Current LED lighting technology is very economical. But when switching on, a very high current peak is generated for a short time. This inrush current is caused by the capacitive behavior of LED lights. In this article you will learn the background of the inrush current and what effects it can have. You will also find ways to limit the inrush current.
- LED inrush current
- Technical background
- Impacts and side effects
- Limiting the inrush current
LED inrush current
Maybe you are in the following situation right now: You have installed the new LED lighting and want to switch it on for the first time. After flipping on the light switch, the fuse will be triggered immediately. The new lamps remain dark. If the inrush current is too high, this can cause the fuse to trip, especially in circuits with several LED lamps.
Of course, a defective lamp could also trigger the fuse. To identify the issue, you should test the LED light sources one by one if possible. If all lamps work in single operation, the high inrush current is the problem.
The inrush current is the electrical current that flows immediately after the lamp is switched on. With old incandescent bulbs, the inrush current is generated by the cold filament and can reach up to 15 times of the normal power consumption. This high current flows only directly after switching on and decreases within approx. 50 milliseconds to the normal current consumption.
Inrush current for LED lights
LED lamps are very economical in operation. However, the problem with the inrush current has been exacerbated by the use of LEDs. A small DC voltage is required for the correct function of an LED. This is generated in most light sources by an integrated power supply unit or an LED driver from the 120V mains supply. The switching power supplies used for this purpose represent a capacitive load.
These switching power supplies generate inrush currents of up to 100 times of the nominal current. However, the extremely high current of LED lights only lasts for a few microseconds. This form of inrush current normally does not cause any problems for the tripping characteristics of the circuit breakers in the household.
However, by connecting several LED lights in parallel, the inrush currents add up. This can lead to the problems described here if a certain number of LEDs are connected in parallel.
Inrush current of LED transformers
An LED transformer is used for all low voltage installations. Typically, several LED light sources are powered by one power supply unit. Each light contains its own LED driver. The driver contains capacitors, which have to be charged when switched on. This charging current is responsible for the inrush current of the light source.
The transformer itself also has a capacitive behavior, which leads to a high inrush current. The effect can also be observed with many plug-in power supplies. When plugged into the socket, a sparking or crackling can often be noticed. The inrush current of the LED transformer is caused by the transformer itself as well as of all the supplied illuminants. The currents of all LED drivers and the transformer add up.
The inrush currents are added up by parallel connection of several LED illuminants to the transformer. From a certain number of lights this can lead to the described problems.
Inrush current calculation
The inrush current can theoretically be calculated using ohmic law. The operating voltage for the calculation is known. However, the internal resistance (forward slope resistance) of an LED lamp or transformer at the moment after switching on is unknown. Therefore the inrush current cannot be calculated due to the missing information.
The inrush current can only be measured with a very fast measuring device on the individual electrical installation. A closer look at the data sheet of the LED driver or power supply can help. But not all manufacturers indicate this value in their data sheets.
Since the calculation of the inrush current is not possible in most cases, only a closer look at the data sheet will help. If no inrush current is given here, a request to the manufacturer could help.
Impacts and side effects
A high inrush current is not untypical due to the capacitive behavior of LED lights. In most applications, this inrush current is not a problem. Difficulties usually arise when a large number of light sources are operated on one circuit. This can have the following effects.
Fuse is triggered
The circuit breakers used today have two protection mechanisms. On the one hand, there is an overload protection, which is triggered by heating depending on time and current. The second mechanism is a short-circuit protection, which works current-dependently and trips very quickly. If the inrush current is too high, the short-circuit protection of the fuse trips.
If the fuse trips regularly when flipping the light switch, the high inrush current is the cause. The limitation of the inrush current described here should help.
Light switch wears out
A high inrush current imposes a mechanical strain on the contacts within the light switch. This can lead to increased wear and thus to a short lifespan of the switch. Your light switch could be affected if it is sparking visibly when switched on or if a crackling noise can be heard.
If the light switch suddenly sparks or crackles after converting to LED, the inrush current is too high. The limitation of the inrush current described here should help.
Relay contacts stick together
Less and less classical light switches are used in modern electrical installations. Instead, a switch actuator is controlled with a push-button or a remote control. This switching actuator usually contains a relay, which finally switches on the circuit with the LED lighting. The relay in turn has mechanical contacts to cause the current flow.
A very high inrush current causes sparking inside the relay. Similar to a classical light switch, the contacts wear out very quickly. In the worst case, the contacts can even stick together. In this case, the lighting can no longer be switched off. The switching actuator must be replaced.
If the relay contacts stick together in the switching actuator, the inrush current is too high. The limitation of the inrush current described here should help.
Limiting the inrush current
If the problems listed above occur, the inrush current of the LED lighting is definitely too high. In this case the inrush current should be limited in a technical way. Common inrush current limiters operate with an NTC resistor or a zero-crossing switch.
NTC inrush current limiter
An NTC resistor is a so-called thermistor. The abbreviation NTC stands for negative temperature coefficient. In the cold state an NTC has a high resistance, in the heated state its resistance decreases. This behavior allows the inrush current of small LED installations to be easily reduced.
An inrush current limiter with an NTC resistor is a passive element. These modules are inexpensive and compact and can be installed in switch boxes or lamp housings.
A zero-crossing switch is an active inrush current limiter. Here an electronic circuit ensures that the connected LED lights are switched on in the zero-crossing of the sinusoidal curve of the 120V alternating voltage. In this way, the inrush current can be reduced effectively and with high repeat accuracy.
An inrush current limiter with a zero-crossing switch is an active module. Zero-crossing switches are designed for larger installations and are usually installed in the fuse box.
The inrush current of LED lights often becomes a problem when several lamps are operated in parallel. Fuses trip regularly or switch contacts wear out very quickly. The inrush current can be effectively reduced with the described inrush current limiters.