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LED power supply switching using NTC thermistors ?
Posted by      04/16/2019 12:37:31     Railduino 2.0


Capacitive load types, such as LED light sources, require increased switching element switching. It is mainly a high starting current, where the current value can reach tens of times the nominal value.

The solution may be, for example, the appropriate dimensioning of the switching elements - for example, by selecting the type of relay that can withstand these high inrush currents - the so-called inrush relays, which, however, are more expensive than the standard relay. Alternatively, limitations of these currents can be used by using NTC thermistors that are not so expensive.


NTC thermistors are temperature-dependent resistors that reduce their electrical resistance with increasing temperature. Thanks to this feature, they are used as current protection devices for electrical and electronic devices, where they easily and efficiently reduce abnormal currents, including inrush current.

The use of a solid resistor as a current limiter always causes a large loss of power. The NTC thermistor reduces the switching current by high initial resistance and as its temperature rises, its resistance drops to a few percent of the original value at room temperature, resulting in a power loss that is lower than using a solid resistor. In other words, the current limiting effect using an NTC thermistor is greater than the effect obtained by using a solid resistor.


Various switching power supplies are often used as power supplies for electronic devices. After the power supply has been applied to the power supply, a large high amplitude start-up current flows into the circuit during charging the smoothing capacitor. Because this switching current can negatively affect the life of the capacitor, damage the switch contacts or destroy the rectifier diode, this current must be limited. As shown in the figure below, starting current limitation can be solved by inserting an NTC thermistor into the power circuit.


Instead of using external relays for switching capacitive loads, a so-called NTC box can be used, which is a device that facilitates the connection of NTC thermistors to circuits for switching these loads.

The advantage is the elimination of another mechanical element as a possible source of failure, saving space in the rack, the cost is lower than the use of external relays, including sockets