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Mains Power LED Indicator Neon lamps can run from 230V AC with a simple series resistor but they’re pretty dim and flickery. LEDs are much better, but you need this extra circuitry to run them directly from a mains supply. By Julian Edgar & John Clarke T here are many applications where you want to run LEDs from mains power; a pilot light near a switch is the most common. There are numerous simple circuits available to do this – typically they use just a resistor, diode and capacitor. However, these circuits all have a major safety problem: the LED is floating at mains potential above Earth. If it’s being run by one of those common circuits, a person touching the outside of the LED’s plastic envelope is relying on the dielectric strength (insulation) of the plastic LED envelope to avoid getting a shock. That’s why many commercial mains switches have the LEDs mounted in bezels or even hidden within the switch and not accessible. But what if you want the LED to project through the faceplate of a wall switch? No LED manufacturer specifies the dielectric strength of the LED’s plastic envelope, so there’s no guarantee of safety. Our simple and cheap circuit overcomes that problem. It allows the LED to be operated from the mains, but the LED is not floating at high and dangerous voltages. The LED’s current can also be easily set. Also, unlike many approaches that run LEDs off mains power, this circuit protects against surge over-current at switch-on, providing a long life. This project is designed for use with mains house wiring, so it must be installed by an electrician. How it came about This circuit came about because, in the house I am building, I am using a 12V system to operate ventilation hatches. These hatches are operated by a DPDT switch triggering a linear actuator. There is a 10mm high-­ intensity green LED on the wall switch plate to indicate when the ventilation hatch is open. Testing showed this approach to be very effective. The LED is visible from many metres away, and because it projects through the switch plate, it can be seen at quite acute angles. To operate different systems, I am also using mains power switches that need pilot lights. In those cases, I was originally using green neons, and the contrast with the 10mm LEDs was profound. To see if the neon was lit in bright light, you needed to peer closely at the bezel. Even in dull conditions, the neon’s brightness was borderline – only when it was quite dark was the neon brightness adequate. Furthermore, it was impossible to see the neon indicator at any angle other than with the viewer directly in front of the indicator. What was needed was a way of running the same 10mm high-intensity green LEDs as the 12V power indicators, but on mains power. How it works In the circuit diagram (Fig.1), capacitor C1 is the main voltage dropping This shows a 10mm LED being driven by the Mains Power LED Indicator. Unlike a neon indicator, it is visible at acute angles and in bright light. 28 Silicon Chip Australia's electronics magazine siliconchip.com.au The PCB needs to be mounted in an IP65 enclosure with cable glands used on the mains power and LED connections. The length of the LED leads depends on the application. Table 1 – selecting capacitor C1 component that also limits current through the LED. Its capacitance provides an impedance at the mains frequency of 50Hz that is 1 ÷ 2πfC, where f is the frequency and C is capacitance in farads. For a 100nF capacitor, this works out to 31.8kW. Ignoring the effect of the relatively small series 1kW resistor, this limits the mains current for a 230V AC supply to around 7mA. More LED current is available with a larger capacitance – refer to Table 1. The parallel 1MW resistor discharges the capacitor when the circuit is switched off. The supply after the series capacitor and 1kW resistor is full-wave rectified by bridge rectifier BR1 and filtered to a smooth voltage by the 470μF capacitor. Zener diode ZD1 limits the voltage across the capacitor to 4.7V. The 1kW 1W resistor is included since, when power is initially connected to the circuit, the mains voltage could be anywhere in the voltage swing of the 230V AC waveform (up to ±325V DC). At initial power-on, the discharged capacitor will briefly present a short circuit. So, if the voltage is high at power on, the capacitor charging current is limited via the 1kW resistor. The zener diode then conducts and prevents the voltage rising much above its clamping voltage of 4.7V. The initial surge current through the zener diode could be as high as 325mA (325V ÷ 1kW). However, this is only an instantaneous current for the zener, and it can easily withstand that briefly even though its rated maximum continuous current is 212mA for its 1W rating. The LED is protected against surge over-current by being driven via the 150W resistor across the DC supply, that in turn is limited in voltage by the zener diode. Under normal conditions, ZD1 does not conduct. This is because, even when using the largest capacitor value LED current Capacitor C1 value 1.4mA 22nF 3mA 47nF 6.4mA 100nF 9.5mA 150nF 13.7mA 220nF for C1 at 220nF, the current through the LED is 13.7mA and voltage across the 150W resistance is 2V. Adding this to the voltage across the LED (typically 1.8V) gives a value less than the zener voltage. Therefore, the zener conducts only when there is the potentially higher current flow at power-up. Without the zener diode and 150W resistor, the LED surge current would be up to 325mA. Assuming 1.8V across the LED, with this circuit, the maximum LED current is limited to 19.3mA when the zener voltage is at 4.7V. Taking this approach gives a long LED life – something simpler circuits often don’t provide. We minimise the risk of electrocution by ensuring that the LED itself is, at most, only a few volts above mains Neutral. Mains Neutral is tied to Earth on the household property, so typically, it is within a few volts of Earth. 1W resistors are used to achieve the WARNING: MAINS VOLTAGE Fig.1: capacitor C1 is the primary component that drops the mains voltage to the 1.8-3.6V needed to drive LED1. The AC is then rectified, with the surge current at switch-on limited by the 1kW resistor and zener diode ZD1. Importantly, the LED cathode is tied to mains Neutral via BR1 for safety. The small PCB (shown adjacent) uses only a handful of components and is quickly assembled. siliconchip.com.au Australia's electronics magazine This circuit operates at mains live voltages. Do not build it unless you are confident working with mains-powered circuitry. Don’t touch any part of the circuit when it is connected to mains power. Fixed wiring installation must be performed by an electrician. Fig.2: we have simulated the circuit using LTspice, and the simulation file is available to download from siliconchip.au/Shop/6/3314 required mains voltage rating, since ¼W types may have a lower voltage rating (eg, 150V). An LTspice circuit simulation for this project is available to download from siliconchip.au/Shop/6/3314 in case you want to see how it behaves and check the LED current with different capacitor values (see Fig.2). Construction We have created a small PCB, coded 10111251, that measures 38 x 56mm for this circuit. Its overlay diagram is shown in Fig.3. Building it will take just a few minutes. Fit the low-profile components first (the resistors and the zener diode), ensuring that the diode’s cathode stripe faces as shown in Fig.3. Next, install the electrolytic capacitor and bridge rectifier, with both components inserted in the correct orientation as shown. The terminal blocks can be installed next, with the wire entries facing towards the nearest edge of the PCB. Finally, mount the large X2 capacitor. The LED leads need to be connected to CON2 using mains-rated wire. Each lead needs to be insulated using Parts List – Mains LED Indicator 1 double-sided PCB coded 10111251, 38 × 56mm 1 64 × 58 × 35mm IP65 polycarbonate enclosure [Jaycar HB6120 or HB6121 (with mounting flanges)] 2 3-6.5mm cable diameter cable glands [Jaycar HP0720 (pack of 2)] 1 3-way 5.08mm spacing screw terminals (CON1) [Jaycar HM3132] 1 2-way 5.08mm spacing screw terminals (CON2) [Jaycar HM3130] 2 M3 × 5mm panhead machine screws 1 length of 7.5A mains-rated wire (for the LED wiring) Heatshrink tubing (to insulate the LED connections) Semiconductors 1 5mm or 10mm LED (LED1) 1 W04(M) 1A 400V bridge rectifier (BR1) 1 4.7V 1W zener diode (1N4732) (ZD1) [Jaycar ZR1402] Capacitors 1 470μF 16V PC (radial) electrolytic 1 X2 mains-rated capacitor (see Table 1 for suitable value) Resistors (all axial, ±5%) 1 1MW 1W 1 1kW 1W 1 150W ½W 30 Silicon Chip Australia's electronics magazine Fig.3: before starting, work out the value you need for capacitor C1. Make sure the terminal blocks and zener diodes are orientated correctly and don’t forget that the wiring for Active and Neutral at CON1 is critical. heatshrink tubing. After that, cover both LED leads with a larger diameter heatshrink tube and shrink it down. The mains connection is made to the two outer terminals of CON1. It is very important that the Neutral and Active connections to mains power are made as shown on the PCB (electricians will be used to such requirements). As it is not used, the centre contact of CON1 can be removed if you wish. A three-way terminal is used so that the Active and Neutral connections are sufficiently separated. Installation As well as being connected correctly, the incoming mains wires need to be clamped to the enclosure using a cable gland mounted on the side directly opposite CON1. The LED leads are also secured to the enclosure using a cable gland that’s mounted directly opposite CON2. The PCB is secured to the base of the enclosure using two M3 screws into the two mounting posts located down the centre line of the enclosure. If you use the corner mounting holes and standoffs instead, use nylon or polycarbonate screws to provide insulation to the outside of the box should a wire come adrift and contact one of the screws. Do not use metal screws that could conceivably become live if a mains wire comes adrift. This project is designed for use with mains house wiring, so it must be installed by an SC electrician. siliconchip.com.au