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My understanding is that inductors in a dc circuit act like a wire and don't do anything.

So what are the ones labeled FLxx in this schematic for? Could they be replaced with a simple wire and have no affect on the circuit function?enter image description here

ohmmy
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2 Answers2

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First, from the way they are specified, those parts are more likely ferrite beads than inductors. For more information about ferrite beads, see this old question:

Regarding ferrite bead

However, the function of the ferrite bead and the inductor is somewhat similar.

In this circuit, they are most likely (it would be more clear if you included the whole circuit and not just a piece) filtering the power supply nets.

They are there to ensure that the current flowing through those wires is mainly DC. The chips that are powered by these nets will have variable current requirement over time. In some circuits, the current requirement can change very quickly. These inductors ensure that when that happens, the extra current (or sudden drop in current) is supplied by the bypass capacitors (for example C42 and C46) rather than by the up-stream power supply.

This reduces time varying currents in the supply lines coming from wherever they are coming from, which is likely to reduce radiation (EMI) produced by this device.

If the supply is noisy (and the noise is in the correct frequency band), these ferrites could also help prevent that noise affecting the down-stream components.

If you replace these inductors (or ferrite beads) with simple wires, the most likely impact would be increased radiated emissions from your system, or possibly interference between different subcircuits in your system (which we can't predict without seeing more of your circuit).

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The Photon
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  • This circuit is from the display circuit of an iPhone. FL18 tested open so I simply bridged the connection as I didn't have a replacement component. Display still didn't come on and I measured 1.6v on that rail. 1.8v was on the supply pin of FL18 before I bridged the connection. Is the drop in voltage due to not having the ferrite bead there or shouldn't affect voltage? – ohmmy Dec 28 '16 at 02:22
  • @ohmmy, There is probably some other problem that caused FL18 to fail. It's pretty common for ferrites to blow like a fuse when a short circuit happens in the load. But if you don't solve that other problem, your device still won't work. – The Photon Dec 28 '16 at 16:14
  • A short circuit (or excess current demand for whatever reason) in the load is also consistent with the supply voltage dropping like you described. – The Photon Dec 28 '16 at 16:15
  • Thank you for your in depth responses I appreciate it. One other question if a circuit uses a ferrite bead rated at 240 ohms 300ma, would it be safe to replace it with a 240 ohms 350ma? – ohmmy Jan 11 '17 at 12:56
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They are there to filter noise or spikes from those signal lines. The designer thought that function was necessary and some of them have resistance so replacing them with a piece if wire is probably unwise.

RoyC
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  • If one were replaced with a piece of wire. Would it lower voltage on that rail or have no affect? – ohmmy Dec 28 '16 at 01:39
  • Without knowing what the circuit is doing it is impossible for me to know that. Notice on the diagram that each of these has a resistance. – RoyC Dec 28 '16 at 08:28
  • This circuit is from the display circuit of an iPhone. FL18 tested open so I simply bridged the connection as I didn't have a replacement component. Display still didn't come on and I measured 1.6v on that rail. 1.8v was on the supply pin of FL18 before I bridged the connection. Is the drop in voltage due to not having the ferrite bead there or shouldn't affect voltage? Apple keeps their schematics pretty vague – ohmmy Dec 28 '16 at 13:07
  • To be clear, the resistance is the value on the right (e.g. 0.4 ohms) not the value (80 or 120 ohms) on the left, which will be an AC impedance at some given frequency. –  Dec 28 '16 at 13:28
  • @BrianDrummond Would it be safe to replace one rated at 240ohms 300ma with one rated 240ohms 350ma? – ohmmy Jan 11 '17 at 03:23
  • Can't tell from those specs. If the DC resistance is also similar, then probably yes. –  Jan 11 '17 at 10:48
  • @BrianDrummond Unfortunately the D.C. Resistance isn't given in the circuit – ohmmy Jan 11 '17 at 12:55