Does ripple current matter for capacitors in an answering machine? - Page 1
Forum section: electronics / repair

author: niemand
author_rank: Contributor
timestamp: November 01, 2021, 03:24:17 am
content: I'm thinking of replacing the capacitors in an old digital answering machine (AT&T 1725) that has recently gone haywire.I am looking at three replacement options for a given capacitor. All three options are 10000 Hrs @ 105°C. One of the differences between them is the ripple current at low and high frequencies:Low frequencies:907.5 mA @ 120 Hz1.056 A @ 120 Hz1.2 A @ 120 HzHigh freq:1.21 A @ 100 kHz1.76 A @ 100 kHz2 A @ 100 kHzThe other thing isimpedance:62 mOhms28 mOhms38 mOhmsWhich is more relevant for my concern here (if any of them are), and which choice should I go for?

author: bob91343
author_rank: Super Contributor
timestamp: November 01, 2021, 05:24:46 am
content: I believe you are overthinking this. Any of those capacitors will do the job. What are the specs on the old parts?

author: niemand
author_rank: Contributor
timestamp: November 01, 2021, 06:08:49 am
content: Quote from: bob91343 on November 01, 2021, 05:24:46 amI have only the voltage rating, capacitance, and temperature rating (85ºC) of the old (original) parts. I know that 3 of the capacitors are made by Nichicon. I am unable to get more detail (and unsure whether more detail is even available) without removing them, and I am uncomfortable removing them before having the replacement parts on hand.

author: james_s
author_rank: Super Contributor
timestamp: November 01, 2021, 07:37:51 am
content: Ripple current only really comes into play with switchmode power supplies and similar types of circuit. There is nothing in a vintage answering machine that is going to make this an issue, modern general purpose capacitors can typically tolerate higher ripple current than anything that was available at the time that machine was made.

author: David Hess
author_rank: Super Contributor
timestamp: November 01, 2021, 09:28:26 pm
content: I disagree with james_s in this case, but not for the expected reasons.Consumer equipment is made as cheap as possible, so whatever capacitor they used, it was the bare minimum required no matter what the application and ripple current rating. Many times now I have repaired consumer grade equipment like chargers where the line input capacitor was just 10s of microfarads and what wore out causing failure not long after the warranty expired. So what does that mean?The more important specification will depend on exactly how the capacitor was used, but you cannot go wrong with either of your choices which will be better than the original cheap part. To improve reliably further, use a higher voltage part so the ripple current rating is even higher. Doubling or even quadrupling the capacitance will likely help also.

author: bob91343
author_rank: Super Contributor
timestamp: November 01, 2021, 10:25:55 pm
content: More capacitance isn't always better. It can increase stress on other components. Safest route is to stick with original values unless you know the parameters well.

author: james_s
author_rank: Super Contributor
timestamp: November 02, 2021, 12:30:54 am
content: Quote from: David Hess on November 01, 2021, 09:28:26 pmI don't see how this is disagreeing. I said more or less exactly the same thing, any of the proposed replacements will work, this is a general purpose application, there is not likely to be much ripple current, I never ran into low ESR capacitors until I started working on switchmode power supplies and CRT monitors. I rarely encountered faulty capacitors before then either.As was said in the other reply, increasing the capacitance too far can cause other problems. I built something years ago and used too large a bulk filter capacitor and it was a nice low ESR part too, I had problems with welded relay contacts and connector arcing due to the high inrush.