Capacitor inrush current - Page 1
Forum section: electronics / beginners

author: eel
author_rank: Newbie
timestamp: March 23, 2023, 09:07:07 am
content: I'm building a fan hub from parts I have laying around and want to figure out how much current will be drawn by the capacitors at start.I've pretty much copied the design of those cheap pc fan hubs from aliexpress, amazon, etc.I looked at capacitor charge current calculators but i'm having trouble with the resistance values. Assuming I use a power plane the resistance is fairly low, does it then just come down to the esr of the capacitor?

author: CountChocula
author_rank: Unknown
timestamp: March 23, 2023, 12:35:07 pm
content: Hi! In the absence of any external factors, the inrush current is a combination of (a) the charging of the capacitor itself under ideal conditions, which is given by the formula I = C dV/dt (capacitance multiplied by the rate at which voltage changes over time), and the cap's ESR, which basically acts as a current limiter (I = V/ESR).External factors could be things like the presence of a current-limiting device (a resistor, for example), or practical limitations in the amount of current that the power supply can provide.Hope this helps!—CC

author: Terry Bites
author_rank: Super Contributor
timestamp: March 23, 2023, 01:02:26 pm
content: Assuming a value for ESR is risky. The quoted values tend to be maximum, if its lower than the spec'd value you might get it badly wrong. Its not a spec you can rely on.A series indcutor might be used to add a predictable di/dt. There's no free lunch of course, the caps may need to be bigger to cope with peak demand.

author: eel
author_rank: Newbie
timestamp: March 23, 2023, 11:27:32 pm
content: Quote from: CountChocula on March 23, 2023, 12:35:07 pmThanks that pretty much confirms what I was thinking. The limits of the power supply is why I asked, even though it's based on a pc fan hub it won't be in a pc so thats probably the limiting factor and all those caps charging at once will draw a bit of current.Quote from: Terry Bites on March 23, 2023, 01:02:26 pmNot super concerned about the demand on the caps, more the power supply.

author: WattsThat
author_rank: Frequent Contributor
timestamp: March 24, 2023, 01:31:37 am
content: Don’t overthink it, any 12 volt supply that can power six fans can certainly charge a 100-200uf capacitor at power up.

author: eel
author_rank: Newbie
timestamp: March 24, 2023, 03:19:41 am
content: Quote from: WattsThat on March 24, 2023, 01:31:37 amThat was my initial thought, then I started overthinking it hahaNext question, if the capacitors and fan connectors are all connected to a power plan am I better off just having one large capacitor since none of them are directly connected to a connector? Or should I have the capacitors connected to the plane and the fan connectors connected to each capacitor via a short trace?

author: Swake
author_rank: Frequent Contributor
timestamp: March 24, 2023, 06:43:53 pm
content: Why does this needs caps at all?I guess your using 'small' 12V PC fans. Add up the amps these fans pull and if the power supply can take that you're good. Else you need a bigger PS er less fans.