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Old 01-06-2018, 02:32 PM   #1
jayjr1105
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Default How to tell Low ESR or not.

So I shop around for caps and sometimes element and mouser don't show if the cap is low ESR or not. It will have an ESR rating like .199 ohm for a 1000uF cap.

Is there quick math that can figure out if it's low or ultra low ESR rated or not?
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Old 01-06-2018, 03:06 PM   #2
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Default Re: How to tell Low ESR or not.

You should look up the spec sheet to find out, and you will also see the ripple current rating, Life span, etc.
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Old 01-06-2018, 04:39 PM   #3
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Default Re: How to tell Low ESR or not.

At the switching frequencies typical in DC-DC converters common on motherboards and video cards and so on, and regular switching power supplies, you can pretty much put an equal sign between Impedance (Z) and the ESR value.

So for any series of capacitors, you can open the datasheet and look up the Impedance measured at some positive temperature and 100kHz (or similarly higher frequency, not 1kHz or lower) and that would be your ESR.

If the datasheet has no impedance listed at a higher frequency (for example it shows only 120 Hz), then that's a sign that capacitor isn't really designed to be a very low ESR capacitor, or a capacitor meant to be used with switching power supplies.

As for actual values... you kinda have to know what the circuit expects. If the circuit has a 1500 uf capacitor which has a typical ESR of 0.02 ohm , then you'll find loads of 1500uF capacitors with ESR between 0.01 ohm and 0.03 ohm and they're all very low ESR and probably good to replace the existing capacitor.

There are also other series of capacitors which are still considered low ESR but you would not be able to use as replacement .. for example a Panasonic FC series 1500uF 6.3v capacitor has an ESR of 0.05 to 0.065 ohm depending on diameter and height choice, and it's still considered Low ESR and you shouldn't use it on the output of a motherboard or video card VRM (the dc-dc converter which converts 12v to whatever the CPU or GPU chip needs)

So this series may not be good for power supplies but may be good somewhere else, you shouldn't "blacklist" it - for example, FC series is pretty well received and appreciated by people that repair audio amplifiers, it seems the amplifiers sound better with these capacitors compared to other series (or the capacitors don't alter the sound too much as more high end / very low esr capacitors can sometimes do).


It would probably be easier to sort by Current Ripple... typically capacitors with high current ripple will also have low to very low impedance / esr (it makes sense, because esr is an internal resistance , so if there a lot of current going through capacitor and the resistance is high, capacitor would overheat, so naturally the lower the resistance the more current a capacitor should be able to handle in theory)
With time and experience you kinda figure out which series of capacitors are suitable for some purpose.
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Old 01-06-2018, 06:55 PM   #4
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Default Re: How to tell Low ESR or not.

Quote:
Originally Posted by mariushm View Post
There are also other series of capacitors which are still considered low ESR but you would not be able to use as replacement .. for example a Panasonic FC series 1500uF 6.3v capacitor has an ESR of 0.05 to 0.065 ohm depending on diameter and height choice, and it's still considered Low ESR and you shouldn't use it on the output of a motherboard or video card VRM (the dc-dc converter which converts 12v to whatever the CPU or GPU chip needs)
is this because the converted voltage is too high for this particular cap? what if said cap was rated at say 16v or 35v (or since this is just a example lets say the cap is rated at ∞VDC )?
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Old 01-06-2018, 07:56 PM   #5
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Default Re: How to tell Low ESR or not.

no, it's because the frequency is too high for the cap to handle.

if the esr is too high for the frequency, the cap cant keep up charging and discharging fast enough and heats up internally.
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Old 01-06-2018, 08:49 PM   #6
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Default Re: How to tell Low ESR or not.

So does that mean esr determines the max frequency a cap can handle
if so can i assume for pwr supplies if i have a very high freq i should be ok with a lower ripple current (Panasonic FR) cause of the switching frequency, but i would assume if the load is expected to have large demand spikes i would also need high ripple eg Panasonic FS series
a less extreme freq would be FK and FM
*i was looking at this post to figure out the use for each series, well if that was correct and i did not misunderstand it
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Old 01-06-2018, 10:25 PM   #7
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Default Re: How to tell Low ESR or not.

that's a pretty close sumation.

the esr is the internal resistance,
if the esr is lower the cap can charge and discharge faster.

at this point it sounds like just sticking the lowest esr possible in everything is a good idea.

and it would be other than one small problem.
if the esr is really low, some power supplies can miss-interperate that as a short circuit and oscilate or shutdown.
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Old 01-07-2018, 08:27 AM   #8
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Default Re: How to tell Low ESR or not.

Well that does not sound like the cap's problem we need a better controller, by this logic my sumption stands
typically where would that threshold be?
the lowest ESR i can find on mouser is 10 mOhms, FR and FM go as low as 12 mOhms and as high as 510 mOhms
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Old 01-07-2018, 09:26 AM   #9
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Default Re: How to tell Low ESR or not.

Yeah. You have to stay pretty close to the original capacitor's properties, and not just replace with the lowest ESR capacitor out there.

There are some voltage regulators or other types of ICs which are designed to expect some minimum ESR from the capacitor in order to function properly.

For example the 1117 series linear regulators will have problems maintaining the output voltage properly and will oscillate if the ESR on the output is less than 0.1 ohm, so you don't want to use ceramic capacitors (super low esr) or polymer capacitors with them ... it's pointless anyway.

So if you use such a linear regulator to power something with 5v, it may be OK to use a 47-100uF 10v..25v capacitor on the output from a low esr series because due to capacitor diameter and height, the ESR would be higher than 0.1 ohm ... it wouldn't be a good idea to use a 330-470uF 35v-50v capacitor from same series because such capacitor - while its lifetime rating would be much longer and last a longer time - would have a too low esr.

For example, from the Panasonic FC series because i have this on hand, not because it's a super great series : http://www.farnell.com/datasheets/2059899.pdf

100uF 25v rated FC : 6.3mm x 11.2mm , 0.35 ohm impedance / esr
220uF 25v rated FC : 8.2mm x 11.5mm , 0.117 ohm impedance / esr
330uF 35v rated FC : 8mm x 20 mm , 0.065 ohm impedance

So 330uF would be no good, 220uF uF would be borderline acceptable, 100uF would be OK. Linear regulators don't require much capacitance on the output anyway.

Some switching regulators are also sensitive to minimum ESR but to a lesser degree
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Old 01-08-2018, 05:15 PM   #10
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Default Re: How to tell Low ESR or not.

I think i realized today why a PSU could consider a it a short, i was thinking it some how lets current flow through it, which made no scene to me since that is the opposite of what a cap does, it reads it as a short cause it charges so fast, so that would mean if i only have a low ESR cap and i need one not as low i can add a 0.5 - 0.1 Ohm resistor to make it work properly for said circuit right? if so what kind of wattage would said resistor need to be (1/4, 1/2, 1) i know how to pick that for sustained loads (eg for a led), but not momentary spikes
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Old 01-08-2018, 06:57 PM   #11
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Default Re: How to tell Low ESR or not.

not a good idea.
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Old 01-09-2018, 11:42 AM   #12
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Default Re: How to tell Low ESR or not.

A switching power supply control loop has components in it that tailor the frequency response and phase shift through the loop to compensate for capacitances, inductances, and R-L and R-C elements in the power stage. One of the critical R-C elements is the output capacitors, where the "C" is the capacitance and the "R" is the ESR.

If you replace an output capacitor with a part whose ESR is substantially different, the loop response will change, possibly making it conditionally stable (dependent on load or ambient temperature) or unstable (oscillation). Additionally, if the ESR is too high the ripple voltage will be greater.

On another topic, when you charge a capacitor with a sudden voltage increase the charging current is limited by the capability of the voltage source and resistances (and inductance, if the charge path is long) in the charging circuit. One of those charging circuit resistances is the capacitor ESR. This effect can be seen in the AC circuit of a PSU that does not have PFC. Clipping an oscilloscope current probe on the Line or Neutral AC wire will show that input current flows only in brief high amplitude spikes that correspond to the tops of the AC sine wave voltage (Unless you know how to do it safely, DO NOT connect an oscilloscope voltage probe into the AC side of a PSU).
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Old 01-09-2018, 12:31 PM   #13
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Default Re: How to tell Low ESR or not.

Ugh... I just found a bad capacitor in an I-inc monitor. It's a 100F 16V, 85C LeLon(IIRC) capacitor and it's a local decoupling cap on the logic board, versus a switching PSU cap.

Should this be a low ESR cap? It measured as almost 2 ohms ESR.

I soldered another capacitor in parallel because there's a huge amount of space on the back side of the board. Surprisingly the PSU board with the 1000F 16V 105C caps, they all test fine so I was quite surprised. Anyway, I tried powering it up after adding that capacitor in parallel, and found the monitor works fine once more, so I called it good (hey, it's my monitor). So it replaced another monitor that I was using that's flickering when freshly turned on.

I think I have more bad capacitors to hunt on this flickery monitor...
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Old 01-09-2018, 01:09 PM   #14
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Default Re: How to tell Low ESR or not.

2 ohm is not very high, for such a small value capacitor and considering it's rated for 85c only. I'd replace with a lowish esr or some more general purpose capacitor, like Nichicon PS or PW series, Ryubycon PX or YXG
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Old 01-09-2018, 03:01 PM   #15
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Default Re: How to tell Low ESR or not.

There are more than just that one capacitor, there were around 4 or so, most of them are around 500 milliohms, but that one stuck out at 2 ohms.

Since adding another cap in parallel appears to fix the problem, I suppose either the cap has strayed in capacitance or that 2 ohms needs to be lower...

I was testing a bunch of 100F caps in my junk pile and most are 1 ohm ESR or lower, many in the 500milliohm range...
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