# Three-phase mains AC/DC conversion to 12 V

I am trying to power an Arduino for a DIY power quality/meter project. It has to work in scenarios without neutral (three-phase delta) so it can still work in case of neutral loss or for applications without neutral.

The goal is also to take advantage of the three phases for supply redundancy and has to work even with a two-phase fault.

My strategies so far are:

• Use three step-down transformers in delta/delta arrangement, followed by a three-phase rectifier six-diode bridge and a smoothing capacitor, followed by a step-down regulator module to 12 V. Rock solid, but bulky.

• Rectify first the three phases with a six-diode bridge, put a high voltage series capacitor after the bridge to remove DC bias, and feed it to an AC/DC power module that accepts a high RMS voltage. I found a module up to 500 V RMS. The problem here would be the frequency of the rectified wave would not be 50 but 300 Hz. That would derate the performance of the transformer in the module.

The second option is less bulky but with a lot of uncertainties. What would you choose? Are there other options you see? Required rated power is 10 W for the system (and for each individual component, since it has to work even with a two phase fault).

• ”not be 50 but 300 Hz. That would derate the performance of the transformer in the module” Why? How about just a power supply which can take 560 V DC input? Oct 8, 2022 at 9:24
• seems that for case 2 the dc component will be huge vl× 1.414 × 1.65 / sqrt(3). leaving not much ac after the ac coupling capacitor for the power module to work with. so i guess case 2 is out of the question. Oct 8, 2022 at 9:36
• No no, without AC coupling via capacitor in series. A real power supply. Also, how come 10 W is needed just for some metering? Oct 8, 2022 at 9:41
• You are either not listening or not understanding. Forget about capacitive dropper for 10 W load. A meter like this should be doable with 100 times less power. That would be doable with a dropper. If you are set on 10 W, you need a 560 V DC capable DC/DC power supply. See Andy’s answer below for one. Oct 8, 2022 at 11:04
• Whatever power supply design you end up using - remember that large 3-phase supplies are typically capable of provide a very large current during fault conditions 10's of kA would not be unexpected. Make very sure that you adequately fuse the power supply inputs to your device and pay close attention to the "breaking capacity" of the fuses you use!. Expecting a piddly little 1A glass fuse to break the arc formed from a high current source is not going to work. I learned this lesson when I tripped a few very large breakers in an industrial facility a little over a decade ago :P Oct 8, 2022 at 13:59

Are there other options you see?

Just use a 3-phase flyback power supply like this NXP one: -

There's even a demo board offered so that you can evaluate it.

The above was the first hit on google when I searched for: 3 phase flyback converter.

The next hit is one from Texas Instruments entitled 20 watt 3 phase E-meter flyback reference design: -

The very next hit is one from ST electronics. Followed directly by a design from Infineon.

I've also seen designs from Power Integrations and I seem to remember one from ON semi. Plenty of designs out there.

Choose what is recommended by the experts and don't try and cobble together a rectifier in order to pick off the ripple voltage via a capacitor.

• that seems like the most elegant and safe solution. i would settle for LT8316 because of the tea1721 lead times. However, I think i found a really quick solution for a prototype. if I use 3 yht4s ac/dc modules each wired phase to neutral and then parallel them with a series diode to prevent backfeeding one another. I don't care about balanced power sharing. plus i could have up to 2 phase fault tolerance and also potentially floating neutral tolerance since the yht4s modules are rated up to 500v rms. just an idea... Oct 8, 2022 at 15:42
• If you are happy that the data sheet says it provides adequate isolation and reliability characteristics and, you are happy about the trustworthiness of the supplier and, the quality system operated by the manufacturer then, good luck. Oct 8, 2022 at 15:50