Low voltage ac to dc rectifier

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hio77

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#242879 18-Nov-2018 17:22

So having a bit of trouble locating decent options within nz (i probably could order a rectifier build for wind turbines from china but I'd perfer something solid...)

I'm looking at taking 24vac as an input (150va)
And convering it out to 12vdc about 5a (over spec for expansion i expect normal draw to be about 2a. )

I'd prefer to not build my own rectifier given i want something solid amd heatsink would be required for that amperage.

Dropping down to 24dcv would be fine too as then its simple dc-dc

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#2128980 18-Nov-2018 18:49

What are you getting 24ac from?

hio77

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#2128982 18-Nov-2018 18:54

andrewNZ: What are you getting 24ac from?

240v to 24vac transformer. Stanard sorta transformer you can get off the shelf at the likes of jaycar is the idea..

Trying to avoid using dc for the run since that really has quite some loss that I'm not really comfortable with.

Lv being opted for so its got far less regs requirements etc.

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richms

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#2128988 18-Nov-2018 19:12

Losses on 24vac are no different to 24v DC.

Just get a 48v power supply at one end, and a cheap "golfcart" DC to DC converter at the other end. 48v will lose less than 24 does.

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hio77

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#2128989 18-Nov-2018 19:15

richms:
Losses on 24vac are no different to 24v DC.

Just get a 48v power supply at one end, and a cheap "golfcart" DC to DC converter at the other end. 48v will lose less than 24 does.

Are you sure? Calculations for 200m says otherwise...

If thats so cool. Dc is easy for me. Ac is a beast that i dont know all too well...

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richms

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#2128990 18-Nov-2018 19:22

Might even be worse on AC if you have something that is only taking current from the crests of the waveform flattening it out.

The thing about AC losing less over distance is when its stepped up in voltage at one end and then down at the other end back in the dark ages when the only way to change voltage was a transfomer which required AC.

There is a corrosion benefit with it being AC which is why its still used on crap LED garden lighting with the resultant flicker that unsmoothed AC into LED gives you. Ive changed mine all to 48v with cheap little DC-DC boards so I could put milight controllers inside the poles of them, have seen a couple of joins totally corrode away on one wire when they were fine for years off the AC transformer, but in saying that the DC is on most of the time but the AC was only there when I wanted the lights on.

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hio77

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#2128991 18-Nov-2018 19:32

Interesting.

Others here heavily reccomended ac over dc to me.

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PolicyGuy

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#2129048 18-Nov-2018 20:58

Possibly a mad idea, but would something from a truck do the job?

Many have 24vDC electrics, and presumably get input from an alternator (variable frequency constant voltage maybe?) so must run through a rectifier with 24vDC output
Maybe?

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Tracer

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#2129052 18-Nov-2018 21:15

Take a look at long-range point-point links in power systems, they're not AC. ;)

Aredwood

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#2129058 18-Nov-2018 21:37

Due to power supply energy efficiency regulations, you generally can't make old school transformer step down power supplies. Which is why it is difficult to find what you want off the shelf. Unless you make it yourself

Another vote for DC myself. Only because you can easily setup a DC UPS at the source end. And also add solar if you want.

If you do still want to go with AC. Just make it yourself. Get your step down transformer, connect it to a bridge rectifier, then put some large value filter capacitors across the BR output. You will then have approx 32VDC. Then just get a DC-DC converter to reduce that to 12V.

Just be aware that most of those DC-DC converters from Aliexpress or similar sources, don't have over voltage protection. So add a crowbar circuit or shunt regulator to the output. So your connected equipment won't be destroyed if the converter fails, and passes its input voltage to its output terminals.

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#2129061 18-Nov-2018 21:48

Usual cause of the excess voltage I have had from them has been the crap trimpot dying, so do not get an adjustable one. One absurd one I saw had a standard PCB 20 turn blue thing sticking out the back of a potted enclosure. So hardly water resistant at all.

If you want to take AC up there, there are many many boards aimed at the audio market that have a bridge rectifier and capacitors on them. If you do get one and its a split rail one you cant use half of it like you might expect tho.

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#2129073 18-Nov-2018 22:10

Aredwood: don't have over voltage protection. So add a crowbar circuit or shunt regulator to the output. So your connected equipment won't be destroyed if the converter fails, and passes its input voltage to its output terminals.

Regardless of any gear listing protection or not, it's fused and shunted.

crowbar circuit i didnt really consider, might sit down and run the numbers on it..

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tripper1000

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#2129114 19-Nov-2018 08:34

I don't think you are going to find the perfect prebuild solution. You're going to have to build some of it yourself.

I've done similar things and I have found AC to be better than DC. Whilst on paper AC and DC have the same losses, I believe most on-paper calculation fail to consider that 24v AC is actually 24v RMS which means the peak voltage is closer to 41 volts, which obviously suffers lower losses than true DC 24 volts. If you are pulling max current the losses would be similar, but you have stated you are over-spec-ing it, so you won't be pulling that peak voltage all the way down to 24v.

tripper1000

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#2129115 19-Nov-2018 08:38

Tracer: Take a look at long-range point-point links in power systems, they're not AC. ;)

Ah yes, but why? Long distance is AC unless it has to go underground or undersea for a long stretch (eg Cook Strait) - they use DC for long under-ground/under-sea runs because it reduces capacitive/inductive losses into the soil/water. At mere 200 meters this isn't going to be a problem.

ResponseMediaNZ

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#2129119 19-Nov-2018 08:53

hio77: So having a bit of trouble locating decent options within nz (i probably could order a rectifier build for wind turbines from china but I'd perfer something solid...)

I'm looking at taking 24vac as an input (150va)
And convering it out to 12vdc about 5a (over spec for expansion i expect normal draw to be about 2a. )

I'd prefer to not build my own rectifier given i want something solid amd heatsink would be required for that amperage.

Dropping down to 24dcv would be fine too as then its simple dc-dc

Would you look to go straight to 12v or do you need to take 24v off somewhere else? I have a couple of options

hio77

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#2129131 19-Nov-2018 09:09

ResponseMediaNZ:

Would you look to go straight to 12v or do you need to take 24v off somewhere else? I have a couple of options

Absolutely. Meets the endgame significantly better really.

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