What are the inductors for

Thread Starter


Joined Sep 27, 2010
Dear all,

I'm trying to get my head around inductors. In some instances (like low pass filters) I think I'm starting to grasp their operation. In the schematic below, however, I have no idea. It is part of a line-following robot schematics and the X4s go back to a MCU's ADC inputs to measure what is on the line and what isn't.

Screen Shot 2015-11-01 at 01.22.08.png

Can anyone, please, help me out and explain the purpose of L2 and L3? Also, I have trouble figuring out where the X3-9 would go, but I expect this to become clearer when I understand the L2 and L3.

Thank you for your time,


blocco a spirale

Joined Jun 18, 2008
They are there to improve the quality of the DC supply by acting as low-pass filters. L3 in conjuction with C19 reduces noise on the supply lines. I assume that X3-9 connects to the output of a voltage regulator.


Joined Feb 24, 2006
X3-9 goes to a 3.3VDC power supply, and X3-10 is the ground return to that power supply. The inductor by itself (L2) presents high impedance to high frequency noise. L3 in conjunction with C19 does two things; L3 presents a high impedance to high frequencies, and C19 provides a low impedance path for high frequency noise to GND. DC current will flow through the inductor unimpeded and have no desire to flow to ground through C19.