Your latest AC-line-powered project requires an illuminated indicator to signify that the unit is connected and powered on. That’s needed for user comfort and troubleshooting if the item seems “dead” when it is on. First, pull 20 milliamperes (mA) from a product’s DC rail to power a red LED. If no rail is available, add a tiny circuit to power the LED. If a suitable rail isn’t available, you could add a small circuit to drive that LED. Both solutions have problems. First, there’s credibility and safety. If the DC rail’s driving circuit has a defect and the LED doesn’t light up, the user may suppose the AC line is shut off and the circuit is not live. The separate indicator circuit is expensive considering what it performs, even if the BOM is short. If you can’t tap a DC rail and need a step-down or regulatory-safety isolation transformer, things are no longer simple and cheap.
There is a cheaper, passive technique to connect an LED to an AC line. “Capacitor-dropper” (cap-drop) supplies are used for low-current DC from an AC source – it drives LED. These circuits raise regulatory and safety considerations since they must be physically isolated from other design elements. To meet UL safety requirements, the entire circuit, and its tiny circuit board (if any) must be physically organized in a way that the user is protected if the chassis fails. This method demands larger and higher voltage passive components. Adding an “AC live” subcircuit with LEDs isn’t as easy as it theoretically seems. A small neon bulb and a current-limiting resistor can be connected directly to the AC line (120 or 240 VAC). This circuit must also be insulated, but it’s so simple and small that heat expands the tubing. Neon bulbs, or lamps, have been around since the early 1900s and are available in a multitude of sizes and styles. The NE-2 lamps are the most popular indicator size. Current-limiting resistor value depends on the neon lamp size, brightness, and spectrum of current ratings. This range is 50 to 220 kilohms (kΩ) for 120 VAC mains, and it doubles for 220 VAC. The power rating for the resistor is reliable because of its order of a quarter watt or less.
The lifetime of a neon bulb is up to 50,000 hours, like the LED. However, neon lamps can withstand vibration, mechanical shock, and repeated on/off operation. They work from -40°C to +150°C and are not damaged by high-voltage static discharges or line transients. Neon lamps are similar to circuit protection gas discharge tubes (GDTs). Neon lights have two symmetrical, non-polarized electrodes. If DC is reversed, the other electrode illuminates (Figure 5). Electrodes alternately light when connected to AC. The eye doesn’t see a flicker.
A neon lamp can be used as an AC power-on indicator without connecting the lamp and resistor and insulating the assembly. As a panel indicator, you can have the bulb and resistor with wire leads, solder terminals, or quick-connect terminals e.g., the N513 standard brightness red neon lamp. Many rocker switches have neon indications to simplify the physical layout, for example, the H8553VBBR2-B. It’s a 15-amp, 120/240-volt, double pole, single throw (DPST) unit with a snap-in panel mount that saves space and simplifies wiring.
Combining a rocker switch with an inside lamp creates an intuitive “human interface” because the switch’s action and illumination are visually and physically linked. Ancient technology can often be the best solution to modern design issues. LED indicators have mostly replaced neon for low-voltage solutions, although neon lamps are still a simple AC-line choice. The lamps come in bare bulbs, panel mounts, and rocker switch packages, increasing their utility and usefulness.