Why use 2 Electrolytics in parallell (Meant SERIES!!)  rather than one? - Page 1
Forum section: electronics / beginners

author: watchmaker
author_rank: Unknown
timestamp: June 01, 2024, 10:32:29 am
content: I am just starting the course sections on capacitors (Real Analog, Boylestad, etc). So it will be a while until I am conversant in the subject.In the meantime, I replaced the capacitor array in my 1990's ultrasonic watch cleaning machine which had failed.While I was there, I decided to replace the electrolytics on the motor timing and control boards (wow, the new ones are 1/4 the size). After the transformer on the speed control board, the original caps in the rectification section were two 50 uF 25 v electrolytics in series to yield 25 uF.Why this rather than a single 25 uF cap?While on the subject, why are there sub 1 uf electrolytics? When I was in shop class (pre IC; tube theory dominated), if I remember right, we were taught the primary reason for electrolytics was their storage capacity. We used ceramics for most things and ECs when we needed large capacitance values (like PSUs).Why are these small value ECs important?THANKS!Dewey

author: Gyro
author_rank: Super Contributor
timestamp: June 01, 2024, 10:48:17 am
content: Confusing... Your thread title says 'parallel' and your text says 'series'. Have you checked the polarities of the two 50uF series capacitors? Putting two electrolytics in inverse series is a cheap way of creating a bipolar electrolytic capacitor.As for parallel capacitors, using multiple capacitors is a convenient way of improving space usage (minimizing headroom, irregular shapes etc. and potentially improving ripple current rating in a given volume.P.S. Watch out for the ripple current rating of your new smaller caps. It's often worth increasing the voltage rating of the replacements in order to achieve a more similar case size (and much longer life).

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 01, 2024, 11:03:11 am
content: Quote from: watchmaker on June 01, 2024, 10:32:29 amThese are basically obsolete, and pre-date the time when high-value ceramics were cheap.High-value through-hole ceramics haven't really caught up, so through-hole electrolytics in the 1-10uf range can still make sense, whearas there is rarely a reason to use a SMD electrolytic in this range nowadays

author: Siwastaja
author_rank: Super Contributor
timestamp: June 01, 2024, 11:04:27 am
content: Parallel: BOM management (same parts can be reused), better surface area to capacitance ratio (better cooling), more flexible form factor for the final product (can place the two in arbitrary positions, fitting the available space better than one larger)...Series: High voltage, absence of parts rated to that voltage. Leakage current balancing is super important, either by careful choice of parts used + derating (e.g., for 300V, two 250V parts in series would work), or adding explicit balancing resistors, TVS clamps of whatnot. This balancing hassle is reason enough to avoid series connection if at all possible.Series is also usable in configurable (by switch) 110V/220V supplies in voltage doubler configuration, but this is getting rare, as active PFC configurations with wide input voltage range is now dominant.Two 25V electrolytics in series sounds weird. Not a good idea because of the mentioned voltage balancing issues. Paralleling two 50V caps would make more sense, for the same physical size/shape.Small electrolytics (instead of MLCC) still make sense when high ESR is desirable (e.g. for stability). Although below 2.2uF or so, one would usually choose a tantalum cap, or MLCC + explicit series resistor. Or, if you use an aluminum electrolytic cap for high-ESR damping, and calculated 1uF is "enough", you could usually still use, say, 10uF instead. Therefore usage of sub-1uF aluminum electrolytics would be increasingly rare.

author: watchmaker
author_rank: Unknown
timestamp: June 01, 2024, 11:37:48 am
content: Quote from: Gyro on June 01, 2024, 10:48:17 amThanks! Corrected title. Yes, they were (and replaced) in series.)

author: watchmaker
author_rank: Unknown
timestamp: June 01, 2024, 11:44:25 am
content: Quote from: Siwastaja on June 01, 2024, 11:04:27 amTHANK YOU for the details on when which is used today.As for the repair, these machines were made in relatively small quantities by a company that started in the 1930s. Mainly sold the clean/rinse solutions (L&R). The boards were still the PCBs with solder traces (this in the 1990s). So I can see they were using up old parts. The caps in question were old Mallories from a 1970s catalog (marked mF for micro farards rather than uF). Yep, I measured them to make sure.And I used double the voltage ratings (this we were taught in shop).The funny thing is these are still approved for use by all watch brands (Swatch to Richemont). Used these 20+ year old machines sell for $5K USD. To replace them with a new machine of the same functions costs $11K. So they are worth keeping running.They are a real technological mishmash. Use a 741 for the speed control (I forget the function control IC), a big hanking motor to rotate the jar table and raise the motor column, spring to lower the motor column (no hydraulics any place), 14 microswitches to tell the boards where things need to be. Fiberglass PCBs with solder traces to reinforce the glued on copper traces. And like I said, these were produced form about 1995 to 2005.The schematics are held close by the person who bought the inventory when L&R orphaned the machines. (I would do the same). But, it is easy enough to backlight the boards, photo them and then make the schematics. I like the repair guy and he keeps the machines going for working watchmakers, so I will not make my schematics public.Dewey