Replacing 10uF 25V electrolytic with ceramic

[1980s_john]

Hi,

I am currently servicing a Wayne Kerr B605 LCR bridge (from the 80s or early 90s) I bought off ebay, and want to replace a couple of failing 10uF 25V axial capacitors with some very long life (5000h minimum) replacements. There seems to be very little choice when it comes to new electrolytics, eg Vishay AHT series which are expensive (£2 each including VAT).

I've read and seen Dave's video (eevblog #33) about modern multilayer ceramic caps which now go up to 10uF 25V and beyond. I guess these have extremely long life but I can't find a number. I see Mouser sell a wide range, but am now confused about which one to go for. For example TDK offer FG and FK series (about 40p to 50p each so much cheaper), but I don't know what the electrical pros and cons are between different series or brands., so please can anyone enlighten me? The application is filtering DC rails so should be little ripple current, so would any type with enough voltage headroom be fine?

Regards,
John
PS Just found Vishay RLL series, 5000h @ 85C and 200,000h @ 40C - and Arrow currently doing free shipping. Still interested in ceramic caps as I have no clue about them and welcome some comments.

[DaJMasta]

It's important to get higher voltage caps than the electrolytic you're replacing - there is a pretty significant derating curve with DC bias on a MLCC cap, so if you just match the uF and voltage of the electrolytic, then you'll effectively end up with less capacity (that is, as long as the rail you're filtering is not like logic level).  The ceramics will probably be lower ESR than the electro, but so long as the circuit isn't dependent on it for stability, it probably won't be a big problem.  Different series will have different geometries and dielectrics, giving you different parasitics and combinations of ratings.... but for most applications these aren't as critical.

It's worth mentioning that you could also run into microphonic effects if the cap is in a high noise or vibration environment, and that kind of issue can be difficult to predict until it's installed.  You could also consider using a tantalum, maybe with a ceramic in parallel to improve transient response (tantalums have a high ESR)... but I think this is more complicated to figure out.  You can also just get long life electrolytics, there are 5000h 105C rated caps out there - and maybe you should look at aluminum polymer caps which tend to have higher temperature/lifetime ratings to regular electrolytics.

[tautech]

Put similar back and if it's in a remote bulk capacitance role standard ESR caps will be fine.
I wouldn't use MLCC for the reasons outlined previously however if you must modify it use tantalum and with a good voltage safety rating.

[Belrmar]

a 10uf might be quite high size , i would reccomend to go towards metalized film resistors or modern tantalum if volume is a concern

[David Hess]

Which capacitors on the schematics are we talking about?  I see 10uF adjustment pin bypasses, low current decoupling, and an AC coupling capacitor made with two back to back capacitors and diodes.  These could all be replaced with solid tantalums or ceramics.  100 volt aluminum electrolytics would be an easy way to extend the operating life of an aluminum electrolytic but forget axial parts; use radial parts and bend one lead back and insulate it.

Watch out for the high voltage coefficient of capacitance for ceramic parts though and definitely do not use them on for that AC coupling circuit.  Once you use 50 volt parts to avoid most of that in the other circuits, they are going to cost as much as a solid tantalum capacitor anyway.