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.
inductors improve dc current and also store energy temporarily! act as electricity resistors in AC and pulsed DC at high frequency