“LED signs are safe due to the low voltage”, a slogan often heard or read in advertisements, might be misleading. Besides high-current hazards, and not just with circuit shorts, hazardous high voltages may be present in the “low-voltage” LED circuit, depending on the power-supply type and construction.
In some cases of remote control, this hazard can even lead to the destruction of components. This discussion stems from some regulatory rules for the LED power-supply circuit designer. But let’s start from the beginning, because this problem surfaced in the last decade. Low-voltage sign applications aren‘t new, and LED neon signs have been used in signage for nearly 20 years. With “classic” low-voltage power supplies – those with a bulky core-and-coil transformer at mains frequency to step down the 115V to 12 or 24V – everything was fine.
But then, as modern semiconductors and electronics evolved, “switching type” power supplies debuted, and they’ve become sturdy enough for 24/7 operation in the last 5-10 years. As discussions now include energy savings and wireless-communication safety, several regulations have been implied by authorities, such as:
- Power supplies for lighting fixtures and signs must have a minimum efficiency of 95% or higher.
- Power supplies must not generate high-frequency interference to wireless communication.
Core-and-coil LED power supplies often no longer meet requirement (1), and (2) has never been an issue because no frequencies higher than mains-line frequency are generated in the circuit. The modern, switching types meet (1), but require costly and bulky additional components to meet (2). The operating principle of “switching” a voltage very quickly (30,000 to 1 million times a second) means the stepdown transformer for the same power rating can be very small and low cost compared to the “classic” types.
However, switching electricity very quickly generates high-frequency/radio-frequency (RF) currents, which must not leave the power-supply box/circuit to meet requirement (2), because the currents cause interference in nearby or remote electronic equipment. To prevent high-frequency currents from leaving the power supply, filter circuits must be used to block the high frequency on the mains and output leads of the power supply. These filter …