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I have a cheap device, which has blown one of its SMD capacitors (no markings) and I was just wondering that what will happen if i remove the cap and put just a jumper wire to its place?

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    \$\begingroup\$ Not a good idea. Best case nothing happens, worse case you short the power supply to ground (e.g. the cap was a decoupling capacitor) \$\endgroup\$ May 6, 2013 at 22:02
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    \$\begingroup\$ Nothing good. Capacitors block DC, wires do not. \$\endgroup\$
    – Dave Tweed
    May 6, 2013 at 22:09
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    \$\begingroup\$ Why the close votes? The question is quite clear and on topic. The downvotes, however, seem completely appropriate. Basically -1 for thinking he could get away with something or that he discovered some new law of physics, despite obviously knowing little about electronics or physics. \$\endgroup\$ May 6, 2013 at 22:55
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    \$\begingroup\$ @OlinLathrop So, are you saying downvotes are for bad questions? What if he had not asked the question and hurt himself? I think that makes it a good question. I would 'maybe' donwvote it because of the lack of clarification of what the capacitor actually is for (even a picture of the circuit would have helped.) But to downvote because you think it's a stupid question is in and of itself stupid. \$\endgroup\$ May 6, 2013 at 23:55
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    \$\begingroup\$ Ah yes, the old "lets be condescending jerks and borderline belligerent to someone asking a question because how dare they not know the same stuff I do" routine... \$\endgroup\$
    – Passerby
    May 7, 2013 at 0:23

4 Answers 4

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I was just wondering that what will happen if i remove the cap and put just a jumper wire to its place?

You would be wiser wondering what would happen if you simply removed the failed capacitor since, for DC, a capacitor is an open circuit.

A capacitor is, theoretically, a wire only at infinite frequency. So, if you're planning on operating your circuit at infinite frequency only...

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No.
Not usefully.
For practical purposes you can consider that a capacitor blocks DC and passes AC.
A wire passes AC and DC.

The fact that the capacitor has "blown" indicates that there is a signifcant energy source present. Usually a cap would 'blow' physically only when there was a problem elsewhere. Using a wire will probably transfer 'problem energy' into even more wrong places :-(.

BUT!!!

How do you know that it WAS a capacitor and not a resistor or some other device? Clear, sharp photo would be of great value. + circuit diagram useful if available.

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  • \$\begingroup\$ This completely ignores how caps are used to smooth dc ripples or brownouts. \$\endgroup\$
    – Passerby
    May 7, 2013 at 5:57
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    \$\begingroup\$ @Passerby - Can you explain your comment? With genuinely no intention of being rude, your comment makes no sense at all. Imagine that we are, talking about a power supply smoothing cap here from eg Vdd to ground. The ONLY line that is not unquesionably correct is "Usually a cap ... only when there was a problem elsewhere" and that depends in the power supply case whether the cap was within spec. A correct cap can die due to manufacturing defect or age. But if it does from over voltage, voltage spikes, unuauallly high ripple current, thos too are caused by "a problem elsewhere". \$\endgroup\$
    – Russell McMahon
    May 7, 2013 at 11:33
  • \$\begingroup\$ The "blocks DC and Passes AC" makes it seem like a special type of diode. It doesn't mention how Caps act like well, like small batteries for DC purposes. Without knowing if Op's cap (aside from actually being a cap) is inline/series or parallel. I just think this is a bit too sparse an answer. I didn't downvote though. \$\endgroup\$
    – Passerby
    May 7, 2013 at 19:33
  • \$\begingroup\$ @Passerby - OK - I now see what your concern is. / "Makes it sound like" is in the ear/brain of the hearer. Without getting into corner frequencies, energy storage and more the "blocks DC / passes AC is about as concise a cap functional description as you'll get in 4 words. I think. I could have said more on such aspects, but The key issue was (I think) to identify how a cap differs from a wire and why it matters. "Stores energy" is definitely useful. But "shorting it out is almost never correct" arguably addresses his main concern. \$\endgroup\$
    – Russell McMahon
    May 7, 2013 at 22:10
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In most cases, it would be a bad idea, but it is impossible to say so without more information. Here are a couple of uses of caps that come to mind:

  1. shunt caps used to clean up the noise. Typically you see several caps in parallel with values vary by a factor of ~10. In this case, replacing a cap with a wire will create a short and will likely cause a catastrophic failure of the device.
  2. RF matching/tuning caps. Caps and inductors are used to create a desired RF impedance. In this case replacing a cap with a wire might fix the problem (the device will likely work but not as well as with the original part)
  3. Caps are used to create some sort of a timed response (e.g. dimmer). Replacing the cap with a wire will likely cause a failure of the device.

So, if you want to try a quick and dirty fix, I would just remove the cap and see if the device works without it.

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No, bad idea. A wire has drastically different characteristics from a capacitor.

Think about it. If the circuit would work with just a connection, then why would someone have gone to the expense of putting a capacitor there? Unless you believe that you have discovered something about electronics that the electrical engineer trained in these things that designed the circuit didn't know, how can you possibly imagine this would work!!?

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    \$\begingroup\$ On the other hand, designers do often place potentially-extra decoupling caps that are "to be safe" / not needed in practice, so sometimes removing the cap won't change device behavior. As others mentioned, the OP's mistake is thinking that the natural replacement for a decoupling cap is a wire, when really it should be an open circuit \$\endgroup\$ May 7, 2013 at 2:19
  • \$\begingroup\$ @Evol: Even leaving it open is not a good idea. If it's a power supply cap, various bad things could happen quickly. If it's a signal cap, the signal will be all messed up. If it's a decoupling cap, you may get away with it for some time, until just the right glitch comes along. \$\endgroup\$ May 7, 2013 at 11:43

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