primary; a closed core, e.g. square would not work because of the peculiarity of my intended experiment.

I think I would be using just tiny white indicator LED lights; I have a bag full of it from my nephew; can't ask him now any "rating" as he is now out of town. I check with online sources and some give 3.2V, 20mA - 0.06 watt and not 3 watt!

Pot-cores are round, not square. See the link I sent you. LEDs should work without a rectifier. Just try them with AC. They will draw current on one of the half-cycles. What have you found so far?

Yes LED may work and operate with AC voltage as its source, but will be dimmer and not be as bright as expected or as per the specifications..

You said “Hope my 6V torchlight bulb could light up.”

There we are... been waiting for this.

It (your 6 volt DC lamp) will likely not illuminate with AC source.

Have you fitted a full wave bridge rectifier?

I am really surprise if incandescent bulbs cannot work on AC. It is based on just joule heating. Maybe you mean 6V AC not sufficient to light 6V DC bulb.

You are correct. Incandecent works on AC and DC.

You could place another ferrite rod in parallel (outside of your coil) with two little pieces at each ends to close the magnetic loop. Glue the pieces together, minimizing the airgaps, and you will have something similar to a "real" transformer core.

You can just use one high intensity LED for your torch.. which would be vary bright and operational using your limited capable power supply...

AMAZON would have several types similar to those used in high intensity flashlights.. Typically operating at about 2 volts DC at which means that you will need to rectify your AC power source.. and the current drawn would also be less than 1/2 amp..

The bulb's brightness or intensity level is typically measured by using the Lumens not by the wattage used on incandescent lamps being a very inefficient device..unlike in the LEDs..

An important reminder For your personal safety,

Just make sure that the stepdown transformer 240vac/ 9vac, that you are going to use is the INDUCTION type transformer, NOT the autotransformer type!

The neon lamp test may work only if the step down transformer, 240vac / 9vac, is of the autotransformer type.... but is dangerous if you happen to touch with your bare hands..

VSAR is correct but he meant "isolation transformer" not "induction type transformer".

Very true..the 9V generation and power transfer is achieved through the inductive coupling of two physically separate magnetic coils or windings ..

The electrical connection is only via the magnetic field which makes the transformer isolated from the main power source..

Thanks. I am aware of the dangers. "Little knowledge kills"

Thanks for all the suggestions. My experiment has not started yet as I have to wait for some other components due to personal circumstances, maybe 2 weeks.

Closing the gaps with pieces of ferrite rods is interesting. My lights can be any type; the "lumens", etc.. won't matter in my intended experiments.

I will post about the success or failure of my experiment when experiment done.

Thanks.

We may have some difficulty with semantics here. When you say "test pen" I suspect you are referring to a non-contact voltage tester (also known by many other names/descriptions). I've used a number of them; my current one is a Klein NCVT-2, which has two voltage ranges: 70-1000VAC, and 12-1000VAC. There may exist others that can detect lower voltages, but I haven't seen one that could measure as low as 6 VAC.

There have been a couple of references to neon testers, which also exist in a pen-type format. A standard neon lamp needs something around 65 Volts before it will illuminate. Again there exist special neon lamps that can illuminate at somewhat lower voltages, but again, nowhere near 6VAC.

Thus the only common devices that will work for your experiment are low-voltage AC voltmeters and the lower AC voltage ranges on multimeters. Since you have a multimeter, forget the other devices.

As someone pointed out, ferrite cores are normally used at much higher frequencies than the power line frequency, and will be very inefficient at that low frequency. This means that your ferrite transformer may be used to verify the relationship between turns of wire and voltages, but the current output will be extremely low. Don't expect any incandescent lamp to illuminate.

Your chances are much better using an LED. You mentioned a white LED; white LEDs generally require more current than single colored ones, so your best bet would be using a red LED (Red is the lowest energy of the visible spectrum, so should require the lowest possible current).

In my country, we use to say a "test pen". I can confirm it is called a tester elsewhere. It is a screw-driver type or with a pointed tip. I can confirm because of the videos saying "tester".

I just read about how a tester work - simply our rubber boots is just the dielectric. I have a complete understanding (I don't really mean to use it seriously). I do have in my possession one rated for 120/240VAC and another 6/12VAC. I am surprise you say don't have low voltages tester.

Just for theory - I may be wrong. My mains transformer secondary 9VAC has no polarity. If I earth my white common by connecting to the neutral of the primary, would I now not have an auto transformer? My understanding now (???) is the secondary 9VAC could also be used "as usual". The 9VAC would now be live 9VAC. Would my 9VAC tester now light up on this 9VAC?

I'll just try with any tiny LED and see if it works - micro currents would be enough for my experiment.

A1) Yes, though <... connecting to the neutral of the primary...> is not an <...earth...> connection. It is unwise to construct an <...autotransformer...> for these experiments without demonstrable electrical training. It is possible for upstream circuit protection equipment to operate and disconnect if the connection is incorrect. Were the upstream <...neutral...> to become disconnected for any reason, all downstream circuitry referenced to it would tend towards the incoming line potential. For that reason, it is safer for the secondary to be referenced to the <...earth...> conductor rather than <...neutral...> conductor.

A2) A <...9VAC tester ....> is not a <..."test pen"...>. It should say either on it, or with the instruction leaflet that came with it, or both, what it will and will not do.

Asking these sorts of questions introduces worry that the concept of RTFM has not been applied, as there is a risk of getting hurt/killed. Please ensure all other CR4 readers are away from the test area when these experiments are being carried out.