# Inductor Question

#### TimCollins

Joined Sep 19, 2011
9
I am trying to understand what this Inductor component does in a DC circuit.

I understand the very basics about it. I know it basically resists changes in current. However, in a circuit that is powered by a 12V DC battery why in the heck would we need an inductor (to filter tiny fluxuations in a DC power supply possibly)? My thought (and maybe I am totally wrong) is that an inductor would only be useful in AC circuits. But I am just not seeing its purpose in a DC application?

The particular inductor I am working with is 100uH. Again, I understand this is basically the measure of inductance, but if I am applying 12V DC then after the magnetic field is established I should expect 12V DC coming out correct?

Alternatively lets say for example I were to plug an inductor of some size into the wall 120V AC coming into an inductor, what would we expect to come out? That's where I am lost, would we get 120V DC out?

Thanks in advance for any help understanding this...

#### crutschow

Joined Mar 14, 2008
27,725
Yes, the only voltage drop across an inductor with a steady dc current would be due to the DC resistance of the inductor windings.

And yes, an inductor can be used in DC applications (or some AC applications) to provide filtering of noise or other undesired signals. It is also is used in DC switching type power supplies, along with an output capacitor, to filter the switching voltage and get (relatively) smooth DC out.

If you apply an ac voltage to an inductor, it will generate a reactive impedance to that voltage which is proportional to the frequency of the ac and the inductance. You would still get AC out of (or through) the inductor, not DC. For DC you would need to add a rectifier (diode) to the circuit.

#### #12

Joined Nov 30, 2010
18,223
Sometimes the inductor is used to stop the circuit board from radiating high frequency energy into the power supply wires. This is "backward" radiation, and therefore backward thinking. That's why beginners miss the idea.

#### jt6245

Joined Feb 18, 2011
21
Sometimes the inductor is used to step up dc voltage. My garage door remote controller is a sample --- it uses an inductor to generate 9v from 3v dc for the rf transmission section.

#### colinb

Joined Jun 15, 2011
351
Sometimes the inductor is used to step up dc voltage. My garage door remote controller is a sample --- it uses an inductor to generate 9v from 3v dc for the rf transmission section.
Well, it doesn't step up dc directly. The circuit to which you are referring is likely an LC circuit (aka LC tank), a resonant circuit that takes an ac input, and uses the reactance of the capacitor and inductor to produce a result such as voltage amplification.

For example, here's an RF transmitter (an RFID reader) circuit that uses an LC tank to amplify the 125 kHz carrier input from 6.6 Vpp to 20 Vpp.
scanlime on forums.parallax.com said:
I use a counter, naturally, and I generate a differential-drive signal on two pins. This gives me 125 kHz at about 6.6V peak-to-peak. I use an LC tank tuned near 125 kHz to amplify this and shape it into a sine wave. Here's my carrier wave, measured at the junction between L1 and C1. Note the scale- it's about 20 volts peak-to-peak!

Joined Dec 26, 2010
2,148
Most common modern DC step-up systems use switching techniques. Although resonant methods can be used, they are not the only possibility. The simplest system to understand simply uses transistor switches to apply a square waveform to the input of a step-up transformer, while the output feeds a rectifier and filter.

Another technique, commonly called fly-back, can work with a single inductor or a sort of transformer: here a switch is used to periodically interrupt the current passing in an inductor, which can develop a higher voltage due to back-emf when the input turns off. The pulses of higher voltage are rectified and filtered.

What is key in every case is that some system creates a varying or switched drive from the DC input so that the coil can function.

#### jt6245

Joined Feb 18, 2011
21
Sometimes the inductor is used to step up dc voltage. My garage door remote controller is a sample --- it uses an inductor to generate 9v from 3v dc for the rf transmission section.
The microcontroller sends out pulse to switching transistor through a resistor,
then the inductor(470uH) back-emf is rectified by a diode and filtered by cap(4.7uF).
This step-up dc voltage is for transmitter(colpitts oscillator) and for feedback control.

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