How to convert 12 V DC to 12 V AC 50/60 Hz PSW?

This started with me trying to power 12 V AC LED swimming pool lights - which are always spec’d as AC for historical (and consequently safety certification) reasons - from a hefty 12 V DC leisure battery. While they work fine using 12 V AC, they don't work off the battery.

Most normal “AC” LED lights have a bridge rectifier on the input, so can run from DC. For whatever reason my pool lights can't. I tried up to 17 V DC just in case the lights relied on some peak threshhold detection, without success. There's clearly some AC coupling involved.

But your idea is a good one.

And closer than you think. It's not "all on you" to make the perfect sinewave; you only need to get into the ballpark and let a transformer clean it up further.

If you can run it through an isolation transformer, you don't even need to really be all that sine-wave-ish. Because having a transformer in the stage will tend to clean up the sine waves and block HV chatter. (at the expense of the suppressed harmonics* heating up the transformer, so you'll need to derate it so it can stay cool). Further, the transformer removes any need for your switched output to even go negative! You could make a sinewave that is 0-12V peak-peak (so 4.24VAC RMS with a +6V bias).

Feed that into a 4.24:12V transformer and you'll have a respectable true 12VAC coming out the other side.

If you can't find that size remember you can use two transformers back-back, with primaries connected to each other. Clever selection of primary:secondary ratios can get you all sorts of places. You just need to make sure you are within the transformer's current limits for each section. So on a 500 VA transformer with a 120V primary, that section is rated for 4.16 amps etc.

* When a sinewave is less than perfect, that acts as harmonics. Read all about Fourier Analysis to understand that. Transformers are tuned for a particular frequency and will resist passing other frequencies; that's how transformers clean up dirty power.

• My supplier isn’t keen to disclose details of the lights, as they don’t want anyone identifying the OEM and trying to source directly. 12 V pool lights are (almost) always rated as 12 V AC – and don’t work with DC. It’s historical, but also something to do with safety certification. I’ve since looked at many suppliers for alternatives and found just a few 12 V DC pool lights, but they won’t fit the light recesses in my pool wall. All things I didn’t know at the outset. Oct 1, 2020 at 8:25
• Thank you for the suggestion around using an isolation transformer for waveform smoothing. It’s not something I’d considered. As you noted there will be losses – so it doesn’t address the efficiency aspirations. But I’ll explore it further. Oct 1, 2020 at 8:25
• @David00 If the supplier wants to play the middleman, then they need to relay your questions and repair parts requests This is routine, and resellers have been doing it for 100 years. Unfortunately a trend we're seeing is companies buying cheap Chinese off alibaba.com, and then demanding Tier 1 (nosebleed) prices for it. That would flag up as complete inability to answer questions or provide repair parts. As far as pool lights not working on DC, that is complete balderdash - historics were incandescent which couldn't tell the difference. Sorry if I am the bearer of bad news. Oct 1, 2020 at 17:04
• I’ve contacted several pool light suppliers, all of whom have said their lights are 12 V AC only, and won’t work on DC. They may just be saying that. But pool lights are so expensive that I can’t go out and buy them just to test them. You can find some pool lights online (often from China) that are DC rated. There are many articles online noting that pool lights are 12 V AC and not DC. And there seems to be a regulatory aspect to this for safety certification. I can see you have a different perspective. Oct 1, 2020 at 21:17