# Limit inrush current

I'm looking into a LED driver and I'm quite curious about the inrush current. Where does it originate from? Is it to load up the input/output capacitors?

Can I limit the inrush current? What's the most simple way to do this, inrush current limiters (ICL), PTCs, or any other way?

Can the inrush current be limited by starting the luminaire in a dimmed mode?

• The most obvious way to limit the inrush is to add a series impedance. This would either be an NTC which under normal operation will get hot and its resistance will fall or for example a fixed resistor that gets shorted out by a relay after the capacitors have charged. Sep 5, 2017 at 7:46
• Okey, thanks for the help. But if i would want to be on the safe side would it help to start at a lower dimmed level(lower current output)? Sep 5, 2017 at 10:05

The inrush current would be commonly caused by the charging of power supply capacitors. These loads would not appreciably change depending on any other circuit setting (such as dimming).

The common ways to limit inrush current are:

1. Using a passive inrush current limiter (basically a resistor whose impedance quickly goes down as it heats up, i.e. an NTC thermistor).
2. An inductor that limits the dI/dt on the line (but care must be taken to avoid ringing and other issues).
3. Using active circuitry that sequences the power up and reduces operational losses. This active circuitry could be a resistor or NTC in parallel with a relay, a pass transistor, or an SCR that is activated some time after power up.

In some cases, some components of the inrush current could be reduced by staggering (i.e. sequencing) the multiple power supplies in a system (the large filtering capacitors at the output of the supply also have to be charged).

This document might provide some more pointers.

• And the cheap way to limit the inrush current is just a low value resistor - perhaps a few tens of ohms. It's chosen to limit the surge a bit, while not getting too hot when the lamp is running. Oct 26, 2018 at 21:45