10uF 35V decoupling capacitor alternative - Page 1
Forum section: electronics / beginners

author: Aykut1
author_rank: Newbie
timestamp: July 29, 2025, 11:59:07 am
content: Hi, I have designed my first PCB, which includes 10uF 35V decoupling capacitors close to the 5V, +5V, +3.3V, and 30V sources. However, I was unable to find this SMD component anywhere near my area, and ordering is taking a lot of time. Is there any other value I can replace this component with? I know the basic working principle of a decoupling capacitor, but I am not sure how does the value range of the component affects. I would appreciate it if anyone can recommend alternative values and also explains how can the range of it is determined or calculated. If it depends on the system I can also share my schematics and design. Thank you!

author: Zero999
author_rank: Super Contributor
timestamp: July 29, 2025, 12:04:35 pm
content: What sort of capacitor is it: ceramic, aluminum electrolytic, tantalum?10µF is a very common value, but for decoupling it's unimportant, although higher is generally better: 15µ, 22µF will be fine. Always go for a higher voltage rating than the voltage rail, especially for tantalum which might need to be double theNote that ceramic capacitors have a saturation effect, like inductors, meaning their capacitance decreases, as the voltage is increased, so you might need to use a much higher voltage rating and value, to achieve the required capacitance.

author: Psi
author_rank: Super Contributor
timestamp: July 29, 2025, 12:41:36 pm
content: Use 10uF 50V that should be easy to find.

author: PGPG
author_rank: Frequent Contributor
timestamp: July 29, 2025, 12:42:26 pm
content: Quote from: Aykut1 on July 29, 2025, 11:59:07 amI don't understand English word 'decoupling' perfectly. For me decoupling capacitors are 100nF I locate as close as possible to IC supply pins.At PCB supply inputs electrolytic capacitor are frequently used. Even mostly I use SMD elements for these capacitors I select THT parts.A practical rule for electrolytic capacitors is that their nominal voltage should be 2x their maximum working voltage.So for -5V and 5V you should use capacitor with at least 10V nominal voltage, but for 30V I would use at least 60V capacitor.Big capacitance (like 470uF, or more) is needed when capacitor is used to keep voltage between charging pulses coming from rectified AC 50Hz input. If supply voltage is already stabilized you can use only small (like your 10uF) capacitors.

author: Zero999
author_rank: Super Contributor
timestamp: July 29, 2025, 01:06:47 pm
content: Quote from: PGPG on July 29, 2025, 12:42:26 pmThose are both examples of decoupling capacitors.QuoteYou don't need to go as high as double the supply voltage for aluminium electrolytic capacitors, on a regulated power supply, unless the temperature is high. 6.3V is fine on 5V logic rails. Tantalum yes, but aluminium is fine with a 10% or so margin.For smoothing capacitors, on a bridge rectifier connected to the secondary of a transformer, you want the capacitor to be rated to at least the maximum peak voltage. The maximum RMS voltage you can expect is the nominal unloaded secondary voltage plus 10% or so, to account for the tolerance of the mains voltage. If the unloaded secondary voltage isn't specified, then the data sheet often gives the regulation factor, a percentage which you need to add on to the secondary voltage to get the unloaded voltage. If this isn't available, then look at other transformers with similar power ratings and make an estimation.For example:12V transformer, 25% regulation factor.VRMSMAX= 12×1.30×1.10 = 17.2VThe capacitor needs to handle the peak voltageVPEAK= VRMS√2 = 24.3VSo a 25V capacitor will so, considering there will be some diode voltage drop and it's not going to be exposed to this voltage continuously. As always keep the capacitors away from the heat sink. By all means consider a higher voltage if there's room, but higher temperature rating is more important, than voltage.

author: Psi
author_rank: Super Contributor
timestamp: July 29, 2025, 10:02:04 pm
content: Quote from: PGPG on July 29, 2025, 12:42:26 pmA decoupling capacitor can mean powersupply smoothing capacitor, or AC bypass cap.Decoupling is used for it's dictionary definition. To separate, disengage, or dissociate one thing from another.It can be disengaging the powersupply noise. Usually 100nF or 1uF, but can be 10uF too.Or separating the AC part of a signal from the DC part.

author: Analog Kid
author_rank: Super Contributor
timestamp: July 29, 2025, 10:13:39 pm
content: I thought the kind of decoupling caps the OP is writing about are generally 0.1μF or so, not tens of μF.At least that's what I've always read. You know, the caps you're supposed to put next to pretty much every IC, between Vcc (or Vdd) and ground, to protect the chips from supply spikes and other crap.Why such large values here?

author: TimFox
author_rank: Super Contributor
timestamp: July 29, 2025, 10:30:15 pm
content: I agree about using 100 nF for “decoupling”, which I interpret as reducing the effect on the Vcc of U2 from current variations/spikes in U1, so you place them close to each Vcc pin of U1 and U2 .“Smoothing”, usually several uF, is used to reduce the ripple on the Vcc rail from the source of Vcc; another term is “reservoir” capacitance, especially for a single huge capacitor on each board.I am sometimes criticized for making careful distinctions in terminology instead of calling every part by the same term.Of course, decoupling capacitors must have very short traces or wires to each device.In higher impedance (high voltage, low current) circuits such as multi-stage tube amplifiers, it was common to insert a reasonable resistor (sometimes inductor) from the device/decoupling capacitor node to the supply rail to improve the decoupling effect, but that is impractical for IC power at 5 V or lower.

author: Psi
author_rank: Super Contributor
timestamp: July 29, 2025, 10:34:49 pm
content: You can decouple the powersupply rail for an IC by placing a 100nF near that IC.You can also decouple your entire PCB from its external power source by putting 10uF/100uF/1000uF caps at the DC input to your PCB. Often we might call this bulk capacitance, but you are really just decoupling the power on your PCB from the external power source.Same concept but the scale/scope is different.

author: Analog Kid
author_rank: Super Contributor
timestamp: July 29, 2025, 10:44:22 pm
content: Quote from: TimFox on July 29, 2025, 10:30:15 pmI think in this case you're perfectly justified in taking care in terminology.Quite helpful, actually.

author: Psi
author_rank: Super Contributor
timestamp: July 29, 2025, 10:49:56 pm
content: I agree that it's always better to avoid using the words decoupling capacitor as a catch all term. Better to save decoupling cap for the rail caps near ICs and use 'bulk capacitance' or 'AC bypass cap' for other cap usage.I was just trying to help PGPG understand that 'decoupling capacitor' can have a pretty wide meaning and that's something to be aware of.

author: TimFox
author_rank: Super Contributor
timestamp: July 29, 2025, 11:07:48 pm
content: When choosing a decoupling capacitor, it is useful to consider the current in each device and its likely variations and frequency. 100 nF is typical for a small digital IC. Similarly, a smoothing capacitor depends on the power supply ripple frequency, and lower-frequency components of the total circuit current draw.

author: PGPG
author_rank: Frequent Contributor
timestamp: July 29, 2025, 11:22:28 pm
content: Quote from: Zero999 on July 29, 2025, 01:06:47 pm12V transformer primary coil have to be supplied by something - typically AC mains. At AC mains surges happen from time to time. These are the moments when having nominal voltage 2x higher than calculated peak voltage will be helpful.

author: Analog Kid
author_rank: Super Contributor
timestamp: July 29, 2025, 11:35:10 pm
content: Quote from: PGPG on July 29, 2025, 11:22:28 pmI don't think those surges are likely to be 2X the line voltage.And if they were, they'd be of such short duration as to not be a problem.So I think 2X capacitor voltages are overkill. As most other folks also seem to think.

author: Zero999
author_rank: Super Contributor
timestamp: July 30, 2025, 07:19:46 am
content: Quote from: PGPG on July 29, 2025, 11:22:28 pmElectrolytic capacitors handle short term over voltages fine. The impedance of the transformer (leakage inductance and resistance) also help to limit the spikes.Open up any 230V mains powered switched mode power supply and you'll find it doesn't have 2×230√2 = 650V capacitors: 400V is the most common rating, occasionally 450V. Perhaps 650V is common in 400V three phase power supplies, but I've never opened one up and wouldn't be surprised if two lower voltage capacitors are connected in series.

author: PGPG
author_rank: Frequent Contributor
timestamp: July 30, 2025, 11:04:35 am
content: Quote from: Analog Kid on July 29, 2025, 11:35:10 pmYou are right. They are not 2x.They are much higher and limited to 2x by protecting elements.One of EMC tests each AC 230V powered device offered at marked have to pass is Surge test. The device get at AC230 supply a serie of 1kV surge pulses of internal resistance 1.2 ohm (if remember well).Pulse shape (rise time/fall time) is standardized.1kV is the minimum value used for devices destined to home use. For industrial devices higher pulse levels are used.Don't think of short times when supply voltage is little higher but of pulses that happen when lightning strikes nearby.Quote from: Zero999 on July 30, 2025, 07:19:46 amRight. In one Chinese supply I opened recently there was 450V input capacitor.At 12V supply I assume up to 15V (accumulator when charged get higher voltage than 12V).So to protect input I use transil that not conduct at 15V and starts conducting at 18V.You can check:https://www.st.com/resource/en/datasheet/sm6t100a.pdfthat for SM6T18A Vcl for 1000/10us pulse is 25.2V (after diode I have at input it gives 24.5V)So I assume that at 12V supply I can use 25V electrolytic capacitor.I have never designed protection for AC230 input. May be you are able to limit voltage to less than 2x working voltage so you use 450V capacitor.

author: Zero999
author_rank: Super Contributor
timestamp: July 30, 2025, 01:36:03 pm
content: Quote from: PGPG on July 30, 2025, 11:04:35 amI haven't designed a PSU with 230V input, but I've seen plenty of schematics and hardware. Many of them don't have MOVs or protection diodes across the capacitors and most of them use 400V capacitors. There are occasions when the capacitors blow up, but more often than not, it's the switching transistors which are destroyed by voltage spikes. Electrolytic capacitors rarely fail due to voltage spikes. They're self-healing; if the oxide is damaged by a voltage spike, it will re reform, as the bit of aluminium oxidises. Electrolytics generally fail due to high temperatures, too higher ripple current, or are just cheap/fake rubbish.If you want to design for long life, then you're generally better off going for higher temperature rating and higher ripple current, than higher voltage. Perhaps it's because higher voltage parts generally are bigger, thus have a higher ripple current rating, which is why the x2 voltage rating myth exists? Maybe it works in practice, up to a couple of hundred volts, but electrolytic capacitors with voltage ratings above 500V are less common, so it becomes impractical. Anyway, it's better to look at specifications which directly will result in longer life and a more robust product, rather than just over-rating the voltage, because it normally has the side effect of the capacitor having a higher ripple current rating.