1
\$\begingroup\$

I've got a bad clock capacitor in my original xbox that I'm trying to replace. It's a 1F 2.5V cap however, I'm not sure what Equivalent Series Resistance (ESR) the new cap should be. I see they sell 1 Ohm, 90 mOhm, 70 mOhm, and 500 mOhm on digikey. Any suggestions? Also how do you feel about the brands Eaton, Taiyo Yuden, or ELNA? Thanks!

\$\endgroup\$
4
  • 2
    \$\begingroup\$ Won't matter for clock backup applications. \$\endgroup\$
    – winny
    May 26, 2017 at 18:31
  • \$\begingroup\$ This site can't advise on brands, that becomes product endorsement. Capacitor ESR matters with pulsed applications that pull large sudden currents from it. But not with a continuous and tiny current draw like you've got. \$\endgroup\$
    – TonyM
    May 26, 2017 at 18:39
  • \$\begingroup\$ ah so in my xbox, this capacitor is just used to hold power to the system settings so the ESR shouldn't matter? As for brand, I thought I heard some were at a higher risk for leaking than others. I'm not looking for someone to try and sell a product but rather state factual information about one. If brand A is more likely to leak than brand B, we should be able to call them out on it. \$\endgroup\$
    – John
    May 26, 2017 at 18:46
  • \$\begingroup\$ I've never had any problems with Taiyo Yuden or Elna products, and I don't remember using Eaton recently. Not affiliated with any of them. Digikey should list lifetime and temperature ratings, that should be a start when determining how long a cap will probably last. \$\endgroup\$
    – Wesley Lee
    May 26, 2017 at 18:56

1 Answer 1

3
\$\begingroup\$

A value of 1 Farad hints at a clock or memory backup capacitor. This will power a chip drawing current in the micro-amps range. Therefore, high ESR will not matter at all.

ESR matters for critical decoupling, regulator stability, and high currents, none of which are of concern in this application. Since you will likely pay more in postage than in the capacitor itself though, I suggest selecting a higher quality model though.

Your criteria should be:

  • Make sure the package fits into the PCB footprint
  • Don't forget about height! Component height can bite when closing the box after doing repairs.
  • Leakage current, which will discharge your cap. Get a cap specified for memory backup, so it keeps its charge for a long time.
  • Lifetime in hours at specified temperature (higher is always good if it's affordable)
  • Select a lower ESR model if it makes you feel warm and fuzzy, but really, it wont matter. 0.1 or 10 ohms ESR will not matter at all, if the chip it powers draws microamps, the ESR voltage drop will be negligible.
\$\endgroup\$
4
  • \$\begingroup\$ great answer!! So when googling around, I see some people claiming the original xbox capacitor has an ESR of 500 mOhm. If I replace with an ESR 1 Ohm you think it would still be ok then? Also, I noticed some brands don't specify the lifetime at temp but rather number of discharge cycles. For instance, the Taiyo Yuden says it will last 100,000 cycles while the ELNA says 1000 hours at 70C. Is one better than the other in that regard? \$\endgroup\$
    – John
    May 26, 2017 at 19:22
  • \$\begingroup\$ John, there is a rule of thumb that says every few ºC you can reduce, you can multiply the lifetime by a factor to get a ballpark of the lifetime at "ambient" temp (I'm guessing your xbox spends the majority of the time at less than 70ºC). I'm not gonna quote the number since I have no way of backing that up. But this way you can estimate approx how long one will last vs the other. Honestly, I wouldn't worry though. Does 100000 cycles not feel enough? Just buy 2 then :P \$\endgroup\$
    – Wesley Lee
    May 26, 2017 at 20:03
  • \$\begingroup\$ Also, the ESR is quoted at a given frequency. At practically DC I bet the ESR of both caps will be identical. So 0.5Ohm won't make a difference for your application. \$\endgroup\$
    – Wesley Lee
    May 26, 2017 at 20:04
  • 1
    \$\begingroup\$ @John. ESR is irrelevant . Leakage current is spec you need to hold a charge for a long time. A battery is even better. Alkaline will last 10 years \$\endgroup\$ May 30, 2017 at 16:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.