Author Topic: Can you use inductor as a jumper in DC circuit?  (Read 1205 times)

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Offline MrOmnos

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Can you use inductor as a jumper in DC circuit?
« on: February 12, 2018, 08:36:06 am »
So I run few pads under my microcontroller so that I can connect  power pin to power pins on other side of the chip. But I don't have a jumper to connect those pads. I found some smd inductors on printer motherboard which are small in size to fit under the IC. I know inductors are short circuit in DC signal. And did a continuity test, it is short circuit.  But is this good for the microcontroller?
« Last Edit: February 12, 2018, 08:38:41 am by MrOmnos »
 

Offline capt bullshot

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Re: Can you use inductor as a jumper in DC circuit?
« Reply #1 on: February 12, 2018, 09:36:47 am »
With microcontrollers and other digital stuff, one doesn't want to put inductors in series with power pins. The power consumption of such stuff is highly dynamic, say it contains a good amount of AC (superimposed to the DC current) that will cause voltage drop across the inductors. These voltage fluctuaction in turn will disturb operation of the uC.
So, don't use inductors (or ferrite beads) as jumpers (for power pins).
 
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Offline TMM

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Re: Can you use inductor as a jumper in DC circuit?
« Reply #2 on: February 12, 2018, 10:08:51 am »
I assume you mean SMD pads? You can actually get 0ohm resistors (=a wire) in various SMD sizes. If you don't have any just solder some copper wire across the pads or if the pads are close enough together just solder bridge it.
 
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Offline station240

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Re: Can you use inductor as a jumper in DC circuit?
« Reply #3 on: February 12, 2018, 10:27:15 am »
0 ohm SMD resistors are marked with just one number "0".
 

Offline Zero999

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Re: Can you use inductor as a jumper in DC circuit?
« Reply #4 on: February 12, 2018, 10:46:00 am »
Just use a piece of tinned copper wire.
 

Offline KhronX

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Re: Can you use inductor as a jumper in DC circuit?
« Reply #5 on: February 12, 2018, 01:35:52 pm »
Well, yes and no. You don't want to insert inductance between the bypass capacitor for the respective power pin, and said power pin.

"Upstream" of that bypass capacitor though, it's perfectly fine :) In some cases, it's even often done (or recommended).

I recall a few years ago, that's how i troubleshot the power problem on the logic board from a TV i had bought (dead).
The HDMI receiver chip had several 3v3 power pins, and each came through a ferrite bead (0603 or 0805), then 1-2 bypass caps, then into the chip.
The 3v3 rail was getting pulled down (pretty much shorted to ground), so one by one, i removed the ferrite beads and re-measured, until i isolated the culprit.


With microcontrollers and other digital stuff, one doesn't want to put inductors in series with power pins. The power consumption of such stuff is highly dynamic, say it contains a good amount of AC (superimposed to the DC current) that will cause voltage drop across the inductors. These voltage fluctuaction in turn will disturb operation of the uC.
So, don't use inductors (or ferrite beads) as jumpers (for power pins).
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Offline T3sl4co1l

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Re: Can you use inductor as a jumper in DC circuit?
« Reply #6 on: February 12, 2018, 01:45:29 pm »
"Upstream" of that bypass capacitor though, it's perfectly fine :) In some cases, it's even often done (or recommended).

Well, not unqualified.  It must be done correctly, otherwise there will be an impedance peak in the supply, at some unlucky frequency.  MCU runs through a few loops of just the right length, and whammo, the supply starts bucking like the Tacoma Narrows bridge.

When the supply impedance meets requirements, it's fine.


On a related but different subject,

You might consider replacing inductors with jumpers, as a cost saving measure when you learn those inductors are no longer necessary.  This actually tends to not work, because the only large jumpers I'm aware of, are nothing more than pressed metal (tin plated brass) hunks, yet they are boutique priced ($0.50+).  The inductor you started with, is probably cheaper!  (Smaller inductors, in normal chip sizes -- at currents typical of chip components, under 8A or so -- are readily replaceable, though are cheaper as well so still may not save much.)

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Offline calexanian

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Re: Can you use inductor as a jumper in DC circuit?
« Reply #7 on: February 13, 2018, 10:48:59 pm »
+1 for bad idea. Unless you have specific filtering needs and a cap at the Vdd pin and some sort of filter network for some unknown reason, you don't want an inductor in series with a logic IC. Bad times. use a piece of wire as said before.
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