Three WTF moments with neon glow bulbs - Page 1
Forum section: electronics / beginners

author: Sredni
author_rank: Frequent Contributor
timestamp: July 18, 2022, 06:05:23 pm
content: As I am an actual beginner in playing with these little glowing monsters, I figured this section would be the right place to post this question.So, I bought some 100 mini neon bulbs on Amazon (don't even think about asking for a datasheet!) and they seem to work just fine (at least for the time being). But I am not so much used to this sort of old tech and when choosing the right number of bulbs (and the corresponding limiting resistance) to put in series in my doorbell I ran into three WTF (Wondrous Technical Facts) moments.I'm gonna spoil the ending: I put three bulbs in series with a 39k resistor and they give that eerily cemeterial glow I was looking for.When testing the series, I used an electrician's terminal block (I am not sure what is the correct term) as shown in this perfectly exposed and well focused picture:What follows is the schematic with the measurements taken in sequence with a low cost multimeter. Note that I am here operating the bulbs near their undecided zone - meaning that a series of four was a bit too much to give reliable illumination, but the problems are still there with a series of three.WTF moment #1Was I so luck as to place the bulbs in increasing order of operating voltage? I don't think so (but I have yet to redo the measurements swapping the bulbs). Is this something to be always expected? Is there a way these lamps know which side the live wire is? (We could use them to make some sort of indicat--- oh, wait a minute!)WTF moment #2Why am I missing more than 20 volts? If I sum the (absolute values of the) voltages across the resistor and the bulbs I do not get all my input voltage. I assume there is a phase problem, but then how do one usually deal when summing voltages with these little demons? (where do I get the phase information?)When I decided I wanted to use only three bulbs, I simply moved the rightmost wire to the previous lamp and repeated the measures:While I am still missing some 10 volts, there is anotherWFT moment # 3How come I have 29 volts on the disconnected lamp?I suppose it's capacitively coupled with... something. After all mains indicators work by driving a current through their invisible capacitance to ground but...Is there I way I could have forecast a ballpark for that value?Can we use it to measure the parasitic capacitance - from what to what, BTW?Ok, I will probably find me a book on neon glow lamps (I already have the GE one, so no need to link it) but maybe in the infinite time before I decide to pick it up and actually read it, someone who played around with this bulbs might shed some light on what exactly is happening. My guess is that we need to take into account the capacitive coupling of everything with everything else, but I am sure some of you guys (especially the old geezers) will have an explanation handy.Anyway, I thought it would have been nice to share these apparent paradoxes...

author: Benta
author_rank: Super Contributor
timestamp: July 18, 2022, 06:20:10 pm
content: You can't do calculations on neon bulb like on filaments or LEDs.The neon bulb has a realtively high turn-on voltage, and as soon as it ignites, the voltage across it drops.

author: Zero999
author_rank: Super Contributor
timestamp: July 18, 2022, 06:31:00 pm
content: Yes, capacitive coupling will explain the voltage where you don't expect it.The live can be determined by measuring the voltage between a conductor and an earthed object. The live will measure the full voltage, whilst the neutral will be close to 0V. You shouldn't rely on the neutral being 0V, relative to earth. It can get disconnected, or even reversed, causing it to be at the full mains voltage.Is the meter measure true RMS voltage? The voltage across the neon lamps and current limiting resistor will not be sinusoidal. This is because the neon lamp is effectively a spark gap. It doesn't conduct, until the voltage across it reaches a certain voltage, then the voltage suddenly drops and it keeps conducting, until the current stops, due to the mains voltage passing through zero. If the meter is only designed for measuring sinusoidal voltages, it will display the wrong reading.

author: David Hess
author_rank: Super Contributor
timestamp: July 18, 2022, 06:42:22 pm
content: At that impedance level, capacitive coupling between the circuit elements, from hot to ground, and especially to the meter input will have an effect. Most voltmeters have an input capacitance of about 100 picofarads if not more.

author: Sredni
author_rank: Frequent Contributor
timestamp: July 18, 2022, 07:43:28 pm
content: Quote from: Zero999 on July 18, 2022, 06:31:00 pmI wonder if it is possible to model it in a tractable way, just to get ballpark guess.But as you say here:QuoteThis will complicate things quite a bit. I will redo the measurements with a true RMS meter (the one I used was not RMS, it really is the most basic of basic multimeters) but most importantly I will make differential measurements with a scope, because I want to see the shape of voltages and current. With four channels I should be able to do two simultaneous voltage measurements.It remains to be seen how much attaching the probes will alter the voltages...

author: Electro Fan
author_rank: Super Contributor
timestamp: July 18, 2022, 10:30:27 pm
content: Might be something of interest in here:https://www.electricalvolt.com/2018/09/what-is-true-rms-measurement/

author: ledtester
author_rank: Super Contributor
timestamp: July 19, 2022, 07:15:51 am
content: Quote from: Sredni on July 18, 2022, 06:05:23 pmI've seen them called "Euro-style" or "Europa" terminal blocks:-https://www.mcmaster.com/european-style-terminal-blocks/-https://www.mouser.com/datasheet/2/418/7/ENG_CD_1776293_P1-2020014.pdfMcMaster-Carr also describes them as "Touch-Safe".The other style with the screws exposed is often referred to as "barrier strip" or just "barrier" terminal blocks.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 19, 2022, 08:56:52 am
content: In the UK they are usually called choc-block or chocolate blocks

author: Terry Bites
author_rank: Super Contributor
timestamp: July 20, 2022, 02:52:09 pm
content: Its unlikely that these bulbs have a consistent strike voltage or dynamic impedance. The electrodes aren't spaced with any precision. Who ever strikes first will have the lower voltage drop leaving the others to fight it out.You could confirm this by plotting the IV characteristic for each bulb.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 20, 2022, 04:37:02 pm
content: Neon lamps, along with other gas discharge lamps, are negative resistance devices. It takes more voltage to start one than to run one. As mentioned already they don't have terribly uniform characteristics (especially with Chinese mystery parts), nor do those parameters stay stable or stabilize much with age. You once could get neons built for logic applications specificically designed to be stable and predictable long term, but they are long gone in any meaningful quantity.Assuming these are running on AC and you can measure it safely with a scope, you should get a pretty informative waveform.

author: floobydust
author_rank: Super Contributor
timestamp: July 20, 2022, 05:17:56 pm
content: Neon lamps were always a bear in applications where you needed a predictable firing voltage. Background radiation, ambient light, temperature - all are big factors. The better (consistent and lower) firing voltage lamps have a radioactive isotope Krypton-85 added (10 yr 1/2 life) but I'm sure that's not happening nowadays. Old 1920-60's gear did use them for opto-couplers - neon lamp plus CdS cell, usually in a linear mode. Nowadays neon lamps are mostly used just as an indicator and quality is low.OP when connecting them in series, you need large value like 1-2MEG bias resistors for each lamp to see enough to ionize regardless of its neighbors.All the details:Using and Understanding MINIATURE NEON LAMPS

author: gnuarm
author_rank: Super Contributor
timestamp: July 20, 2022, 09:43:36 pm
content: Quote from: Zero999 on July 18, 2022, 06:31:00 pmI'm not so sure it does. There might be something if he had his finger on just one probe, but 20-30 volts seems far too much for a probe on a neon bulb. Does it read the same laying on the table???Also, he said he's using a "low cost multimeter", which may be cheap enough to be an analog meter with a 20,000 ohms per volt, which may be enough to load down the circuit. It would explain why the sum of the parts is less than the total.

author: Sredni
author_rank: Frequent Contributor
timestamp: July 21, 2022, 01:08:22 pm
content: Ok, I redid the measurements.1 resistor, three neon bulbs (L1, L2, L3)This is what I get with the nonRMS multimeter (Uni-T136b)When live is on the resistor's sideVtot R L1 L2 L3 L1L2L3235 53.3 64.0 59.8 53.7235.8 52.9 57.7 53.0 52.1 (183.9)When neutral is on the resistor's sideVtot R L1 L2 L3 L1L2L3236.1 52.0 52.1 58.6 62.3 (184.7)As you can see, the nearer the lamp is to the live wire, the higher the (average) measured voltage.And these are the measurements with a true RMS meter, the Uni-T139CVtot R L1 L2 L3 L1L2L3234.4 66.5 55.6 57.5 56.0235.2 66.7 56.7 57.8 56.0 (177.1)235.2 66.4 57.1 58.0 56.2(In the last term of the second row, I probably miswrote 56.2 as 52.0 -edit: I checked, it was 56.0, now corrected)When I flip live and neutral I get the following RMS measuresVtot R L1 L2 L3 L1L2L3234.9 66.2 56.0 58.2 57.1 (177.7)Note that the numbers in parenthesis are actual measurements across the series. And, no, they do not add up, but the discrepancy is lower than with the cheaper multimeter. I also have to stress that these are measurements taken in sequence with one instrument, and the values were not exactly stable over time, even if they always ended up being close together.Then I had a look at the waveforms, and it's clear why the numbers do not add up. I will load the screenshots later.

author: TimFox
author_rank: Super Contributor
timestamp: July 21, 2022, 01:48:41 pm
content: The voltage waveform across a neon bulb in this circuit is far from sinusoidal.When using a true RMS voltmeter, note that the RMS voltages for the components along such a chain do not add directly, but the variances (square of the RMS voltage) should.

author: Sredni
author_rank: Frequent Contributor
timestamp: July 22, 2022, 04:11:17 pm
content: Ok, here are the waveforms. Each screenshot show channel 1, channel 2 and their difference.All screenshots are taken when the live wire is on the 39k resistor's side.This is the voltage across the resistor (I computed the value of the current):Channel1 shows the live sinusoid with respect to gnd - it's about 240V peak and the missing (nearly) 100V peak will be seen on the neutral in the last pictures. If rescaled to be the full 235Vrms sinusoid, we can see that the whole series (R-L1-L2-L3) fires up at about 248V and turns off at about 193V).Voltage across the first lamp, the one near the resistor:Voltage across the middle lamp:and voltage across the last lamp, the one attached to neutral:(I switched to hires for the acquisition because I had unreliable waveforms on screen)

author: ledtester
author_rank: Super Contributor
timestamp: September 14, 2022, 10:32:24 am
content: Quote from: Sredni on July 18, 2022, 06:05:23 pmYou might be interested in this recent video:Playing with Neons (NE2 and MTX90) -- Usagi Electrichttps://youtu.be/M6P-adLrXBIAt around 8:30 you'll find a bunch of neon bulb application circuits including oscillators, logic gates and counters.Also watch that channel for follow-up videos.

author: tggzzz
author_rank: Super Contributor
timestamp: September 14, 2022, 10:55:43 am
content: Quote from: floobydust on July 20, 2022, 05:17:56 pm... And yet the Solartron LM1420 DVM uses a neon as itsvoltage reference.Having said that, the front panel contains a CAL trimpot. Connect the internal Weston cell and twiddle until the display ofshows 1.019

author: strawberry
author_rank: Super Contributor
timestamp: September 14, 2022, 06:55:54 pm
content: invented 50V bidirectional zener diode

author: langwadt
author_rank: Super Contributor
timestamp: September 14, 2022, 10:18:13 pm
content: https://youtu.be/M6P-adLrXBI

author: srb1954
author_rank: Super Contributor
timestamp: September 15, 2022, 12:00:16 am
content: Quote from: tggzzz on September 14, 2022, 10:55:43 amBack in those days there were special neon voltage reference tubes that gave pretty respectable stability suitable for a a DVM of that calibre. The initial voltage tolerance on those tubes wasn't all that good hence the need for a trimpot to cal the instrument.

author: james_s
author_rank: Super Contributor
timestamp: September 15, 2022, 12:34:58 am
content: Quote from: BrokenYugo on July 20, 2022, 04:37:02 pmBack when neon glow lamps were used in a lot of night lights it was common for the older ones to only work when the room was lit. They'd get weak and require some external photons to kick start them.

author: TimFox
author_rank: Super Contributor
timestamp: September 15, 2022, 01:55:58 am
content: Quote from: srb1954 on September 15, 2022, 12:00:16 amOne example is the 991.https://frank.pocnet.net/sheets/049/9/991.pdf

author: David Hess
author_rank: Super Contributor
timestamp: September 16, 2022, 02:45:00 am
content: Quote from: james_s on September 15, 2022, 12:34:58 amThe start voltage is lower when the neon lamp is illuminated by an external light source, which can be a problem in some circuits. To avoid this, neon lamps which have a radiation source inside the glass envelope, typically Krypton-85, are available to make the starting voltage consistent.