Is 12v safe? 100A power supply. - Page 1
Forum section: electronics / projects

author: mrflux
author_rank: Contributor
timestamp: April 28, 2015, 11:35:05 pm
content: Hello!I'd like to light up a large area with basically 500m of LED strips at 12v. I realize that the strips will have to be cut into seperate pieces, as the strip itself obviously won't be able to carry 100A. I'm sure there are many ways to do this better, e.g. power them with many smaller power supplies.But my question is theoretically concerning safety: The PC PSU that I'm using for testing can supply 40A and seems perfectly safe to touch, although it provides quite a spark when shorted. And as I understand it, 12V is not sufficient to start conducting current through the skin.But if someone gets wet and shorts the circuit or likewise, they'll be able to carry 100A?Should I use fuses?Many smaller power supplies? (40A is still dangerous?)Or simply make sure that the strip is completely isolated/waterproof?Any links or recommendation about further reading will be appreciated!PS. I'm thinking to buy an "HP DPS-1200FB 12V 100A PSU". Would it be safe to touch the terminals on this?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 28, 2015, 11:51:33 pm
content: I don't recall if there are regulations concerning the routing of high amperage (but otherwise SELV) circuits. When the power is high, there is the possibility of fire hazard, regardless of the voltage, so you definitely want fuses for each branch (per strip or per group, but not merely per power supply output because that would be useless against a current-limited type).It's probably desirable to try to wire the strips in series. This will quickly grow beyond SELV, but the wiring need not be any fancier than domestic 120V (or 240V, for that matter) wiring practices.Tim

author: retrolefty
author_rank: Super Contributor
timestamp: April 28, 2015, 11:52:24 pm
content: As you have figured out, 12 vdc is not an electrical shock hazard, however that high a current flow carries several hazards including burns and fires. In the military I was required to remove all jewelry including rings when working on our equipment. Having a wedding band arc welded across a high current source would turn incandescent immediately and would be a very bad and long day.

author: sacherjj
author_rank: Frequent Contributor
timestamp: April 29, 2015, 12:05:53 am
content: I'm running a 150A 12V to charge 144 devices. I can't charge all at the same time, because I need MORE POWER than the 1.5kW. I am using 24 device distribution boards with MOSFET switches for each one. This allows me to segregate the power a little. I use 6 main fuses that are set for the full value of one of those boards (50 A).I'm using very high quality fine stranded 4 gauge wire that is from a car stereo install supplier. Somewhere around 1000 strand to make up the 4 gauge. I'm monitoring the current output of the supply with a shunt. I made power traces on the back of the board and solder 1/16" x 1/2" copper bar along the traces for both 12V and GND. (4 total bus bars, because I have 12.) This is soldered to the board by using a propane torch to heat up the bar and then solder it. (More like plumbing soldering than electronics soldering.)If you fuse along the way (either actual fuses, or PTCs) and put large current fuses RIGHT AT the supply to handle any supply cable shorting. Also, unless you can handle the voltage drop, seriously over spec your supply wiring and traces. When playing with 20A, you can use 12 gauge wiring because that is what they use in house wiring. But you'll find out that you may have more than a volt drop over a couple feet run and some connectors and solder joints.I've load tested some things with 14 gauge nichrome wire. (This is the foam cutting stuff that is usually 28-32 gauge.) 6 strands to make a massive 1.5kW heating element. Think of everything you are dealing with like a potential high power toaster heating element where ever you run it. Then make precautions that fuses will blow before any of those lines get enough current to get hot. Most people don't deal with mains wiring into your home's breaker box. But that is the level of current we are talking here and it is no joke.As to your touching of the terminals. You should have the supply installed in a way where that isn't possible. Most people won't we harmed by touching 12 volts. But fun things happen when someone crowbars the two terminals with a wrench. Hopefully, your supply does a graceful shutdown. The Meanwell power supply I'm using has large bars with a bolt location on the output. I bolt 6 4 gauge cables to that, then run directly to fuses. Then these terminals are covered. Anything shorting out past the fuses will spark a little, then blow the fuse.

author: pickle9000
author_rank: Super Contributor
timestamp: April 29, 2015, 12:13:53 am
content: This is an old story (40+ years when I heard it) basically an auto mechanic with an open wound (cloth bandaged and bloody) and holding onto a chassis ground. The other hand on the starter solenoid he slips with a wrench or whatever an drives the positive 12 volt lead into the palm of his hand and dies. Vehicle not running. Path would be hand to hand internally.I've often thought yup could possibly do it but have never found any news story or reference. In any case it's interesting to think about.

author: ajb
author_rank: Super Contributor
timestamp: April 29, 2015, 12:29:59 am
content: Unless all of your led tape is located in a very small area, distribution losses are going to be substantial at 12V. Its worth looking at placing several smaller power supplies through the installation so that the bulk of your power distribution is done at higher voltage and lower current which will decrease I^2R losses and give more consistent voltages at the LEDs (particularly if you're using RGB tape).

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 29, 2015, 03:25:14 am
content: I have done a teardown on one of those PSUs and it's a very good choice.https://www.eevblog.com/forum/reviews/teardown-hp-dps-1200fb-12v-100a-psu/As for protection, just a bunch of fuses are all you need.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 29, 2015, 05:19:49 am
content: Quote from: mrflux on April 28, 2015, 11:35:05 pmSending 100A over 500m (1500 ft, 1/2 Km) just by itself is a bad idea independent of any other factors. Note that when power is sent over any distance, it is always prefered to use the highest practical voltage to reduce the current (and therefore the losses through the wires.)QuoteYes, 12V is typically safe from chances of electrocution.QuoteEven if they had each hand or foot in a bath of mercury, you can't send 100A (or even 1A) through a human (through the skin) at only 12V. That is not really what the hazard is here.QuoteCertainly. But it has nothing to do with electrocution. It has everything to do with limiting the possibility of burning up the power supply, or shorting the wire, overheating, and causing a fire.QuoteAs mentioned before, sending low-voltage, high-current over distances is a BAD idea.QuoteThat is a completely separate and independent issue. You have revealed no details that would help us understand whether weather would be an issue??QuoteYou need to learn about Ohm's Law and study some practical circuits and implementations.QuoteYes, it would probably be safe to touch the terminals. But that is not really a very useful question.

author: recjohnson
author_rank: Newbie
timestamp: April 29, 2015, 11:01:00 am
content: Richard has given a very detailed reply. Yes, 12 volts are not a shock hazard but a 100-amp source surely carries a risk of starting a fire unless you cover up all exposed conductors. Waterproofing is definitely required if there is a chance of getting your strips wet (rain, garden sprayer etc). However, if you take care of insulation and ensure that the strips or conductors don't hang free, there should be no problem.Fusing the individual branches is good idea.

author: Zero999
author_rank: Super Contributor
timestamp: April 29, 2015, 12:27:05 pm
content: Quote from: T3sl4co1l on April 28, 2015, 11:51:33 pmThe insulation rating on the LED strips may not be suitable for anything more than SELV.Connecting them in series as a good idea but how about going for strings of four and use 48V as a compromise between current and safety?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 29, 2015, 12:41:10 pm
content: Unless all strips radiate out from a single point, multiple distributed PSUs will be more efficient and avoid long runs of heavy cable.Also, use 24V LED strip if you can.

author: mrflux
author_rank: Contributor
timestamp: April 29, 2015, 02:18:42 pm
content: Thanks for all the great feedback! WowBasically, it will be used outdoors, and I'm just trying to understand at what point it actually becomes unsafe.Would 24V still be safe? At what point does it become unsafe? Let's just pretend that some child decides to take out the cables and put them in his mouth.I was thinking to coat the LEDs with transparent acrylic paint to protect them from corrosion etc.But perhaps water, after many years of wear and exposure, will find a way to enter. What, then, would happen if they carry, say, 24, 36 or 48V.Does anybody know the maximum recommended length for normal Chinese LED strips? 5m? 10m?At 20mW per LED, a 300 LED 5m strip would carry 500mA of current at 12V.In summary:Use fuses to avoid fires or damageUse 24V to reduce current and thereby required dimensions of wires etc.Waterproofing to avoid fires - but no risk of personal injury?Overdimensioned power cables to reduce energy loss

author: ajb
author_rank: Super Contributor
timestamp: April 29, 2015, 02:51:24 pm
content: I wouldn't go more than 5m without an additional feeder, even that may be pushing it if you're using high-density strip. In particular with RGB tape, if you go more than 5m you'll run into problems when displaying white--as the voltage drops, the green and blue LEDs fade out first due to their higher Vf, so the color shifts red as you get further away from the feed point. If you wind up with the tail end of a 5m strip right next to the head end of another strip, the difference can be noticeable. Remember, there's no particular reason you can't feed the middle of a section, which means you can do up to a 10m strip with only one feeder.As for weatherizing, you can get silicone encapsulated tape, which is a bit of a pain to terminate since you have to cut away the coating to get to the pads, but would likely be easier than trying to encapsulate the entire strip yourself--and of course you'll need to seal up the joints after you've made them. If you're installing this in such a way that it's going to be completely exposed to the elements, though, you need to pay attention to things like the UV and temperature stability of encapsulants and adhesives, even cable ties to make sure the whole thing doesn't just go to pieces in a few years.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 29, 2015, 03:32:15 pm
content: You need to test to determine acceptable lengths, voltage drop etc. You can feed tape at both ends if you really want a long run without breaks.The advantage of 24V is you halve the voltage drop for a given power.A problem with big PSUs is that the current needed to set fire to something would trip out a small PSU, but can be well within the normal operating range of a big one, so the PSU wins.Suitable fusing at the point where big cables branch off to smaller ones solves this but doesn't help with distributing high currents without excessive voltage drop, or needing expensive, cumbersome cabling.

author: Zero999
author_rank: Super Contributor
timestamp: April 29, 2015, 05:04:36 pm
content: Quote from: mrflux on April 29, 2015, 02:18:42 pmThe child will get a shock if they put the cable's conductors in their mouth, irrespective of the voltage, it could be 1.5V and they'll still feel a tingle.As far as safety is concerned, DC voltages are generally less hazardous than mains frequency AC.The shock hazard is proportional to the current which not only depends on the voltage, but the current path through the body and the resistance between the internal organs and the conductor. If your skin is very dry and you're not in contact with a large surface area of conductor, you'd have to up the voltage of well over 100VDC before you'd even receive a shock. If you have wet skin and are in contact with a large surface area of conductor, then the voltage required to shock you will be much lower. Safety standards vary but generally take into account the environment and whether the short circuit current will cause a fire/arc flash hazard.https://www.tlc-direct.co.uk/Book/7.16.2.htmhttp://www.premiersolutions.co.uk/images/17th%20Edition%20Regs.pdfThe LED strip should be suitably rated for outdoor, otherwise they'll suffer from water ingress and corrosion problems and all exposed connections should be covered so children can't play with them.

author: macboy
author_rank: Super Contributor
timestamp: April 29, 2015, 05:22:16 pm
content: You have already figured out that you can't connect 500 m of LED strip and expect it to work, and you intend to use multiple short sections. Consider the length of cabling needed to connect the furthest section back to your 12V source. What will be the voltage drop through that, and the corresponding brightness reduction? Since you will use a common power supply, you can't compensate for voltage drop in individual runs.I would recommend using point-of-load (POL) power supplies for each pair of strips - one strip in each direction. You can distribute say 48 VDC (nominal) which is still considered SELV and "safe". Your current will be much less, so power loss will be much, much less (power loss is proportional to I2). Each POL supply is a DC-DC converter that is immune to a reasonable voltage drop in the line, providing stable output voltage despite it. You not only have much less current to carry around, but you can also tolerate much more voltage drop before issues occur. This will allow a much smaller gauge wire to be used to distribute the power. The savings from that can be substantial, likely considerably more than the cost of the POL power supplies.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 29, 2015, 05:32:19 pm
content: Or, for that matter, connect the LED strips in SERIES. Four strips in series would operate on 48V. Since the most common strips have connection points at each end, it is easy enough to "daisy-chain" them in series.@mrflux has not revealed whether he is "flying" these strips suspended overhead on cables, or whether they are fixed to some existing structures, etc. There is much design detail influenced by factors which are not in evidence here.Furthermore, those short, self-contained "12V" LED strips aren't the most efficient things for large-scale installations as they burn up a significant amount of power in the current-limiting resistors. A larger-scale lighting design would likely use current-controlled power supply units to greatly increase efficiency.

author: mrflux
author_rank: Contributor
timestamp: May 01, 2015, 12:41:48 am
content: I tested out the strips, and being powered with 12V in one end, with 5m length, the current was 2.15, equivalent to 86mW per LED.When I powered the strips from the middle (thanks @ajb !), the current went up to 2.6 amps, equivalent to 104mW per LED.But when I cut off a small strip of 12 LEDs, the power went up to 160mW.So, in short, I will definitely need to compensate for the voltage loss. Except, in doing so, I would expose the first LEDs to a higher voltage.(each segment of 3 LEDs is powered in parallel, to allow for the whole strip to be powered by 12V)What a mess. I guess this means that each strip shouldn't be longer than about 1m.I'll probably end up making my own strips with a thicker copper layer, and connect each segment in series at 48VSo then I'll have to find another power supply. Hmmmm.Would it be difficult to make a 230 VAC to 48 VDC PSU for LED lighting at 1200w?

author: DanielS
author_rank: Frequent Contributor
timestamp: May 01, 2015, 01:15:13 am
content: Quote from: mrflux on May 01, 2015, 12:41:48 am(-)48V is a very common supply voltage for telecom gear. There should be no shortage of options in the 1-10kW range.

author: bitwelder
author_rank: Super Contributor
timestamp: May 01, 2015, 05:55:37 am
content: Quote from: mrflux on May 01, 2015, 12:41:48 amDepending on how the strip is packaged, you should be able to apply the power at one end and measure the voltage at both ends, so you'd know exactly how much voltage drop you have along the line

author: mrflux
author_rank: Contributor
timestamp: May 01, 2015, 11:00:40 am
content: Thanks @bitwelderFor anyone interested, I tested the voltage drop across a 5m strip (end to end)Voltage at beginning: 12.13 VVoltage at end: 9.38 V

author: Fungus
author_rank: Super Contributor
timestamp: May 01, 2015, 01:05:39 pm
content: Quote from: mrflux on May 01, 2015, 11:00:40 amYep, that sounds about right.Getting power down this strips is difficult. To maintain 12V all the way along the strip you typically need to connect the power supply to both ends of the strip (and even then the voltage will usually still droop a bit in the middle.You might even need to connect a third power wire to the center of the strip if the strip gets too hot for wherever you're installing it.Quote from: Richard Crowley on April 29, 2015, 05:32:19 pmWon't work - see above.Multiple smaller power supplies is a better idea than one big one. With one central power supply you'll have big problems getting the power to the strips without loss (a few meters of cable can easily drop a couple of volts unless it's really heavy gauge). Do plenty of tests to check voltage losses along the wires before you do the final installation.If you really have to use one big one the the answer to "safety" is to put a fuse in each wire coming out from the power supply (and youwillneed multiple wires - sending 100A down a single wire isn't going to work out). Put the fuses right next to their strips so if a strip goes off it's easy to find/test/replace the fuse.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 01, 2015, 01:42:40 pm
content: I agree that trying to use 5m long strips in series is not practical. But you can see that using 5m strips in parallel is not practical either, because the conductors through the strip are NOT big enough to support the entire 5m length.Those strips are manufactured in continuous lengths and chopped up into 5m pieces for convenient sale on a reel. Do NOT assume that just because the strip is 5m long that it is practical to use as a single continuous strip. As you have demonstrated yourself, the strips are NOT capable of passing enough current to support the entire length.I would chop up the strips no longer than 1m each regardless of whether you use them in parallel (12V) or in series (48V or whatever).

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 01, 2015, 06:02:52 pm
content: Quote from: Fungus on May 01, 2015, 01:05:39 pmActually there is a way that could work.Wire them in series by connecting the +ve to end A of the first strip, -ve end of end B of that strip to +ve of end A of the second strip etc.You will still have the voltage drop along the strip, but it will be equal for all LEDs, as all LEDs will have the same total length of strip, so at least they will all be the same brightness. You can then compensate for the drop by increasing the supply voltage . It won't be super efficient, but it will work, and brightness will be consistent.

author: Fungus
author_rank: Super Contributor
timestamp: May 01, 2015, 07:01:11 pm
content: Quote from: mikeselectricstuff on May 01, 2015, 06:02:52 pm...the lack of "super-efficiency" means that when you put them in an enclosed or poorly ventilated space they will heat up and die.https://www.eevblog.com/forum/blog/eevblog-735-mailbag/

author: Didix
author_rank: Regular Contributor
timestamp: May 01, 2015, 08:09:55 pm
content: Many years ago I attended a safety course for car mechanicians in the army.They showed us images from the emergency room.People that shortened a car battery with their wedding ring, necklace or metal watch band.Although this is 38 years ago I will never forget those mutilations.For example the ring finger, at the place where the ring was, only the blackened bone was left.There is the danger! (well, one of them)-Didix

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 01, 2015, 08:11:27 pm
content: Quote from: Fungus on May 01, 2015, 07:01:11 pmLED strips aren't hugely efficient to start with, but they have plenty of surface area. A bit extra heat from drop on the tape conductors isn't going to make much difference.

author: Jeroen3
author_rank: Super Contributor
timestamp: May 01, 2015, 09:29:06 pm
content: There should be plenty mains-AC to DC outside-rated power supplies available. You can get potted supplies for LED's. The only thing you have to do right are the connections, but that would be required anyway. If you get IP67 they will even work inside your fish pond, assuming you get IP67 connectors.Another possibility is buying led strips rated for a higher voltage. Those cheap waterproof silicone strips will have half the LED's failed after a year anyways.Using a single power supply will require a lot of wire. And I don't know the thefts trend in your country, but over here copper is stolen a lot. They even take the overhead lines and gas pipes.