# Conversion of electric power between AC and DC **URL:** \ **Category:** Factual Questions\ **Created:** [March 4, 2018, 11:58pm UTC](https://boards.straightdope.com/t/conversion-of-electric-power-between-ac-and-dc/810029 "2018-03-04T23:58:54Z")\ **Posts on this page:** 8\ **Page:** 2
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [March 5, 2018, 10:20pm UTC](https://boards.straightdope.com/t/conversion-of-electric-power-between-ac-and-dc/810029/21 "2018-03-05T22:20:11Z")
> [@jz78817](#): > > a 1F capacitor about the size of a soda can would be 15-20 volts at most. They’re better than that now. For instance, take [this supercapacitor](https://www.mouser.com/ProductDetail/AVX/SCMT22D505MRBB0?qs=sGAEpiMZZMsCu9HefNWqpmIe8hz%2fozG9duPVREl%252bSGKfzmfVSgvC0Q%3d%3d). 5 F, 5.4 V, dimensions of 22x25x12.5. Wire 5 in series and you have a 1 F cap @ 27 V with dimensions of 22x25x62.5. That’s 34 ml, while a soda can is ~330 ml. That said, this is a supercap and has significantly higher internal resistance than the usual electrolytic type, so it’s not useful for all applications. ---
**Author:** ![SamuelA](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@SamuelA](https://boards.straightdope.com/u/SamuelA)\ **Post date:** [March 5, 2018, 10:38pm UTC](https://boards.straightdope.com/t/conversion-of-electric-power-between-ac-and-dc/810029/22 "2018-03-05T22:38:19Z")
> [@Chronos](#): > > It really is amazing how quickly 1 F capacitors went from “size of a garbage can and costing thousands of dollars” to “fit in the palm of your hand for a buck or two”. Kinda. I got excited when reading about super/ultra-caps but my ardor soon cooled when I looked at them for a project. They have 2 huge flaws : 1. Unlike capacitors, they have a very finite **cycle limit**. More than batteries but you can only charge/discharge them so many times. 2. The energy stores is proportional to C **V** ^2, and most super/ultra caps have a very low voltage limit! Their energy storage capacitor is garbage! For the project I was looking at, there was just no way to use them instead of batteries. The only places I think they might be interesting is to use them as an alternative to batteries for some low power device that gets recharged often. Like an outdoor sensor that has a microcontroller and wireless, and a small solar panel. The ultracap would keep it running through the nights and give it a decade + effective life for the overall package. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [March 5, 2018, 10:48pm UTC](https://boards.straightdope.com/t/conversion-of-electric-power-between-ac-and-dc/810029/23 "2018-03-05T22:48:23Z")
Well, like most new things, they are not so much better than the alternatives that they offer a complete replacement. They fit in a useful niche between batteries (high energy density, medium power density, low cycle count) and traditional capacitors (low energy density, high power density, high cycle count). I looked into supercaps to make a coilgun once, but the numbers didn’t work out. In terms of power density, li-poly RC batteries actually worked out better for the cost, but have certain downsides like a more complex charging circuit. In the computer industry, supercaps are widely used as a very short-term uninterruptable power supply. Solid-state drives may have data in a volatile cache that hasn’t yet been written. If the power goes out, it’s possible to corrupt the drive or otherwise lose data. The supercap gives a second or so of power; just enough to write the volatile data and shut down cleanly. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [March 5, 2018, 11:14pm UTC](https://boards.straightdope.com/t/conversion-of-electric-power-between-ac-and-dc/810029/24 "2018-03-05T23:14:41Z")
I’m using supercaps (6x 10F, 2.7v in series) as part of a safe-shutdown circuit for a Compute Module - based embedded controller. Since the device needs to be able to recover from an unexpected power failure, I needed a way to power the Compute Module for no more than 1 minute after a power failure was detected, to give it time to write data to Flash, and to flush all buffers. I considered batteries, but the supercaps had a much simpler charging circuit, and more importantly, had much, much better high-temperature longevity. Since this device is outside, in a non-temperature-controlled housing, it can easily reach 125°F (or more) in the summer. This temperature will cook pretty much any rechargeable battery in a few years. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [March 5, 2018, 11:20pm UTC](https://boards.straightdope.com/t/conversion-of-electric-power-between-ac-and-dc/810029/25 "2018-03-05T23:20:24Z")
So-called supercaps have their place. But they are sometimes used inappropriately for people who only understand capacitance and voltage ratings. if the design requires the caps to deliver a lot of energy over a short period of time, supercaps will often fall flat on their face. That’s because the ESR and ESL for supercaps is very high. In layman’s terms this means that, while it’s true a supercap can store a lot of energy, getting the energy _into_ and _out of_ the cap is a relatively slow process (compared to other types of caps). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 5, 2018, 11:27pm UTC](https://boards.straightdope.com/t/conversion-of-electric-power-between-ac-and-dc/810029/26 "2018-03-05T23:27:43Z")
But still fast compared to batteries. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [March 6, 2018, 12:25am UTC](https://boards.straightdope.com/t/conversion-of-electric-power-between-ac-and-dc/810029/27 "2018-03-06T00:25:11Z")
> [@Chronos](#): > > But still fast compared to batteries. still application-dependent. just like a lot of other situations, with capacitors or batteries it’s: 1) capacity, 2) high charge/discharge rate, 3) size/density. and you can only pick two. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [March 6, 2018, 12:32am UTC](https://boards.straightdope.com/t/conversion-of-electric-power-between-ac-and-dc/810029/28 "2018-03-06T00:32:36Z")
> [@jz78817](#): > > still application-dependent. just like a lot of other situations, with capacitors or batteries it’s: 1) capacity, 2) high charge/discharge rate, 3) size/density. > > and you can only pick two. Which is why supercaps are often paired with batteries. The batteries provide the deep storage, and the caps are used to supply quick bursts of current. [Previous page](https://boards.straightdope.com/t/conversion-of-electric-power-between-ac-and-dc/810029.md?page=1)