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CZK 999
VAT incl.

825.62 VAT excl.

NTC thermistors, limiting start-up currents when switching capacitive loads (LED sources)

6x NTC thermistor, 5 Ohm, serial / parallel connection possible

NTC thermistor 6x 5 Ohm Parallel connection possible Serial connection possible
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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. 

Features and functions:

6x NTC thermistor - 6 channels, each with an NTC thermistor
5 Ohm - the nominal value of the thermistor is 5 Ohm
- 8,5 A - max. current (0-65°C, R = 0,073 Ohm)
Parallel / Serial connection - thanks to the connectors, serial / parallel connection according to load can be easily set


What to consider before using the NTC box

To use the NTC box, it is necessary to know at least the maximum continuous current to the load (LED power supply). For NTC box, the maximum permissible continuous current is 1A (approx. 250W load). It is therefore possible to switch the inrush currents of about 30A (typically 30 times the rated current for LED power sources). If it is necessary to use larger permanent currents - eg 2A (load approx. 500W) it is suitable to use two connected NTC box channels - ie to divide the current between two NTC thermistors.


4x LED ceiling lights, each luminaire has its own 230V switching power supply. One lamp has a power of 18W. In total, 4x18W = 72W

The total continuous current to the luminaires is about 0.31 A. However, the start-up current can be up to 10A when switched on.

Considering the conductor resistance to the luminaire, including the transient resistances and the internal resistance of the power source to about 1 Ohm, using one NTC channel will result in a start-up current limitation of nearly five times - using a NTC thermistor of 4.7 Ohm.

In our case, the current is reduced to about 2A and the switching element (e.g. relay) in the control device can withstand this value.

Connecting the NTC Box to the Railduino Module

Relay output Q7 controls the LED power supply via the NTC box that limits start-up current.
Manual NTC Box CZ verze 13.01.2022 DOWNLOAD
CE Derclaration of conformity CZ/EN verze 1.1.2019 DOWNLOAD