Replacing a capacitor with one with lower ESR

Discussion in 'General Electronics Chat' started by _dan_, Sep 24, 2015.

  1. _dan_

    Thread Starter New Member

    Dec 10, 2013
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    Hi all,
    Searching through the suppliers lists of caps, I see there are branded caps like Panasonic NHG series with lower rated life 1000h, and no information about ESR at some price and little bit cheaper caps like Samwha or Nichicon that are written to have longer life 5000h or 3000h with low ESR (0.08 Ohm for the Samwha). I see this as function of the ESR - lower resistance, lower heat generated from ripples and so higher lifetime.
    My question is - can one use low ESR caps instead of general purpose caps, is there some issue with it?
    And is the brand name important factor - as far as I suppose branded caps have real - worst case - values and lower end brands use some exaggeration during rating?
    Thanks for the time!
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
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    Using a cap with lower ESR in normally no problem. I like certain brand names like Panasonic and Nichicon because I have found that I can trust their published specs.
     
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  3. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    Generally when replacing any kind of cap you try to stick name brands and low ESR and higher temps to be on the safe side .. The only problem with this is making sure the size is the same or will work in your application.. I have a couple repairs where I had to make caps fit
     
  4. recklessrog

    Member

    May 23, 2013
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    I remember a quote from some years ago that said "The reliability of any piece of electronic equipment is dependent on the quantity and quality of the electrolytic capacitors it contains"
    If a capacitor has failed, it makes sense to look at it's environment and replace it with one of higher quality.
     
  5. dannyf

    Well-Known Member

    Sep 13, 2015
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    You can use whatever you want.

    There are cases where low ESR isn't desired.
     
  6. peter taylor

    Member

    Apr 1, 2013
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    A 12 V DC, high power switching motor speed controller that I saw in a magazine uses a low Equivalent Series Resistance (ESR) capacitor (2,200 μF) to dampen inductive spikes across the motor winding's.

    Apparently, a normal Electrolytic won't do the job.
     
  7. _dan_

    Thread Starter New Member

    Dec 10, 2013
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    Thank you Lastraveled
    I will stick to Nichicon then, because they are at moderate price and got positive feedback here :)
     
  8. _dan_

    Thread Starter New Member

    Dec 10, 2013
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    I also remeber a teacher saying that warranty period of an electronic product should be calculated based on capacitors lifetime
     
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  9. _dan_

    Thread Starter New Member

    Dec 10, 2013
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    That is my wondering - which are those cases
     
  10. Lestraveled

    Well-Known Member

    May 19, 2014
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    The ESR of electrolytic capacitors when used in pulsed applications, like switching power supplies, is critical. The parameter that degrades first from stress is ESR. When the ESR increases the capacitor will begin to dissipate heat. Total failure will soon occur.

    ESR is the lumped resistance of a capacitor and is a good indication of its quality. I am having a hard time imagining an application where a high ESR would be desired.
     
  11. recklessrog

    Member

    May 23, 2013
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    You only have to look at the number of a certain sky HD box power supply failures to see how often capacitors with high esr cause failures. There are several companies that sell a package of new high quality replacements for them.
     
  12. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hello there,

    In most cases we want low ESR because that means better efficiency. But in some cases low ESR is not good.

    A good example is a well known circuit called a boost regulator. It has been shown that if the capacitor ESR goes too low it could cause instability. That means if you had a cap with some less than good ESR in a boost regulator that was working fine and you replaced it with a very low ESR cap, it could start to oscillate. It's kind of complicated to explain why this happens, but it happens.

    Another fairly well known (more or less) case is in positive LDO linear regulators with a PNP output stage. The ESR has to be within a certain range or else you might see oscillations which would wreck havoc on the rest of the circuit. You can actually find this information right on some of the data sheets for these regulators.
     
  13. dannyf

    Well-Known Member

    Sep 13, 2015
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    When you are driving into a capacitor, you want the impedance to be as high as possible for stability reasons. So any case where a capacitor is a load in a negative feedback system, too low of an ESR could be an issue.
     
  14. Lestraveled

    Well-Known Member

    May 19, 2014
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    Now why would anyone want that??
    - In a conventional power supply the capacitor is supplying current in between the cycles. Lower ESR is better.
    - In a switching power supply low ESR is very important.
    - In a tuned circuit lower ESR equates to a higher Q.
    - In the feedback components of an op-amp, again lower ESR equals a higher Q.
    - In decoupling and surge suppression lower ESR is better.

    What do you mean by, "When you are driving into a capacitor, you want the impedance to be as high as possible for stability reasons." ??

    A capacitor is a capacitor.
     
    Last edited: Sep 24, 2015
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  15. wayneh

    Expert

    Sep 9, 2010
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    And you can always design in a resistor if it's really important. I think a design relying on some non-zero ESR of the capacitor is bad form.

    The last pile of capacitors I bought came from Newark (several were on sale and almost free) and included Panasonic and Nichicon.
     
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  16. umphrey

    Member

    Dec 1, 2012
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    I was looking at a charge pump power supply 1-2 years ago. Open loop regulation +15V to -15V with 2 caps in series with the output voltage. In that case lower ESR meant higher output voltage. That's the only time I can think of where I really wanted to match the ESR of the new cap.

    Also if using it as a shunt filter lower ESR will probably mean higher cap current, which isn't a problem 99% of the time but I wouldn't make a broad statement for that application. Every once in awhile a designer will rely on cap ESR instead of external damping.
     
  17. _dan_

    Thread Starter New Member

    Dec 10, 2013
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    Low ESR are mainly required at high frequencies like in SMPS, for linear regulators otherwise are not necessary - like the case of 50-60Hz .
    The only issue I can think of now, besides what MrAl said about LDOs, is inrush current. Since its very fast transient process it wouldn't heat the capacitor, but may require some sort of delay like NTCs for big values of capacitance.
    Unfortunately general use cap data sheets don't have much information about ESR, so judging by wikipedia it may be 10 times higher or more, than low ESR caps (100 times compared to Sanyo OsCon). This meaning the inrush current with low ESR caps will be 10 (100) times higher. I am interested if someone has some accurate data about this?
     
  18. dannyf

    Well-Known Member

    Sep 13, 2015
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    Not likely. ESR reflects a bunch of factors and is not a "fixed" thing like your resistor's resistance. That's why their actual values don't matter nearly as much as their general values.
     
  19. Lestraveled

    Well-Known Member

    May 19, 2014
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    Dan
    It is very common to see a large NTC thermistor in series with the AC input on switch mode power supplies that operate off of the mains. This is to prevent fuses and diodes from popping when it is powered on. But it is not about the ESR of the power supply caps, it is simply about taking a capacitor from zero volts to 300 volts in a few cycles. It takes a lot of current. The joules of energy in a capacitor goes up by the square of the voltage. This means that a 100uF cap at 15 volts has .011 joules of energy, while a 100uF cap at 300 volts has 4.5 joules of energy. the voltage went up by 200 but the energy went up by 400. ESR is a microscopic player in this scenario. But if these capacitors have to supply current to a switcher, their ESR now becomes very important.

    You are asking very good questions. You are getting to know when a low ESR cap is required.
     
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  20. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi there Les,

    Apparently you did not read post #12.

    In your list of 'good points' you included most of the desirable effects of low ESR, but there are also undesirable effects. Here is a quick view that is more intuitive...

    .still editing.
    .have been interrupted, will have to come back to finish this post.
    .in the mean time, please read post #12. thanks.
     
    Last edited: Sep 26, 2015
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