Capacitors for DC smoothing, Low-ESR or not Low-ESR?

[Agent24]

I am posting this here as my original problem has branched off into another question and I think I'll be more likely to get a good answer with this post.


I want to replace all capacitors in my ADSL Router (Dynalink RTA1320)

I have decided on Panasonic FM for three of them, but I am having problems with the last two.

They are 1000uF 25v and I believe are for DC smoothing as they are just after the bridge rectifier and a couple of coils.

They are G-Luxon SM series which are apparently just general purpose, and the datasheet doesn't even list ESR.


Half the problem is the size of mine are not listed in the datasheet, so I suspect they are a custom job.

The other part is that nothing with the same ratings will actually fit inside.

16v versions are short enough, but is this OK? The voltage I measured across them was only 11.4v, but I read elsewhere that for this kind of application the rating should be higher (Not sure if this is true)

Also, the current ones aren't apparently Low-ESR but would a Low-ESR type be better used here?

What about ripple current? What sort of ripple does a smoothing capacitor have to deal with?

[yyonline]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

I'm guessing the device uses a 12V power adapter? I don't see any transformers which could be stepping up the voltage, and your reading confirms this. So 16V is fine, provided they have the same or higher ripple rating compared to the SM capacitors.

[yyonline]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

What about ripple current? What sort of ripple does a smoothing capacitor have to deal with?

Sorry for the double post, I couldn't figure out how to edit and quote at the same time...

Based on the SM datasheet...10x16mm at 25V is 355mA ripple. So your replacement needs to be able to handle that.

You mentioned NHG in the other thread. NHG exceeds this rating, making it a good replacement.

[Agent24]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

It uses a 9 volt AC power adaptor.

[shovenose]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

yeah so 16v is totally fine,

[yyonline]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

If you're measuring 11.4V, I'd check the output voltage of your power supply by measuring it. I don't see how it would be increasing the voltage unless the supply is putting out too much to begin with. I don't often work on things with external power supplies, so someone who knows more than I do on this is free to correct me if I'm wrong.

[Agent24]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

Without load the 9v AC adaptor measures 10.5v AC.

[Radio Fox]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

If you're measuring 11.4V, I'd check the output voltage of your power supply by measuring it. I don't see how it would be increasing the voltage unless the supply is putting out too much to begin with. I don't often work on things with external power supplies, so someone who knows more than I do on this is free to correct me if I'm wrong.

The router uses an ac adapter. The 11.4v Agent24 measured in the first post is dc. The voltage isn't increasing, it's being rectified & smoothed.

[Agent24]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

I thought the rectified DC would be lower because of the silicon voltage drop?

[Radio Fox]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

I thought the rectified DC would be lower because of the silicon voltage drop?

Yes, it would be if it wasn't for the smoothing caps keeping the output voltage at it's peak value, rather than the RMS value.

As the ac adapter is unregulated, let's assume that under load, it produces it's designed output of 9v ac. As you say, when it hits the bridge rectifier, you will lose around 1.2v (0.6v lost across each conducting leg of the bridge), reducing it to 7.8v. This voltage is pulsed dc.

The effect of the smoothing capacitors is to "fill in" the gaps between the pulses, giving a virtually continuous peak voltage reading, which is 1.414 times the RMS voltage, giving 7.8 X 1.414 = 11.03v.

[Agent24]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

Thanks for that, it makes more sense now!

I ordered the capacitors on the 25th, just got to wait until they get here... coming from the UK - RS says it'll be 8-10 days...

[PlainBill]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

Yes, it would be if it wasn't for the smoothing caps keeping the output voltage at it's peak value, rather than the RMS value.

As the ac adapter is unregulated, let's assume that under load, it produces it's designed output of 9v ac. As you say, when it hits the bridge rectifier, you will lose around 1.2v (0.6v lost across each conducting leg of the bridge), reducing it to 7.8v. This voltage is pulsed dc.

The effect of the smoothing capacitors is to "fill in" the gaps between the pulses, giving a virtually continuous peak voltage reading, which is 1.414 times the RMS voltage, giving 7.8 X 1.414 = 11.03v.

Excuse me, there is a slight error in the calculation. The DC voltage is the peak AC voltage (9V x 1.414) less the voltage drop of two diodes (1.2V) = 11.53 volts. Doing the calculation in reverse order (as you did) results in predicting the output voltage .5 V lower than what it will actually be.

PlainBill

[Agent24]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

Put the new capacitors in today.
(https://www.badcaps.net/forum/showthread.php?t=12175)

I looked back to when I bought it and it has been less than a year and half.. hopefully the new ones last a bit longer..

[Pyr0Beast]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

Heat kills caps.

Also 6000 hours (one year) would be quite good for the caps if this stuff runs 24/7

[Agent24]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

It's not 24\7, I usually switch off at night.

[Pyr0Beast]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

That would save power&money.

Still, it isn't enough.

Bad design

[PCBONEZ]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

Heat kills caps.

Also 6000 hours (one year) would be quite good for the caps if this stuff runs 24/7

The Endurance Lifetime given in data sheets doesn't have anything to do with how long caps last.

It is basically how may hours you can abuse them WITHOUT anything going out of spec when returned to nominal conditions.

Manufacturers make no claims in data sheets [and usually at all anywhere] as to how long they will last after the abuse or with no abuse.
.

[Pyr0Beast]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

The Endurance Lifetime given in data sheets doesn't have anything to do with how long caps last.

It is basically how may hours you can abuse them WITHOUT anything going out of spec when returned to nominal conditions.

Manufacturers make no claims in data sheets [and usually at all anywhere] as to how long they will last after the abuse or with no abuse.
.

It has everything to do with that. If they seriously get out of spec their life has ended and that is all what matters. Unsuitable for application and suitable for recycling/landfilling.
>> Load Life : 105 ̊C 2000~5000hours.


Dried up cap can still hold a charge but it's usability equals zero.

ESR and other characteristics are defined at rated temperature.

[PCBONEZ]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

It has everything to do with that. If they seriously get out of spec their life has ended and that is all what matters. Unsuitable for application and suitable for recycling/landfilling.
>> Load Life : 105 ̊C 2000~5000hours.


Dried up cap can still hold a charge but it's usability equals zero.

ESR and other characteristics are defined at rated temperature.

Most people think that.

That's because most people don't know what 'Endurance" aka Endurance Lifetime" aka "Load Life" aka "Lifetime"[in a data sheet] means.

- - - You just proved you don't.

~~~

There are 8760 hours in one year.
- According to you, 5000 hour rated caps die in 208 days of 24/7 use.

.

[PCBONEZ]

Re: Capacitors for DC smoothing, Low-ESR or not Low-ESR?

There is a formula to estimate Useful Lifetime from Endurance Life but according to Chemicon [I think it was] that equation only has a 40% Level of Confidence.