Reducing 12v DC to 5v DC

[Lasermad]

I am pushing 240 volts into a power supply , it outputs 12v DC

Now i need to run cable with the 12v for 50 meters and reduce the 12v to 5V

any easy cheap way to do it ?

the 5v needs to be pretty clean stable power as its pushing into a
driver board which sends 5v to a actuator and then reduces to 3v for a small red laser

Ideas with sources would be appreciated

[CountFunkula]

How much current do you need on the 5V side? How much current can the 240->12VDC supply?

LM7805s are handy for this kind of thing.

[The_Doctor]

Forget it. To carry useful power, even a few hundred milliamps, at 12VDC for 50 metres means the power loss due to voltage drop will be so great you might not have enough left at the far end. It won't be clean, and your only shot is to use VERY heavy cable which you will not want to pay for. Even if you did it, your 50m longwire antenna (which is definitely what it is, even if you're in the remote Highlands) will make sure that the power is not clean on arrival even if it started out that way.

You have mains, so start there, Buy 50 metres of the lightest BRAIDED SCREEN mains cable you can get, likely 6A rated. The braid will keep external signals out, and the mains safely in, and it forms a limited armour that will resist rodent attack or anything else small and hard to defend against any other way. It will also increase tensile strength. Once you have that cable you can drop the voltage and output your DC with any cheap regulated PSU you like at the far end. (Use a residual current breaker at the source end if you're concerned for high safety.)

Even if for some reason you can't use mains you'll still need a higher voltage than 12V. At least 48V. Copper is so expensive now that you could buy the best step-up/step-down boxes you can justify for the required power, they'll still be cheaper than 50 meters of heavy cable, and they'll help clean up the power on arrival too.

I was going to cite some figures for resistance of cable but that's not easy! Wikipedia have an article lacking any figures at all because they're so hard to get. All they say is that it varies between cable makers, which is worse than useless. What matters is that low volts for power over large distances is a very bad idea for several reasons.

EDIT:
An extra way to be safe, and still cheap, is to use an isolated transformer to drop to 110VAC, like builders use for tools, but a small one, like those used for bathroom shaver sockets. This works because you can be sure that both live AND neutral will not be referenced to Earth. Run them either through the braided cable I mentioned, or even through a (very solidly built) balanced audio cable IF it is rated for the voltage and current that will pass through it. So long as there is no risk of confusing it with some other cable you're in business, but braided mains cable might be cheaper. Most cheap switchmode PSU's that can put out 5VDC will operate off 110VAC just as well as 230, and your high volt line will be far safer than direct mains. Ground the screen as for real mains cable, just make sure that there is no connection for the live/neutral wires except for the loop down the cable's inner conductors and back. And add a residual current breaker anyway if you're still anxious about it. (But not an ELCB, because those refer to the Earth you won't be using in your supply wiring). The screen is just a limited armour in this case, but ground it anyway. It will keep RF signals out that way, and help with safety too.

[badger1666]

Hey Paul you can get dc-dc convertors off eBay that will do that job fine
Make sure you use good thick sheilded cable for the 50m run, and don't run near any mains cables

[The_Doctor]

The only thing those will do in this case is avoid the extra loss involved in trying to drop it with a linear regulator.

There's no point in ANY cheap eBay regulator if the price of admission is 50 metres of heavy copper cable! You'll spend many tens of quid to save a fiver!!! And it might not even work.

I'll repeat what might be the best idea from my big post:
Get a bathroom shaver outlet, use the isolated 110VAC pair down a thin braided screen cable rated for the voltage and current of the 110V line. Then ANY cheap switchmode wallwart might be good enough. 5VDC supplies that run on 'universal mains' voltages get chucked out in the trash more often than not. Very easy to get those, and more cheaply than any other source. They are usually 'double insulated' and fully isolated from ground, which makes them perfect with this method. Just ground the screen anyway for extra safety.

[The_Doctor]

How much current is drawn by the actuator and the small red laser? If you're sure the total is well under half an amp you might get by with 50 metres of 2.5 square mm twin and earth mains cable with 12V through it, and a 12V to 5V DC converter intended for car electronics at the far end. Those will cope with any signals induced onto the long line and clean the output well enough. I've used some rated for 15W, so 3A at 5V. Ebay, from China... Go for more power capability than you need to keep the reliability high. Be wary though, they might not tolerate the voltage drop in the cable if it's more than a couple of volts.

There's a table here: http://www.tlc-direct.co.uk/Figures/Tab4.7.htm
So long as it's under an amp you might get by using 2.5 square mm with no more than a couple of volts dropped, but if I were doing it I'd rather use a scrapped shaver outlet and lighter cable and not have to find stuff needing shipping from China just to keep the costs down. The moment you decide you want a few amps (stuff happens!), that expensive run of mains cable will be useless, and the thin 110VAC line will still work well.

[Lasermad]

Ok Doctor , i appreciate the detailed answers

this i think is going to have to be one of those suck and see projects

If all works im likely to be building 50 units so shaver solution not
ideal , i have ordered 4 different types of converter from eBay
3 from china and one from uk ... to experiment and compare
( thanks chris )

The red laser within unit is just a 5mW unit so power draw
is minimal and also i was quoting 50 meters as worse case scenario
most units will be 3-30 metres away .. only a few would be 30-50
metres

As regards wire i guess a decent possible solution is to source
the power cable CCTV guys use as they seem to run long
cable runs and i guess they cant afford too much interference ?

[The_Doctor]

The red laser within unit is just a 5mW unit so power draw
is minimal and also i was quoting 50 meters as worse case scenario
most units will be 3-30 metres away .. only a few would be 30-50
metres

As regards wire i guess a decent possible solution is to source
the power cable CCTV guys use as they seem to run long
cable runs and i guess they cant afford too much interference ?

That changes things, makes it easier.. Several fixed-type installs of low power might work with straight 12V, a 5mW laser even with a linear regulator might want only 50mA. Probably better to use LM317 though, they do better regulation than 7805. If you use caps and diodes (long cables are inductive enough to need them added) as specified in the data sheet it will clean up the power well enough. I don't know what the actuator wants but it looks like any screened cable with two cores intended for balanced lines might do this. Just look for some good compromise between low cost, and low wire resistance. (My idea with the isolated 110V line was based on assuming this was a one-off project likely to grow).

If you're going to use DC-DC converters it might pay to look for some weird voltage no-one wants to use, so long as the far end can use it. Any kind of step-up will help, and likely be paid for by savings on thinner cable. With a buy-up of 50 you might get bulk discounts.

[loopee]

Hey Lasermad

Have a look at using network PoE injectors/, they will do up to 100 meters and will give you 12 volt at 1-2 amp using standard Cat5E cabling and as an added bonus you get Ethernet with it

This will keep your power very clean and stable

Have a look at the Tycon power website http://www.tyconpower.com/products/POE.htm#POE

And more specifically at TP-POE-HP-48GD for the injector and POE-SPLT-4812G for the extractor

you will still need to do the other voltage reduction but this can be done as other have stated with regulators or DC-DC converters

They're slightly expensive but what you spend here will reduce the cabling costs and probably equal out

[dsli_jon]

..'nother idea (IF you can have 'localized' 110/240 or whatever it is, *at* the laser-pj end-point..) I've used in the past for even reeealy-long runs, with perfect-success, would be to get a 'lil DC-DC 'SSR' and then a small, 5V supply, right there with the laser-module, and use the 12V, over your 'long-haul', thru the SSR as a 'local-switch'..

..As long as whatever voltage drop was above the SSR-threshold, 'gating' that direct 5V would certainly be the 'cleanest' provision for the laser, and quite simple / no heat-dissipation concerns, etc.

I just love SSRs..

.02
j