What's the difference between "neon" and "cold cathode" sign tubes? - Page 1
Forum section: electronics / projects

author: Ben321
author_rank: Frequent Contributor
timestamp: October 28, 2023, 11:06:43 pm
content: I know what neon lights are. They are tubes filled with different gasses (usually either neon gas, or a mix of mercury vapor and argon) and coated with various materials to generate lights of different colors, and are usually shaped to look like different symbols or objects.I also know what CCFL (cold cathode fluorescent) tubes are. They are similar to neon tubes in concept, but are usually smaller and use lower voltages and currents, and are used for illumination (sometimes for decorative purposes like on a car or inside a clear computer case, and other times for practical uses like LCD screen backlights), and usually are straight tubes, rather than shaped into decorative symbols.But what are these so called "cold cathode" sign tubes? I know they use transformers very similar to those used with neon sign tubes, but with more current. Neon transformers and cold cathode transformers usually are over 5kV (unlike CCFL high voltage power supplies which are usually under 2kV), but while neon sign transformers usually output 30mA (sometimes 60mA I've seen), the cold cathode sign transformers usually output 120mA or more.I also know that unlike neon sign transformers, where all the recent ones are required by the UL to have a ground fault interrupter on the HV output, these cold cathode sign transformers do not have this requirement.Yet despite the differences in the specs for their transformers, what are the actual technical differences between the neon sign tubes and cold cathode sign tubes. Google isn't really helping me, because whenever I search for any information on "cold cathode" technology, I end up getting search results for CCFL technology. I hope somebody here can explain to me what's up with these cold cathode sign tubes (which is a technology I hadn't even heard of prior to doing research on the UL requirements for neon signs).

author: amyk
author_rank: Super Contributor
timestamp: October 29, 2023, 03:05:15 am
content: Neon refers to the gas, cold/hot cathode is the electrode design. They're all a type ofhttps://en.wikipedia.org/wiki/Gas-discharge_lamp. Most if not all neon tubes for signage are of the cold cathode type.

author: Gyro
author_rank: Super Contributor
timestamp: October 29, 2023, 10:12:32 am
content: There's no difference between the internally phosphor coated sign tubes and CCFLs apart from size and diameter. They are all cold cathode. Neon sign tubes are the only cold cathode ones that use the fundamental red colour of the Neon gas itself but they have exactly the same cold cathode structure as other sign tubes.CCFLs are much small diameter and driven by a high frequency inverter (hence the more efficient operation. They are also typically built into equipment (eg laptops), so take the input of the inverter from a low voltage DC supply and protected by the case.Mains sign transformers are more likely to be used in relatively exposed conditions (even outdoors) and they tend to be more remote from the sign, connected by a couple of High voltage leads, The signs themselves tend to be more exposed, eg. sandwiched between a couple of sheets of perspex in a shop window, with just rubber boots over the HT connections. This accounts for the stricter electrical regulations.

author: Wolfram
author_rank: Frequent Contributor
timestamp: October 29, 2023, 11:13:14 am
content: Cold cathode in the context of Neon usually refers to tubing of larger diameter, often used for architectural lighting. The term is used loosely and without any formal definition, but it tends to refer to 20 mm and larger tubing, often running at over 100 mA. Fundamentally it's the same as regular neon tuning and smaller diameter CCFL tubing.

author: S57UUU
author_rank: Regular Contributor
timestamp: October 29, 2023, 04:00:55 pm
content: If I remember right, there is a big voltage drop at a cold cathode, so bad efficiency overall. Cold cathode lamps are usually not used for illumination. (Maybe with HF drive (in scanners), I'm not sure).To reduce that drop and improve efficiency, you can heat the cathode like in a vacuum tube, usually in direct mode (the filament is also the cathode).

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 29, 2023, 07:00:47 pm
content: I think CCFLs also have a bit of mercury, whereas neon tubes don't.

author: Zero999
author_rank: Super Contributor
timestamp: October 29, 2023, 07:06:31 pm
content: There the same thing, as mentioned above.Neon tubes are mostly red-orange, but the discharge also gives off a little UV, so they can be used with a phosphor to make other colours: blue for shocking pink, green for more of an orange, or a deeper red. Technically this is cold cathode a fluorescent lamp, but with neon, rather than mercury.Other colour 'neon' tubes are now made with mercury, with a phosphor, forming a cold cathode fluorescent lamp.

author: Ben321
author_rank: Frequent Contributor
timestamp: November 04, 2023, 11:38:14 am
content: Quote from: Wolfram on October 29, 2023, 11:13:14 amThanks for the info. Your response is the only one out of all of them that is actually about the type of lighting I'm talking about. What do you mean by architectural lighting? Also I don't think it's loosely defined in terms of technology. The UL rule that says that "neon sign transformers" over 3kv must have a secondary ground fault interrupt, also says that "cold cathode transformers" don't need to have this feature. And I don't think the UL is referring to CCFL backlights for LCD screens. I think they are referring to a more powerful type of neon lighting.

author: S57UUU
author_rank: Regular Contributor
timestamp: November 05, 2023, 05:11:40 am
content: "Cold cathode" and "hot cathode" are meant very literally. A cold cathode is just a piece of metal, while a hot cathode is actively heated. That's why fluorescent tubes (including CFLs) have two terminals at each end of the tube, connected to a filament inside.Ups, now I see that the question was the difference between neon and cold cathode, not cold and hot. Sorry. Neons are just a variety of cold cathode. Maybe the difference is that "neons" are driven by line frequency transformers, and "cold cathodes" by inverters?

author: Zero999
author_rank: Super Contributor
timestamp: November 05, 2023, 09:46:13 am
content: Quote from: S57UUU on November 05, 2023, 05:11:40 amStrictly speaking, neon lamps contain neon gas, which produces most of the light. In reality, neon tubing used in signs is often not neon, but mercury, with a phosphor. It depends on the colour.Newer neon signs don't use neon lamps or fluorescent lamps, but LEDs. This is an example of technology changing, but the name remaining the same.

author: MathWizard
author_rank: Super Contributor
timestamp: November 07, 2023, 05:11:21 am
content: So how dangerous are CCFL's if you break them if they might have Mercury ? I'm guessing you still definitely don't want to get a shock from 1.

author: LaserSteve
author_rank: Super Contributor
timestamp: November 07, 2023, 08:19:45 am
content: Cold Cathode generally ment a tube for room lighting.Backfilled with Argon/Mercury and using a cool white phosphor.The intent was to have a very long life tube for places you cannoteasily service such as department stores, malls, cove lighting, or high baywork spaces. Also had the advantage that the local neon sign shopcould easily produce room lighting including custom shapes.For the mass produced CC lamps, a special socket was used at each end with a builtin safety switch to terminate the HV if the lamp was removed or the tube broke.This switch placed considerable mechanical pressure on the tube, which is ok becauseglass is incredibly strong in compression.The lamps I remember were 8 foot long. A properly constructed coldcathode tube could achieve much,much longer lifetime over a classicalflourescent lamp with hot cathodes. On the order of 5 to 10 years.A neon sign tube in the US is designed for typically a 20 mA or 30 mAconstant current HV transformer. CC lighting ran at much higher currents.However the install cost was high compared to a hot cathode rapid start lamp.CCFL in modern terminology are usually very small tubes with a tiny diameter used for back lightingLCDs.I have not seen a working room sized cold cathode lighting install in over twenty years.Edit, I did. I did taw a CC install, today when I went to vote.The new elementary school has them in the Gym.W. McFarlane-Moore developed some of the first large scale room lighting using the fact thatcarbon dioxide emits a beautiful pale bluish white light without a phosphor.Moore tubes needed a fill system as the CO2 broke down over time.See:https://signsofthetimes.com/cold-cathode-hints-part-i/https://www.lighting-gallery.net/gallery/displayimage.php?album=2496&pos=42&pid=82794Your common NE2 indicator lamp is also cold cathode, not having a heater to cause enhanced emission of electrons.Steve

author: MathWizard
author_rank: Super Contributor
timestamp: November 10, 2023, 08:02:40 pm
content: Now I would like some light bars in the wall/ceiling corners of some rooms in my house. I had cheap LED strips before, but they burned out years ago.Just looking up CCFL light bars, I mostly get LED ads. I get the feeling the LED's ones will a lot be cheaper.

author: Zero999
author_rank: Super Contributor
timestamp: November 10, 2023, 09:32:10 pm
content: Quote from: MathWizard on November 10, 2023, 08:02:40 pmJust go with LEDs and under power them, if you want them to last longer.

author: tooki
author_rank: Super Contributor
timestamp: September 24, 2024, 03:52:29 pm
content: Quote from: S57UUU on November 05, 2023, 05:11:40 amJust an FYI, after redoing a display of signage tubes (one each of neon, helium, krypton, argon, and xenon): modern neon drivers are not line frequency. Modern ones are high-frequency (e.g. 20KHz) with constant-current control. AFAIK we haven't used the line-frequency ones in a while. Line-frequency transfomers are big and heavy, so I suspect that most neon used since the 90s has been with modern high-frequency supplies.