Aren't LEDs DC power?

[Dick in Wisconsin]

Isn't there some kind of AC to DC transformer in each and every ceiling LED fixture? Wouldn't it be more cost effective to put a single AC to DC transformer at the beginning of each circuit and then run DC wiring to each light?

Seems to me the LED lights would be less expensive, lighter weight, and the wiring cheaper.

The transformer/power supply could be put in a box adjacent to the switch at the beginning of each circuit.

Do I have this right?

 

[Bert_]

Each individual bulb still requires something to regulate current. Simple led's meant to run on 12v often use a resistor, but that leaves some efficiency on the table. 120v led's usually have the current limiting built into the ac-dc converter.

 

[pepi]

No, the tubes are wired direct to the line, AC. The LED tubes are built so the AC becomes usable DC power. It's done with diodes, bridge or half wave, never took an LED tube apart to look at what's going on, no need to.

A transformer steps up or steps down AC it is always AC. Only when the power is rectified will it be come DC. Today diodes are used, it can be a bridge rectifier, flat wave when viewed with a scope. Or it can be half wave, DC output at on one side of the sign wave.

That's a ruff kind of explanation..... However the answer to the question is no.

Pep

 

[Dick in Wisconsin]

No, the tubes are wired direct to the line, AC. The LED tubes are built so the AC becomes usable DC power. It's done with diodes, bridge or half wave, never took an LED tube apart to look at what's going on, no need to.

A transformer steps up or steps down AC it is always AC. Only when the power is rectified will it be come DC. Today diodes are used, it can be a bridge rectifier, flat wave when viewed with a scope. Or it can be half wave, DC output at on one side of the sign wave.

That's a ruff kind of explanation..... However the answer to the question is no.
Pep

OK, there isn't a transformer. But there is a device or process at EACH LED light to convert the electricity to 120VAC to 12VDC.

So wouldn't it be more cost effective to make the 120VAC to 12VDC conversion ONCE and then run less expensive low voltage wiring throughout the structure? Or am I missing something?

 

[ddawg16]

OK, there isn't a transformer. But there is a device or process at EACH LED light to convert the electricity to 120VAC to 12VDC.

So wouldn't it be more cost effective to make the 120VAC to 12VDC conversion ONCE and then run less expensive low voltage wiring throughout the structure? Or am I missing something?

We might get there eventually.....One could argue that all lighting in a house is DC. But then you have to decide if you want 12V or 24v. Then you have the issue of dimming. If you want to dim it, then you need something in between to do it.

Converting 120Vac down to 12 or 24 Vdc is actually easy. Diode and a couple of caps....one of the cap is in series....it regulates the current....the diode rectifies it (don't really need a full wave bridge)….and the other cap filters out the AC. The voltage drop across the LED light sets the clamp voltage.

In relative terms of cost, you are looking at maybe $0.25 in materials?

If you want it dimmable, then you need a few more components.

However.....using DC to power ALL the lighting in your house? Not exactly cost effective....and don't forget, DC is not as efficient as AC.

 

[u3b3rg33k]

We might get there eventually.....One could argue that all lighting in a house is DC. But then you have to decide if you want 12V or 24v. Then you have the issue of dimming. If you want to dim it, then you need something in between to do it.

Converting 120Vac down to 12 or 24 Vdc is actually easy. Diode and a couple of caps....one of the cap is in series....it regulates the current....the diode rectifies it (don't really need a full wave bridge)….and the other cap filters out the AC. The voltage drop across the LED light sets the clamp voltage.

In relative terms of cost, you are looking at maybe $0.25 in materials?

If you want it dimmable, then you need a few more components.

However.....using DC to power ALL the lighting in your house? Not exactly cost effective....and don't forget, DC is not as efficient as AC.

That's not how it's generally done.

What's done is with buck converters (with a full wave rectifier if they're not cheap, half wave if they're cheap) to drop the voltage down and make it DC. what one would call a switch-mode power supply. no one puts linear rectifiers in lighting if they want to get their energy star sticker or meet their cost targets.

 

[theoldwizard1]

OK, there isn't a transformer. But there is a device or process at EACH LED light to convert the electricity to 120VAC to 12VDC.

An LED is a DIODE ! A diode is design to convert AC to DC (well, okay, they "block" current going in one direction), so you your nice pretty sine wave only has the the half of the "hump" on the positive side of zero (with a big gap in between). An LED just happens to emit light.



Diode can handle pretty high voltage (it is in the spec sheet) with the worst case being the voltage they have to block (which they turn into heat). A typical "forward voltage drop for an LED is 1.85V to 2.5V. What is most important, as mentioned by Bert, is maintaining a constant current.

LED Christmas lights "cheat" they have no electronics to regulate anything and use the fact that there is no such thing as a perfect diode (the "equivalent circuit" looks like a perfect diode in series with a resistor and and a voltage sink). I can't really explain it properly, but if you hook the "correct" number of similar diodes in series to 120VAC, they work !

 

[Bert_]

Most led's don't run on 12v most are higher. Also voltage is not the important part, each led needs regulated current.

The driver, as it is often called, is not just there to change voltage.

 

[Dick in Wisconsin]

Most led's don't run on 12v most are higher. Also voltage is not the important part, each led needs regulated current. The driver, as it is often called, is not just there to change voltage.

Couldn't all of that come from a device after the 120VAC switch that then sends the power down a low voltage line to each light on the circuit?

 

[ForceFed70]

So wouldn't it be more cost effective to make the 120VAC to 12VDC conversion ONCE and then run less expensive low voltage wiring throughout the structure? Or am I missing something?

Not at each LED, but at each fixture there will be a power supply(or multiple) yes. Same as florescent with their ballasts.

I don't think you'll ever see something like you're proposing. The reason is because LEDs don't need constant voltage, they need constant current. It's not as simple as feeding DC to the fixture unless there is further power conditioning happening within the fixture at which point you might as well do the AC to DC conversion while you're at it. There are no standards specifying the amount of current - every fixture is different.

Having said that - I did recently work on a hospital renovation for a very expensive imaging suite. Hospitals use low voltage whenever possible to reduce risk of equipment electrocuting a patient. We did just that for this suite - put in a central DC power supply and wired it into the walls just like you would with 120V rather than putting a DC power supply at each piece of equipment.

 

[Dick in Wisconsin]

Having said that - I did recently work on a hospital renovation for a very expensive imaging suite. Hospitals use low voltage whenever possible to reduce risk of equipment electrocuting a patient. We did just that for this suite - put in a central DC power supply and wired it into the walls just like you would with 120V rather than putting a DC power supply at each piece of equipment.

Very interesting.

 

[u3b3rg33k]

An LED is a DIODE ! A diode is design to convert AC to DC (well, okay, they "block" current going in one direction), so you your nice pretty sine wave only has the the half of the "hump" on the positive side of zero (with a big gap in between). An LED just happens to emit light.



Diode can handle pretty high voltage (it is in the spec sheet) with the worst case being the voltage they have to block (which they turn into heat). A typical "forward voltage drop for an LED is 1.85V to 2.5V. What is most important, as mentioned by Bert, is maintaining a constant current.

LED Christmas lights "cheat" they have no electronics to regulate anything and use the fact that there is no such thing as a perfect diode (the "equivalent circuit" looks like a perfect diode in series with a resistor and and a voltage sink). I can't really explain it properly, but if you hook the "correct" number of similar diodes in series to 120VAC, they work !

forward voltage drop is where the heat comes from, not the blocking.

 

[Platonic Solid]

Couldn't all of that come from a device after the 120VAC switch that then sends the power down a low voltage line to each light on the circuit?

You are correct. That's the whole premise behind POE (Power Over Ethernet) LED lighting. It's not new, just taking a while to gain market share.

 

[u3b3rg33k]

Not at each LED, but at each fixture there will be a power supply(or multiple) yes. Same as florescent with their ballasts.

I don't think you'll ever see something like you're proposing. The reason is because LEDs don't need constant voltage, they need constant current. It's not as simple as feeding DC to the fixture unless there is further power conditioning happening within the fixture at which point you might as well do the AC to DC conversion while you're at it. There are no standards specifying the amount of current - every fixture is different.

Having said that - I did recently work on a hospital renovation for a very expensive imaging suite. Hospitals use low voltage whenever possible to reduce risk of equipment electrocuting a patient. We did just that for this suite - put in a central DC power supply and wired it into the walls just like you would with 120V rather than putting a DC power supply at each piece of equipment.

hmm. the local hospital here has 120V in the beds.

 

[laser3kw]

most LED tubes are actually an array of LEDs.

 

[fartymarty]

LED Christmas lights "cheat" they have no electronics to regulate anything and use the fact that there is no such thing as a perfect diode (the "equivalent circuit" looks like a perfect diode in series with a resistor and and a voltage sink). I can't really explain it properly, but if you hook the "correct" number of similar diodes in series to 120VAC, they work !

But since they are diodes they are flickering on and off 60 times a second and only get power flow in one direction. Swing one (LED Christmas light powered by AC) at the end of a string quickly in an arc back and forth with some wrist action and you'll then see the individual on and off action. Then for fun do the same thing with an older type tungsten bulb. The tungsten bulb is getting power flow on and off 120 times per second but doesn't lose it's glow between power spikes so you'll just see an arc of light.

 

[Barnabas]

It is the way it is now because 120 volts is the standard for lighting circuits in the USA. Creating new fixtures to run off that standard makes it easy for folks to replace an old 120V lighting fixture with a newer 120V lighting fixture. Until 12V DC becomes a standard in house wiring, you wont see this except for proprietary systems.

On the other hand, in a car, the standard is 12V DC. So there you can add 12V DC lights anywhere you wish.

So to answer the OP question, yes, you can do that, there just isn't a universal wiring standard that makes it practical.

 

[PhysicsDude]

One problem with the "DC from the wall outlet" solution is there are a lot of different voltages used by different devices, and if you have to convert DC-DC voltages, you're ruining any theoretical efficiency you would gain by having a central DC power supply.

Most large, efficient LED lights have all the LEDs in a long circuit(usually 1 or 2 circuits depending on the fixture), using 50~100 DC volts or somewhere around that range.

We all are used to working with 12V LED ribbon lights because they're extremely common and cheap to work with. But if you open up a professionally built light fixture that internally have a ribbon-LED type light source (like canless recessed lights) you'll find they almost never use 12V lights, its one long chain of LEDs and the LED driver is sized accordingly (36v, 54v, 72v, etc).

If you hypothetically have 100 volts worth of LEDs, putting them all on a single circuit is more efficient, than making 8 or 9 12V LED circuits. So a large light fixture (like say a 32W 4' LED tube) is almost certainly more efficient running off 120v ac converted to 100v DC internally, than running off 12V or 24V, not to mention the increased efficiency of the power loss between the panel and the fixture when using 120vac.

Likewise small electronics such as phone chargers commonly run off 5V, and 5V is way too low to be efficient in any sort of infrastructure. Having a 24VDC or 50VDC or 100VDC or whatever central power supply to run your lights is of little or no benefit to consumer electronics like phone chargers, computers, TVs, etc.

TL;DR no one DC voltage solves all the problems, and modern AC-DC converters or switch mode power supplies are cheap and efficient enough, its not worth the infrastructure change. Most of your common LED bulbs have a sufficiently good rectifier circuit that probably costs 20 cents to make and fits inside an E26 edison screw connector.

 

[cybrdyke]

Isn't there some kind of AC to DC transformer in each and every ceiling LED fixture? Wouldn't it be more cost effective to put a single AC to DC transformer at the beginning of each circuit and then run DC wiring to each light?

Seems to me the LED lights would be less expensive, lighter weight, and the wiring cheaper.

The transformer/power supply could be put in a box adjacent to the switch at the beginning of each circuit.

Do I have this right?

These products already exist, so you're on the right track. Also, UL has already written some new articles to keep things safe.
You can find these products in commercial applications. There's 2 main systems being used. One of them has already been mentioned. Power over Ethernet or PoE, is basically connecting fixtures with low voltage cables (it's more complicated than that, but you get the gist). The other is an electrified ceiling grid for places that have a suspended ceiling. The t-bar grid is supplied with low voltage DC to bring power to lights, speakers, etc that are mounted in the accoustical tile spaces. Armstrong is the leader in this technology.
Both technologies are pretty cool and starting to get some traction.
So, DickinWisconsin, if you'd have asked this question about 5 years ago, you might be a millionaire today!!
CD

 

[theoldwizard1]

Couldn't all of that come from a device after the 120VAC switch that then sends the power down a low voltage line to each light on the circuit?

Yes, but the real trick is supply the proper CONSTANT DC CURRENT to each diode in a long string. If you are using sufficiently thick enough wire (so that voltage drop is negligible) or the string is short (4') AND the constant current power supply can keep up with the load (when LEDs are added or removed).

 

[theoldwizard1]

There's 2 main systems being used. One of them has already been mentioned. Power over Ethernet or PoE, is basically connecting fixtures with USB cables ....

NO ! PoE uses standard RJ45 connectors and standard CAT5/CAT6 cabling. For 100 Mbit Ethernet or less, only 4 of the 8 wires are used. The spare wires are used to carry DC power.

Even more confusing is there are MULTIPLE PoE standards with varying voltage and power capabilities.

Almost everyone knows that plain old USB 2.0 and 3.0 can supply 5.0V @ 1A (many sources can do 2A). USB-C can supply higher voltages and current.

 

[MattT]

So wouldn't it be more cost effective to make the 120VAC to 12VDC conversion ONCE and then run less expensive low voltage wiring throughout the structure? Or am I missing something?

You're missing the fact that when you reduce voltage you increase current. Using 14AWG wire as an example you'd be limited to around 180w which ain't much especially if you include fans in the mix.

Having said that - I did recently work on a hospital renovation for a very expensive imaging suite. Hospitals use low voltage whenever possible to reduce risk of equipment electrocuting a patient. We did just that for this suite - put in a central DC power supply and wired it into the walls just like you would with 120V rather than putting a DC power supply at each piece of equipment.

Are you sure that was done for patient safety? Being an imaging suite I'm wondering if it might really be to reduce electrical interference. Like another poster said they've got 120v all around the beds.....................

 

[cybrdyke]

NO ! PoE uses standard RJ45 connectors and standard CAT5/CAT6 cabling.

Yes, RJ, not USB. I edited my post.

 

[ForceFed70]

Are you sure that was done for patient safety? Being an imaging suite I'm wondering if it might really be to reduce electrical interference. Like another poster said they've got 120v all around the beds.....................

Yes, I'm sure. Hospitals, including this one have 120VAC everywhere. But they're more concerned with areas where equipment will be in direct contact with a patient. Often in "wet" scenarios. What's done in an OR for example is much different than at the bedside. Bedside still have IV pumps and patient monitors but that equipment is specially engineered for the purpose and not in such a difficult environment.

The imaging equipment costs millions and is well shielded against 60Hz AC interference although steps are taken to limit. Steel conduit everywhere for example. This is one of those things where the rooms/building is designed around the equipment. It's not just the imaging equipment that comes into play, it's the gantry system and power adjustable patient platforms too.

 

[Specracer]

Yes, dedicated fixtures for many years have used an outboard transformer and LV wiring (long before LED). Now many under cabinet, and cove strip lighting has outboard drivers and LV wiring to the LED arrays. Yes using this through an entire house may make sense on paper. However it comes down to standards and the "normal". In the US a house is wired 110v with romex (or EMT). A circuit is usually shared between outlets, and lighting. Separating this would complicate the wiring increasing labor, which might out pace the costs of the extra drivers. You also have to consider the wall switch (110v) etc If I was building a house, I would stay with the "normal" of romex everywhere. So if you have 110v to your individual fixture, you need a driver integrated into the lamp.

 

[Platonic Solid]

FWIW, I started a thread about this back in 2015
https://www.garagejournal.com/forum/showthread.php?t=278883

 

[checkthisout]

Ha ha I mentioned this too in a past thread.

Thing of it is, this LED stuff is still relatively new and the homebuilding simply hasn't caught up with the technology yet.

I think.

The savings might not be there due to the need to upsize wiring and essentially have a 2nd panel box for the 12V plus a master transformer etc. It sounds smart in theory but in practice it might actually work out to be a net loss.

 

[ishiboo]

It's funny because in Europe they think it's funny we use ONLY 120v due to its efficiency, and you are proposing a new 12v bus!

It has nothing to do with AC or DC. It has to do with transmission effiency. The same wires carry more current the higher the voltage. As Matt said, 14ga wire would only get you 180W (actually at 80% it would be 144W) versus the 1440 watts you now have with 14ga - 10 times less using the same wire!

LED bulbs/fixtures do not just use a resistor for current control... for longevity and efficiency they use LED drivers whose production is far cheaper in quantity than large amounts of copper wiring and a dedicated copper bus. In addition, uptake is dependent on mass market appeal - being able to throw these bulbs into any fixture you have makes all the difference.

Vehicles use 12v because there are few high wattage requirements, distances are very short, half the wiring (the ground) is provided by the chassis, tradition, etc.

 

[theoldwizard1]

Vehicles use 12v because there are few high wattage requirements, distances are very short, half the wiring (the ground) is provided by the chassis, tradition, etc.

Since the later 70s, I have heard folks talking about 48V as the new vehicle standard. The cost of the battery kills it.

 

[dogdog]

why does everyone assums that DC is 12V ?

 

[cybrdyke]

why does everyone assums that DC is 12V ?

I dont know...habit, I suppose. It might be the most common.
The powered ceiling grid system that was mentioned previously is 24VDC and the controls are wireless.
CD

 

[Platonic Solid]

why does everyone assums that DC is 12V ?

Good point. None of my fixture arrays use 12V. I stay mostly between 18V - 36V.

 

[dogdog]

there were talks about driverless LEDs... I think they have already began to selling those ....
I remember bigClives mentioned about it last year or two....

https://www.ledsmagazine.com/articl...93-efficiency-and-no-perceptible-flicker.html

 

[American Locomotive]

24vDC with 14 AWG wire wou'd get you 360 watts of power, which would be enough for just about any residential lighting application. However, something to keep in mind is that each LED still needs its own internal switching power supply driver to regulate the current to the LEDs. It only adds a 1-2% efficiency penalty to have the LED driver rectify the incoming AC to DC.

Another important thing is that modern switching power supplies have a "sweet spot" for efficiency. This is typically around 50-80% of their rated load. Efficiency tends to drop way off at very low loads. So if you have this big honkin' DC power supply in your basement with only one or two lights on, you're going to be wasting a ton of power. You'd be far more efficient running an LED fixture with its own internal power supply/driver.

I think a better solution for saving money and reducing wiring costs would be a revision to electrical standards allowing for 16 or 18 gauge "LED lighting" circuits with 10 and 7 amp breakers.

I dont know...habit, I suppose. It might be the most common.
CD

It's only the most common in the sense that automobiles use it. Almost all low-voltage DC stuff in the industrial world is 24V.

 

[75gmck25]

I don't know the history behind it, but every military tactical vehicle (and aircraft, I think) I've worked with has been set up to provide 24 volt power for radios and other accessories.

IIRC, the older mil-spec vehicles (duece and a half, etc.) also ran the truck off 24 volts, but when they started buying commercial vehicles (CUCV, etc.) to get lower prices they installed a split system where the vehicle uses 12 volts, but the alternator still charges two batteries that provide 24-28 volts for the equipment.

Bruce

 

[joe_padavano]

An LED is a DIODE !

Beat me to it!

 

[wes73]

Ubiquiti Uses POE for their new LED panels and even have an option for dimmers, though controlled through a Unifi LED Controller.

https://unifi-led.ubnt.com/

 

[Falcon67]

NO !
Even more confusing is there are MULTIPLE PoE standards with varying voltage and power capabilities.

There are multiple levels of PoE devices within the PoE standards. For network stuff .af or .at is 95% of it. The newer .bt thing are for really glomming lighting onto switching. So now instead of counting power "calories" at the panel board, we're trying to shove lighting control, etc into the wiring closet. That already has heat dissipation and maybe power issues.

I can't wait for PoE microwaves and washing machines.

We have looked at some of the far out occupancy, LED, network controlled type lighting with modular office panelized wall type stuff and there is nearly zero payback on any of it inside anyone's lifetime. The words "now you're just showing off" and "racing for trophys" applies. IMHO.

Would rather see low voltage LED type power run parallel or similar to existing. Leave the fancy control to wifi wall switches with optional occupancy sensing.

 

[ishiboo]

There are multiple levels of PoE devices within the PoE standards. For network stuff .af or .at is 95% of it. The newer .bt thing are for really glomming lighting onto switching. So now instead of counting power "calories" at the panel board, we're trying to shove lighting control, etc into the wiring closet. That already has heat dissipation and maybe power issues.

I can't wait for PoE microwaves and washing machines.

We have looked at some of the far out occupancy, LED, network controlled type lighting with modular office panelized wall type stuff and there is nearly zero payback on any of it inside anyone's lifetime. The words "now you're just showing off" and "racing for trophys" applies. IMHO.

Would rather see low voltage LED type power run parallel or similar to existing. Leave the fancy control to wifi wall switches with optional occupancy sensing.

Not really a use for a low voltage bus, it doesn't give that much of an advantage. Let's keep the 120v we have now! POE lighting seems to make a lot of sense for applications where you need control over individual lights, have changes to make where low voltage is a benefit (do not need to be a licensed electrician), etc... not really a benefit for the masses.

 

[HeadsUp]

I'll just leave this link here.

https://en.wikipedia.org/wiki/War_of_the_currents