Any disadvantages of using small capacitors in high current circuits? - Page 1
Forum section: electronics / projects

author: DylanM
author_rank: Contributor
timestamp: October 17, 2012, 09:14:42 am
content: I'm currently designing a high current 12 volt regulated DC power supply. The transformer I will be using can provide quite a hefty current, up to 41.7 Amps.I've found all the components I need except the smoothing capacitors. According to some quick calculations, I'll need around 220000uF to effectively smooth 40 Amps. Capacitors of this size are rare and expensive, so I was thinking of using multiple smaller capacitors in parallel instead, maybe 5 x 50000uF for example. Would there be any disadvantages to doing this? In theory it seems like it should work, but these capacitors are so small it's hard to believe they would cope in such high current environments.So, would it be okay to use multiple small capacitors, or will I just have to shell out some cash for one great big one?Thanks

author: Kremmen
author_rank: Super Contributor
timestamp: October 17, 2012, 10:22:43 am
content: Multiple smaller caps is a preferred solution anyway, because that way you can distribute the ripple current over several physical components.In fact, you should do sufficient analysis of your circuit so that you know what the ripple current actually turns out to be, and can make proper component selections based on that. The ripple is a killer in high current psu applications. You will want to use reasonably low ESR caps to avoid heating them due to ripple losses, with resultant electrolyte loss and failure over time. Rifa, Cornell and similar mfg's make suitable components for this kind of use. They won't be cheap however.Also at these currents you should pay attention to power factor correction already. A traditional transformer-rectifier-caps front end will have an awful power factor at such average current levels.Then there is inrush current limiting. If you really install 220 mF of capacitance, you definitely do want to limit the inrush current. Not doing so is a sure way to make the fuse manufacturers rich (and the neighbors angry).What kind of transformer? If a toroid, and with those caps you are approaching where you might wish to limit the magnetization inrush as well. With a traditional EI core, maybe not but give it a thought anyway.

author: Psi
author_rank: Super Contributor
timestamp: October 17, 2012, 10:38:39 am
content: Multiple caps in parallel is almost always better than one single cap.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: October 17, 2012, 11:07:10 am
content: A linear supply at at such high currents ,whats it for ? Perhaps it would be better to consider LC smoothing rather than just using caps at these currents .chokes are much more reliable than large caps at the end of the day.

author: SeanB
author_rank: Super Contributor
timestamp: October 17, 2012, 04:00:22 pm
content: Have a look at Photonic's latest destruction of an arc lamp supply, which used a choke to limit inrush current and do output smoothing as well. With this suppy the inductor would most likely be as big as the power transformer.

author: DylanM
author_rank: Contributor
timestamp: October 17, 2012, 08:45:45 pm
content: Quote from: Kremmen on October 17, 2012, 10:22:43 amThanks for the tips. I'll look for low ESR caps and try to limit inrush current in some way or another. And it is actually an old EI core transformer, so I might disregard that last bit unless I come across a reason to do so.Quote from: kevotronic on October 17, 2012, 11:07:10 amIt's not necessarily just for one project. I just have a nice old 0.5 kVA transformer laying around that I figured I would put to use by making it into a heavy duty power supply.Quote from: SeanB on October 17, 2012, 04:00:22 pmThat seems a little overkill. Couldn't a simple 5W resistor be used, or one of those little general purpose choke coils? Besides, the power supply will normally only draw a maximum of 2 Amps on the primary side, so couldn't I just install a 4 Amp fuse and hope for the best? Or would the inrush current still be an issue?

author: SeanB
author_rank: Super Contributor
timestamp: October 17, 2012, 08:55:32 pm
content: Choke is between bridge and the capacitors, not on the input side. It needs to have a big air gap to keep it from saturation. On the primary side a slow blow fuse and a NTC inrush limiter are recommended.

author: Kremmen
author_rank: Super Contributor
timestamp: October 18, 2012, 06:46:38 am
content: Personally i hate those NTC based limiters. While they do kinda work there are drawbacks. Firstly, the idea of constantly cooking a component during normal operation is repulsive (personal opinion) and produces unwanted heat you need to get rid of. Secondly, in a case of rapid restart, such a limiter doesn't limit anything (objective truth). After power off, the NTC will stay hot for some time, allowing whatever surges to pass freely on immediate restart. So it is not a general solution for surge limitation.As an example, attached is a diagram for a limiter i designed for a big CNC mill servo drive. This one limits the inrush of a 4.5 kVA primary transformer but is equally applicable for other purposes. It has been designed fail safe so as not to fry the limiter resistors in case of power loss to this circuit.

author: Kremmen
author_rank: Super Contributor
timestamp: October 18, 2012, 03:31:27 pm
content: Quote from: AcHmed99 on October 18, 2012, 12:23:29 pmActually the relays shown in this circuit are not the load bearing ones. They are control relays for much larger 3 phase contactors that sit in the main circuit cabinet in this application. One of those connects the main power and the second one bypasses the limiter resistors after the set delay. In this kind of application it is most straightforward to use potential free contacts for such switching, allowing arbitrary circuit arrangements.Of course the relays on the board may be used to switch the load directly as well, but then single phase only and with the power/current limitations you noted.

author: SeanB
author_rank: Super Contributor
timestamp: October 18, 2012, 06:41:30 pm
content: I did the same on my variac, but just used a capacitive dropper and a 48V relay to bypass the resistors, along with a thermal fuse just in case. There is 100uF across the bridge to do the delay, it takes a second or so to charge up enough to switch the relay on, and disconnects fast enough on power off. The inrush current was not enough to blow the 10A slow blow fuse, but would be enough to occasionally trip the 20A breaker in the panel.

author: Zero999
author_rank: Super Contributor
timestamp: October 19, 2012, 09:16:47 am
content: Are you sure it's worth the trouble?Have you looked at the price of a 12V 40A SMPS?How does it compare to all of those capacitors, an inductor, rectifier, regulator and all the heat sinking required to dissipate all of the excess heat?