Is my 15KV neon light transformer broken ? - Page 1
Forum section: electronics / beginners

author: BravoV
author_rank: Super Contributor
timestamp: March 20, 2019, 01:24:17 pm
content: Just scored this for free, and to be honest I was suspicious and curious why it's free ?But I couldn't measure it as I scored it while I was on traveling, so I paid just the shipping charge to send it back home.With primary winding is rated for 220VAC at 2.25 Amp, while secondary with middle tap connected to chassis with 15KVAC at 30 mA, photo & details below.Took out my LCR meter (4 W) and started to measure all windings with these results :Question :- Is it normal for the primary winding's DCR at that low ?- Also while still on DCR mode, measured the resistance between primary vs secondary, the LCR meter showed infinite resistance, but sometimes once in a while flipped/flickered to 50 to 60 Mega Ohm reading, shorted inter-windings ?

author: rsjsouza
author_rank: Super Contributor
timestamp: March 20, 2019, 01:33:58 pm
content: I don't have my NST here to test, but 2.5 ohms on a 450VA transformer is not really unusual. The AC impedance would be higher.The resistance between pri-sec is a bit more worrisome, but at those levels the ohmmeter can be very susceptible to external factors.If you have a Variac, all your questions will be answered.

author: Fraser
author_rank: Super Contributor
timestamp: March 20, 2019, 01:36:06 pm
content: This thread may be of interest. You can calculate the inductive reactance of the primary.https://www.physicsforums.com/threads/mains-transformer-with-1-4-ohm-primary-winding.924900/HT transformers can arc across to the primary winding if they are not on separate bobbins and this used to happen in oscilloscope transformers. I would not expect a readable resistance at the voltage used in your meter test though. More likely it is your meter reading an infinite resistance and responding in an unpredictable way due to RFI etc.If in doubt, connect a 240V 25W light bulb in series with the primary winding when first applying power to the transformer. If the secondary RCA’s over to the primary no great harm will occur. It is not uncommon for these Neon drive transformers or output connecting cable insulation to fail, causing the classic flickering Neon sign. Yours may well be OK.Fraser

author: BravoV
author_rank: Super Contributor
timestamp: March 20, 2019, 03:15:50 pm
content: Quote from: rsjsouza on March 20, 2019, 01:33:58 pmYeah, cause I was expecting pri-sec should have infinite resistance, but what external factors are you talking about ? As the transformer was not connected to anything except the LCR meter.Quote from: rsjsouza on March 20, 2019, 01:33:58 pmStart at really low voltage at primary using variac, and slowly crank up while measure the secondary ?To be honest, all my meters are maxed out at 1KV, and quite scare as this is lethal voltage, still thinking how to properly isolate the output terminals before I start powering it.

author: BravoV
author_rank: Super Contributor
timestamp: March 20, 2019, 03:26:37 pm
content: Quote from: Fraser on March 20, 2019, 01:36:06 pmThanks for the link.So, my primary reactance based on my measurements above ...Reactance := 2 x pi x freq x inductance= 2 x 3.14159 x 50 Hz x 0.414 Henry= 130.06So at 220 VAC the primary current will be -> 220V / 130.06= 1.69A ?Much lower than the rated current (2.25A) as printed at the label isn't it ?Quote from: Fraser on March 20, 2019, 01:36:06 pmAre you saying at both isolated windings behave like sort of antenna, that caused that flickered high resistance reading as I experienced ?Quote from: Fraser on March 20, 2019, 01:36:06 pmNoted, will do, but as previous post, I will find a safe isolation caps 1st at that two HV terminals before powering this thing.Also currently I don't have any HV probe to test it's output, really tempting to build an adhoc Jacob Ladder though, just to test it's output.

author: chemelec
author_rank: Contributor
timestamp: March 20, 2019, 04:45:54 pm
content: I have Numerous Neon Transformers, but Being in Canada, they are all rated at 110 VAC for the Primary.Here is a Picture of My Jacobs Ladder in Operation.Look Closely and you can see the Arc.

author: rsjsouza
author_rank: Super Contributor
timestamp: March 20, 2019, 05:12:12 pm
content: Quote from: BravoV on March 20, 2019, 03:15:50 pmAs all LCR meters I know put out AC at various frequencies, any capacitive coupling may cause it to falsely read current flow, thus skewing the measurements.Obviously that you can simply try to test with a regular DMM's ohmmeter (which only puts out DC) and see if you have any of such effects in your measurement.Quote from: BravoV on March 20, 2019, 03:15:50 pmExactly. Start at zero on the output of the Variac and increase it but never go over 14V.For your transformer the turns ratio is15.000V/220V= 68The maximum input voltage then is:1.000V/68= 14VThe best way to experiment with this is across a very large area and clean of debris and other junk - if you want to do this on your bench, make sure it is clear of pliers, components, vises, screwdrivers, metallic boxes or plates, etc. Give wide clearance between wires on the high side and, if possible, keep them lifted off the ground. Don't rely on probes and use alligator clips on connections - you can also insulate these connections with insulating tape.Once you get the high voltage bug, you can't shake it anymore off you.

author: BravoV
author_rank: Super Contributor
timestamp: March 21, 2019, 05:03:24 am
content: Quote from: rsjsouza on March 20, 2019, 05:12:12 pmGood idea, I think I have a small low wattage step-down transformer 220V -> 12V tucked somewhere, might be handy coupled with variac, thanks.Quote from: rsjsouza on March 20, 2019, 05:12:12 pmNoted, yeah, HV bug, starting to worry about it too as this is my 1st venture at > 220V.

author: beanflying
author_rank: Super Contributor
timestamp: March 21, 2019, 05:15:10 am
content: Unplug anything sensitive EVERYWHERE get a few bits of heavy copper wire with a few bends 10-12mm gap opening up and then stand back and TESTI ran an old scope on the Mains and a Clamp meter to check current and it was fairly clean and in spec so I am now less cautious when playing.Mine is actually a 277V 60Hz Primary which runs fine on the local approx 250V 50Hz. Secondary was rated at 15kV so it will be down a bit.Still planning to refurbish it and do something about safety of the ladder so it can be used in public lest any human smoke come out.https://youtu.be/aGucgApUb64

author: BravoV
author_rank: Super Contributor
timestamp: March 21, 2019, 07:14:19 am
content: Quote from: beanflying on March 21, 2019, 05:15:10 amDamn, I think I got bitten by Jacobs Ladder bug, really, I didn't have any plan on this when I acquired it, as it was sitting gathering dust and was offered to me for free.I guess you mean refurbish it, just by remove the rust and re-paint the whole box only right ?Cause I opened the top cover, the whole transformer and all it's wiring is sort of submerged completely into a solid hardened asphalt, looks like there is no simple fix if there is a problem inside, without getting the nasty work of removing that black thingy 1st, attached below how it looks inside.

author: beanflying
author_rank: Super Contributor
timestamp: March 21, 2019, 08:14:45 am
content: Yep external Scrape and paint. Most of them are Bituminous filled or similar. I have already stripped and cleaned the terminals and insulators as much as possible as mine was well used.A few basic cautions 15kV will travel over moist surfaces! Assume everything around the transformer can conduct when you are first testing it.Play Safe

author: BravoV
author_rank: Super Contributor
timestamp: March 21, 2019, 08:18:34 am
content: Quote from: rsjsouza on March 20, 2019, 05:12:12 pmUpdate, tested using Fluke 189, manual range at Mega Ohm range, it started to flicker from infinite resistance to about > 200 Mega Ohm and slowly , I mean like at few seconds starting to crawl upward like 230 .. 240 ... 250 and so on.

author: BravoV
author_rank: Super Contributor
timestamp: March 21, 2019, 08:25:37 am
content: Quote from: beanflying on March 21, 2019, 08:14:45 amThanks for the reminder, yeah, I will be very cautious and triple check at every connection made, and probably quadruple checks all over again before I power this thing. Also it will be put above a stool in the middle of the room, instead on top of bench table.Waiting for my order to arrive of few meters of HV motor spark plug wire at local motorcycle repair/modding shop.

author: Psi
author_rank: Super Contributor
timestamp: March 21, 2019, 09:45:14 am
content: if you're worried the primary might short you could put a resistive load in series with NST primary for the first power up.A high wattage mains voltage incandescent lamp is a good load for this, if you still have one around somewhere.If anything shorts out the lamp will just light up and nothing will go bang.Note that you will have to remove this lamp before trying to get arcs from the secondary. The 15kV 30mA will be too much to draw through a normal 100/200W lamp. So the lamp will lightup if you try get any arcs.The usual failure mode for a NST is one of the two output sides going short.So you get an arc from one side to case but not the other side to case.

author: Zero999
author_rank: Super Contributor
timestamp: March 21, 2019, 10:33:37 am
content: Quote from: BravoV on March 20, 2019, 03:26:37 pmThe primary's inductance should be much higher than that. The magnetising current will be tiny, compared to when it's loaded.The problems is the inductance of the transformer will be frequency dependant and meters often test at a much higher frequency, than the mains.I think the transformer is most likely fine. As mentioned above, connect it to a spark gap, turn on the power and keep well back. Note that15kV at 30mA, is a high enough voltage and current to be lethal!

author: BravoV
author_rank: Super Contributor
timestamp: March 22, 2019, 02:29:31 am
content: Quote from: Psi on March 21, 2019, 09:45:14 amOk, noted.Also, the case connection from middle tap, its just for ground connection came from the mains ground line, am I right ?

author: BravoV
author_rank: Super Contributor
timestamp: March 22, 2019, 02:31:00 am
content: Quote from: Zero999 on March 21, 2019, 10:33:37 amI guess I will have to try it myself, will be using variac, and still waiting for engine spark plug cable to arrive.

author: Psi
author_rank: Super Contributor
timestamp: March 22, 2019, 06:41:15 am
content: Quote from: BravoV on March 22, 2019, 02:29:31 amCorrect, HV center tap is mains earth.The manufactures are cheap, or maybe smart, and get around some rules for insulation/isolation of high voltages by making +/- 6kV outputs instead of one 12kv output.A single 12kV output would require more insulation and be more expensive to make.