What does an inductor on the + input supply rail do?

Thread Starter


Joined Sep 28, 2010
Hi everyone,
I've been breaking down circuits to learn from and on some automotive applications I've noticed an inductor on the input rail!, it doesn't go to ground or anything like that, its just straight on the input rail.

to circuit<-----ooooo-----> + 12v Input

Like this! I know an inductor acts like a capacitor, so why would you use an inductor when you could use a much cheaper capacitor ?


Joined Aug 15, 2007
Actually, inductors are the opposite of capacitors. Whereas caps block DC, inductors pass it.

But without seeing the complete circuits, anything else would be just guessing.


Thread Starter


Joined Sep 28, 2010
It went straight to a 5v voltage regulator, then the rest was a charging circuit.

But why was it there? I am trying to understand this?

Also, what do you mean that capacitors block DC? I thought capacitors just filtered spikes and held a small current for use with dips and peaks?

I thought the inductor was there to possibly act as a filter of some type to minimise spikes?


Joined Oct 2, 2009
In an automobile, the ignition coil produces large amplitude signals at radio frequencies.
In many applications it is not unusual to use both an inductor and a capacitor to reduce high frequency noise in the power supply line.

Thread Starter


Joined Sep 28, 2010
I get it now, thanks guys! But I still don't understand why a charging type circuit would need to filter certain frequencies.

It seems to be a filter for high pass frequency. The link from Bertus helped me understand!

Cheers guys!


Joined Dec 26, 2010
In my opinion, you really would need to understand this before even thinking of making any projects to be run from such a supply.

There are at least two basic reasons for having a filter inductor in the power line to something. As already mentioned, there may be external sources of interfering signals, sometimes at such high levels that they may do damage. A filter may be used to block out or at least reduce such interference. The fact that beginners are often unaware of the often very serious extent of such disturbances in an automotive power supply situation is one reason why it is inadvisable for them to try to make such things.

Another reason for having filtering is that the device itself may contain possible sources of interference to other equipment sharing the supply. In this case filters may be used to try to eliminate or reduce interference coming out of the device and on to the supply.
Inverter circuits for powering mains equipment from a DC input for instance may generate radio interference.