I have a bunch of old neon indicator lamps recovered from old AC devices and used to indicate power status.
I wonder: is there any reason nowadays to use the neon lamps instead of LEDs? I see they are still widely available.
LEDs are cheaper even after considering the need for a diode in AC applications. They blink half as often in AC applications, but emit much more light per watt.
In what cases are neon indicator lamps still preferred?
Neon bulbs use microamps of current when fed through a dropping resistor directly from the AC line. LEDs need 10× to 100× the current and can't be fed AC directly.
I think you'll find neons have more resistance to ESD, transient overvoltage, and high temperatures.
You can do many interesting things with neon lamp circuits, all of which are curiosities in 2019 (well maybe they're useful if you need EMP resistance, but I suspect we'll have bigger problems if that's a factor). Neons wear out, for one thing. From the above document:
When I were a lad, I designed a monitor system for Linotype machines (yes, that long ago) using a neon bulb oscillator triggering a then-newfangled C106 SCR (in that horrible package with the 0.05" nominal wide leads), to operate a stepper relay with precious metal contacts multiplexing thermocouples.
I don't see any real reason other than to save a few pennies to use neons as indicator lamps. An NE2H runs at around 2mA, which is sufficient to give more brightness from a decent LED (not all packaged indicators use decent LEDs, however). The neons will wear out if they're powered constantly (rated life is 12,000-25,000 hours which is less than 3 years 24/7).
Have a look at this video by BigClive, the light effect (scroll towards end of video to see it) would require a significantly more complex circuit and/or obscure components if you would use LEDs and tried to get the same (random) light effect.
This is the circuit that is used:
I admit that this isn't a "killer application" of neon lamps but more a fun project.
Not an EE answer but aesthetics.
LEDs look uglier. Neon lights occupy more bands in the yellow orange spectrum while LEDs only have a few spikes. See this Q&A on spectrum of LED bulbs (hint: they're just blue LEDs with Phosphor coated lens).
Although more than just a simple lamp, The Nixie tube was a a popular display in the 50s which was used in various technical instruments like voltmeters and frequency counters. It's revival is in part due to its unique, vintage look. Plenty of examples of DIY hobbyists making their vintage displays.
Hellbus [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
If your equipment is mains voltage, no low-voltage DC available, a neon is a lot easier. As Dave Tweed said, for one thing the resistor can be smaller. Neons can also tolerate the odd voltage surge, and other out-of-spec conditions. An LED has a delicate and tiny structure necessary to it's function. A neon is just some gas and 2 bits of copper.
Things like kettles, radiators, etc, that are mains-only, typically use neons. If it's cheap garbage from Ebay / Amazon, they might (ab)use an LED cos it's cheaper and they don't care about reliability. But why do they always choose blue?
Have you actually tried building mains neon vs LED indicators? I bet you haven't.
Here, try a project. Sometimes, a leg of split-phase or 3-phase AC power will drop out at the poletop. Either neutral gets lost, causing L1 to rise and L2 drop, or a leg is lost, causing L1 to be backfed through 240V appliances. Make a very useful gadget that grabs L1, L2 and N, and visually indicates whether L1-N and L2-N are very coarsely the same voltage, and L1-L2 is coarsely 2x the others. The user switches off breakers and watches to see if that affects brightness one to the other. Having 3-4 lights and having the user watch for differences in brightness is fine. Durability a million hours. Needs to be simple and very low parasitic load, energy budget 720mW (3ma). Go.
However, your point is fair that 99% of electronics innovation is done on the low-voltage side of the wall wart. Neon has no place there; it would need an active circuit driving it, and that compares disfavorably with LED.
Neon and LED are different indicator types for different regimes of electricity. When you argue against neon, you're arguing against mains voltage as a supply source. As a big fan of houses having low voltage DC systems so they can happily exist in power-out situations without generators or inverters -- I can't really disagree.
The current requirements of a Neon are low enough that they can generally be driven directly from the mains with a simple and cheap series resistor.
If you try to drive a LED in a similar way you have at least two problems.
So neons tend to win out in "pure mains" applications, things like indicators on sockets, plugs, power strips, wall switches, surge protectors, heaters with simple electromechanical thermostats and so-on.
If a device is intelligent enough to have a microcontroller on it then LEDs generally win out.
The need for neons can be related to the industry. For example, equipment that wears out in nuclear facilities must typically be replaced with identical gear, because the cost of qualifying new equipment to operate in the system is so great that it makes no sense to use anything but the old design. If the old design had neons, the replacement typically will too.
The need for neons can also be cosmetic. I build nixie tube clocks for friends and use neons for the colon separating hours and minutes. The color closely matches the color from the nixie filaments, and LEDs just wouldn't look as good.
They also have very low current draw and so are sometimes used as indicators in older designs in emergency power or ride-through applications.
Most regular LEDs have about 2 Vdc drop in forward mode and need about 1 mA to light up. although 10 mA is recommended with standard LEDs.
In addition, they can only withstand about 2 Vdc in reverse mode, which means that they need a rectifier diode in series to protect them when the ac voltage changes polarity.
In this configuration, the LED lights up only half of the time, which may reduce its brightness; this could be good or bad depending on the application.
Neon lamps use a 100 K to 250 K, 1/4 W resistor (see http://www.farnell.com/datasheets/57560.pdf).
Neon lamps are not that bright and in certain applications may not stand out when lit, especially in highly lit areas.
With time, neon lamps develop flickering and electrode polarization (only one of the electrodes glows).
Neon lamps are available in various base sizes (see https://www.bulbtown.com/neon_lamps_and_light_bulbs_s/909.htm). Some even come with an internal resistor sized for a specific operating voltage.
Several answers have already discussed the technical reasons, but they don't discuss the applications where those reasons would apply.
A lot of times, you'll see Neon indicators in industrial-grade machinery, for three reasons, two already mentioned:
As a result, industrial applications using these higher-voltages (note that ANSI still considers 480V a "low-voltage") can save power and cost with appropriately sized neon's. (Sure, it's minimal savings, but typically you need a reason to change something, not keep it the same.)
There are some other curious aspects of industrial systems that generally hold true as well:
When these aspects are considered, it should make it easier to see why a good chunk of (especially older) industrial-grade equipment would prefer neons to LED's. Especially when considering that if a neon "dies", it's often just extremely dim (vs. completely off as an LED typically is) so it still has minimal functionality.
As an example and anecdote, when I was building the glass manufacturing machines, one feature our machines had was that when plugged in, there was an indicator system that told you if the 480V outlet you plugged into was wired properly. It was extremely cheap to build with Neon's (a few lights and resistors), but would have been slightly more costly (and complex) to build with LED's. (Sure, considering the cost of the machine it was barely noticeable so who cares, but we had no reason to use LED's there and this system has been used by the company for decades, so why spend extra time and money designing a new one?)
Source: I used to maintain and build industrial machinery for a plastic injection molding facility, and a glass machine manufacturing facility.
This is a very niche use, but some neon lamps can be used as logic elements due to the voltage difference in the striking vs maintaining voltages.
You can read about somebody using this, rather than a microcontroller, to control a clock:
Is there a reason? Sure. Suppose you're making a relay-based computer, with no semiconductors, and wanted memory, say PROM. Each bit is a dip switch, or a little jumper wire. But how do you connect those to data lines? You need a voltage controlled switch, like say a relay... or a gas discharge tube, and neons definitely fit the bill, although normally you'd be driving them pretty hard that way (10-20mA) even if for short pulses. So, neons don't quite fit the bill, you need gas discharge tubes if you care about longevity, but I'll share a secret: you can use a conducting GDT to trigger a neon bulb, just for the visuals. The current needed to trigger a subsequent relay will be carried by the GDT, while the neon bulb looks pretty.
With diodes or neons or GDTs, you can drive the memory in a cube matrix form, with just one relay per each line/plane per dimension, e.g. a 16x8 matrix can select a byte from a kilobit (128 byte) array. The address decoder tree for such an array takes ~48 relays. Without GDTs, you'd also need a thousand coupler relays in addition to the address decoder tree, and other circuitry such as bus driver relays remains the same.
The high-ish voltage needed to strike GDTs that are cheaply available (~90V) is not as nice to deal with as the low (5-48V) voltages needed to trigger bus coupling relays, but a GDT takes way less board area and is cheap vs. a relay - if you find a good deal on auction sites.
It's hard to buy large (>1000) lots of fast relays for $0.10 each (I need them fast - 1ms or faster when new) - you find one deal or two and then it's crickets for 6-18 months. Low voltage GDTs for $0.10 each show up in reasonably sized lots about every 2 months, although the recent downturn has somehow doubled the supply. You'll get the couple thousands pieces you need much sooner than you otherwise would. That's still about 2/5ths compared to distributor pricing in >1ku quantities, so if it's a hobby product that you're putting together yourself, you have plenty of time to trade for dollars, since assembling the PC boards with hundreds of parts per board seems to take forever. But even primary market distributor prices for GDT's aren't crazy - you can get one for under $0.50 in 1k quantities.
They may weigh about as much as the smallest of mass-manufactured SMT low voltage relays because of the ceramic envelope, but you can pack them 5x as densely, and they are easier to solder, too.
Disclaimer: There are many ways to design things that work well, and the above isn't by any means a golden goose, nor even a wooden goose, and it only works well enough anyway.
I remember using neon indicator lights as crude, inaccurate, but simple voltage references.
The reality is that in today's electronics there are not the voltages present for neon operation. And there are for LEDs.
