Watts Volts Amps some explaining

[The_Miller]

Copy pasted from a topic that drifted of topic because of the confusing nature!

Thanks for the great posts guys and feel free to ask/add

@DB Custom

2 cells in series keep the mAh capacity as listed on the cell, but double the available voltage. If you don’t have a buck driver converting the 8.4V back down to the 3.5-3.7Vf of the 3V emitter you’re gonna blow the emitter with too much voltage.

Current is the amperage the emitter pulls from the cell. The only way you’re going to get 100mA draw is if you limit it with a driver or a resistance. With a resistor holding current down to 100mA draw, yes, the 1000mAh cell should do around 10 hours run time, more or less (due to the cell losing charge rate in a non-linear fashion)

When you put 2 cells in series and double the voltage you can’t feed the same emitter as a single cell, as it turns out our Li-ion cells are close enough to the required forward voltage of the emitter that they can run in direct drive. But 2 cells are way over the normal 3V emitter’s abilities and will fry it in a nano-second.

Conversely, if we use a 6V emitter like the XHP-50 or XHP-70 then a single cell can’t give the required operating voltage. So the 2 cells at 8.4V do the job, with enough overhead to overclock the emitter and let it do some 7.3Vf at a high amperage rate, producing a lot of heat in the process. So if we want to limit the overclocking we need to control the amperage, which in turn will keep forward voltage down and heat at a specific controlled point as well. In the case of the L6, I stacked an R140 resistor on top of the 2 R182 already present for an amperage of 6.36A and a lumens output of 4600 or so. More current will produce more heat, which will adversely affect the L6’s ability to maintain temperature and allow a rapid drop in output due to overheating of the emitter. This is because the 8.4V of the 2 cells is close to the maximum forward voltage the 6V emitter is capable of. The differences here are due to the 6V emitter and it’s higher capabilities.

So as you see, it’s not a gain of capacity from stacking the 2 cells in a light requiring a 3V supply (as in XM-L2, XP-L, XP-G2) it’s the halving of the voltage by the driver (Buck driver) that allows a corresponding reduction in amperage draw, which gives that apparent doubling of run time.

A few rare lights have the ability to run one cell or two cells to the same 3V emitter, and this is what’s causing the confusion. When fed 2 cells this driver goes into Buck drive with constant current, halving the voltage to the emitter so it doesn’t blow. These types of drivers are usually only around 86% efficient, making heat in the conversion process to eliminate the excess voltage. So there isn’t a doubling effect due to the heat losses. This kind of driver is what’s causing the confusion, no doubt.

Layman’s terms are all I know, I’m not an electrical engineer by any means and only know what I do know about it from building some 300-400 flashlights. I may not be explaining it well as I don’t have all that book sense other’s here have. I do know what works though, and have managed to build some pretty intense flashlights. I’ve managed to blow my share of emitters in the process.

@brjones

I’ll use the water analogy, even though electricity doesn’t really behave like water. But for understanding voltage, current, and wattage, it works well enough.

You’re standing beside a stream out there in Maine.
How much water is flowing by you?
Well, that’s a product of both the girth of the stream, and the speed the water is flowing.
Wide stream but ‘slow flow’ can amount to the same amount of water as a narrower stream that’s flowing fast. Same amount of water. But the speed of the water is voltage. Width of the stream is current. TOTAL AMOUNT OF WATER = WATTAGE.

So wattage really is the amount of actual energy going through.
Wattage doesn’t care about volts or current. It’s just total energy.
That’s why watt-hours IS a better way to rate battery capacity, especially of things like laptop batteries, which come in a variety of voltages.

This is the only electric equation I have memorized, but I use it all the time, we know this now:
Volts x Current [by current, I mean Amps] = Watts.
4-volt battery running at 1 amp = putting out 4 watts.
1-volt battery running at 4 amps = putting out 4 watts.
^ The 4-volt battery running at 1 amp, will be having an easier time of it, yes?

The other maxim which works in physics, and in life: Energy always flows from high to low.

So when you have that small stream, but it’s flowing fast, that’s because it has a good downhill drop (head, I think it’s called). Like would be needed for an old water-powered sawmill. Or you’re shooting a tremendous amount of water through a straw or something. High voltage, but low current. Like the little 12V alkaline battery in a garage door remote. Pretty high voltage for such a little battery, but it can’t put out a lot of total wattage, due to its size. (Interesting note on 9V and 12 alkaline batteries: they are actually 6- and 8-cell mini batteries, themselves in series.)

The opposite would be like a big alkaline 1.5-volt D cell. Low voltage, but can put out a decent current.

Well, it’s harder to build a water wheel with a lot of water that doesn’t have far to drop and isn’t flowing very fast. It helps to have both, but you need that drop (voltage).

So as things flow from high to low to get things done, it’s easier to start with a higher voltage, and let it drop, than to try to artificially make the voltage higher, so it can still drop later on. Every stage introduces extra inefficiency and complexity. And boosting voltage (boost circuit) is not as easy or efficient. It sucks. But it is that way.

So, batteries don’t like to produce a lot of current, apparently. But then again, introducing a lot of cells makes its own complexity. So, it’s a balance.

Single-cell lights take advantage of the fact that many LEDs need a voltage of less than ~3 volts to run them. So, a simple Lithium cell works well, and keeps it simple. Energy flows from high to low, so you’ve got to have your power source of a higher voltage than what it’s dumping into. It’s like water flowing downhill. Water is not going to flow uphill. So a 4V li-ion cell won’t light up an LED which needs 6V or 12V to run. You have to be higher. It’s like gravity, kind of.

These higher-power Crees are made to need 6 volts or even 12. So let’s say 6. So yeah, you need to feed that LED a voltage higher than that (within their specified ideal range for the LED). Two li-ion cells in series (about 8 volts) will do that. But just using ONE CELL that’s TWICE AS BIG as one of the previous two cells (same amount of “batteriage”), well it’s only producing 4V, so it’s never gonna light up that 6V LED. Trying to get 7 or more volts out of a 4-volt cell is kind of like trying to make water run uphill. In electricity, you can do it, but you will have to cannibalize some of that energy that makes the water wheel run, to run a pump that sends some water higher than the stream already is, so that bit of water can run down fast with that bigger drop it now has. That new compact, single-cell XHP35 (which needs 12V) is rare beast, but perhaps a harbinger of things to come. The main unusual thing about the light is that major boost circuit.

So, to close, if you have two cells in series, or just one cell that’s overall the same amount of actual battery material, they will in the end put out the same amount of total energy (watts), but you just want the volts to be in a range that’s at or a little above whatever the LED needs to run. Being below that threshold calls for less-common, less-cheap, more-fancy circuitry.

But having much higher voltage for no reason is not good either. I’ve seen plenty of LED flashlights that use two 18650’s in series, even though the LED is perfectly capable of being powered by less than 4 volts (one li-ion cell). They then have to use a buck circuit to get those 8 volts down to 3.x volts or whatever. To me, that’s really dumb (and inefficient) too. The reason they do this (my understanding) is because metal is cheap to buy in tubes. And if they want to make a flashlight with more runtime (two 18650’s, for example), it’s cheaper for them to slide two 18650s down a tube (lining up in series, which has the consequence of doubling the voltage, not just runtime), than to figure out a way to make the cells line up side-by-side to they can be easily wired Parallel to each other.

I have a policy of NOT buying a light that uses multiple cells in Series, unless there is an actual voltage need for those multiple cells. First, I don’t like long or big flashlight bodies! But really, running lithium in series is much less-safe than just a single cell. When running in series, those multiple cells basically act as one… but they are not one. They are different, and if one cell starts to have a problem, the consequences can much more quickly take a bad turn than when it’s just one cell crapping out.

I too liked the Small Sun ZY-T08 (dual-cell but parallel, side-by-side), but missed out on getting a properly-modded one from MtnElectronics when available. Other manufacturers could take a cue from that type of side-by-side body strategy for either parallel or series. (And well, I guess that model is “inspired by Fenix” TK series, so I suppose some mfr’s DID take notice.)

And at some point, the higher you raise the voltage at all (regardless of how many cells), the less-safe things become when you’re dealing with significant amounts of electricity. At some point, it’s like those jets of water that can be used as ‘knives’. At some point, you don’t need much water at all, to do some damage, if that small stream of water is flowing fast enough (has enough pressure behind it—that’s Voltage), like a laser of water. For me, this mark seems to be over somewhere around 50 or 60 volts, especially with larger power supplies. We’re not really there with LEDs yet. But it is coming, with electric vehicles.

If there’s something outstanding you think I might be able to elucidate, please let me know.

@Texas_Ace

Here is the best way I have found to describe this subject to people over the years, I copy/pasted it from here due to the fact I could not say it much better.

https://www.quora.com/Whats-the-difference-between-amps-watts-and-volts

Voltage is like the pressure in a garden hose with a hand operated spray nozzle at the end.
Even when the nozzle is turned off there’s still pressure in the garden hose. Even when a light is switched off there’s still voltage in the energized portion of the circuit.

Current is like the rate of flow of water in the garden hose, say how many gallons per minute (or in our case mAh) are moving in the hose. When the nozzle is turned off, there’s still pressurized water in the hose, but there’s no flow.

When a light is turned off the electrons are still in the wire, but they aren’t flowing — there’s no current.

Power / Wattage is the product of pressure and flow rate. If you were spraying the water into a bucket, it’s like a measure of how quickly the bucket is filling up.

If you have a big hose with lower pressure, or a small hose with high pressure, you may have the bucket filling at the same rate (if the numbers are right).

Electrical power is the product of voltage and current, and it’s measured in watts

You could have a light bulb operating at 50 volts and 2 amps of current…

50 volts x 2 amps = 100 watts

You could have another light bulb operating at 25 volts and 4 amps of current.

25 volts x 4 amps = 100 watts

Even though the light bulbs have slightly different construction and are hooked to different energy supplies, both would be burning 100 watts of power, and both would have the same brightness.

@Barkuti

@kiriba-ru

I think defenitions of constant voltage and constant current should be added to the first post.

0K; I’ve just taken a very brief tl;dr overview of this subject, and neither I am an electrical/electronic engineer so, please bear this in mind.

What do the initials CC & CV (Constant Current & Constant Voltage) usually refer to/mean?

Essentially: they refer to non necessarily exclusive different modes of operation for power supplies. A led driver is a specific kind of power supply.

CV: Constant Voltage mode of operation is based on the principle of keeping the supply output voltage constant (within a certain specification) at least up to the rated current. Therefore, output current will match load impedance up to the PSU’s limits.

CC: Constant Current mode of operation means the supply will tune the output voltage to whatever is required for such current to flow through the load. Of course, practical power supply designs may have their limitations, and this means their DC/DC converters will only be able to operate within certain voltage ranges (the wider the better). These supplies usually achieve this through sensing the voltage drop across a current shunt (a very low precisely known value resistor set in series with the load), and comparing this value to some sort of reference voltage in their integrated circuits in order to determine to what amount raise or lower the voltage at the output so the voltage drop seen at the shunt remains constant.

0K, wanting some sleep. Suggestions/additions welcome.

Cheers :partying_face:

[kiriba-ru]

I think defenitions of constant voltage and constant current should be added to the first post.

[Texas_Ace]

@brjones

@Texas_Ace

Current is like the rate of flow of water in the garden hose, say how many gallons per minute (or in our case mAh) are moving in the hose. When the nozzle is turned off, there’s still pressurized water in the hose, but there’s no flow.

Sorry, but I have to correct this [no disrespect, it’s good overall… love your sig]. Current is not how many gallons per minute. In your analogy, voltage would be the speed of the water, and current would be the width of the hose. Current is not the total amount of water. The TOTAL amount of water coming out (gallons per minute) is WATTS. “Speed” of the water is caused by pressure (or drop of gravity, which is like pressure), so yes, voltage can be viewed like water pressure. It is easier for a beginner to visualize “speed of water flowing” than “pressure of water”. That’s why I state it this way. In reality, when things are moving, they are essentially the same thing. But yeah, that pressure is there when the water is not flowing, too. Current is the “width” of the flow, which doesn’t say how fast it’s flowing. Both the pipe width and the speed or pressure of the water together determine how much actual water comes out.

Like, literally you could have a gigantic, single 1.5V alkaline battery, just like one AA cell, but the size of a car. It would be literally a “wide” battery. That gigantic, one-cell alkaline 1.5V would be able to put out a HUGE amount of amps, but they would still only go at 1.5V. So, like a very wide stream that doesn’t have a lot of pressure behind it (or is flowing slowly, but still can deliver a lot of water). So you can have a big car 12V battery, or a small spotlight 12V battery. The “wider” battery just has more “stuff” in it, it can just do more chemical reaction to make electricity. A smaller battery can’t keep up, the chemical reaction is happening as fast as it can. The wider car battery can put out more watts, because it can do more chemical reaction (current). But both lead-acid batteries will always be producing their current at 12V, because of their lead-acid chemistry. So: same volts, but more amps (wider stream/wider battery/more chemical reaction—but same chemistry), equals more watts (more power). Just more chemical reaction going on at that 12V chemistry.

In reality, it’s about the balance. You don’t want anything too extreme, or not making sense. You want the voltage to be high enough to do the job (get the water flowing, or make that circuit work), then you want enough current to do the amount of work you need to do. The voltage should be ideally at or just slightly over what the LED needs to operate. That means the electricity will flow. Then you decide how much current you want to pump into it. If the voltage is too high, the voltage will need to be adjusted down with circuitry—but that circuitry is simple. It is easier to dam up water, than to pump it uphill. But ideally, your battery chemistry (voltage) and motor (or LED or whatever), will be good matches for each other.

Everyone knows that “volts * amps = watts” is oversimplified, and that there are variations in the real world. You have to walk before you crawl. Trying to explain the nuances of Power Factor to people who don’t know what an Amp is—-is why this (manifestly necessary) diversion went on so long to begin with… The original answers were too technical/detailed. Not overly-simplified enough. So people weren’t understanding. I wouldn’t have understood either; I read those earlier posts. All indications are that the confusion is probably cleared up now, so complaints are belated. I doubt many beginners would read a thread on “Whatts a Watt?”, so this info is perfect where it is. I have found often the most valuable info I’ve ever picked up in forums (not just this one) has been supposedly “off topic”.

I was in a hurry and just copy / pasted the first explanation I found that compared it to water flow, I didn’t even completely read it honestly.

Although I think you are basically saying the same thing as what the quote said.

Water pressure / Speed (same thing in the case of electronics 101 for beginners) = Voltage.

Amps / Current = Hose size

Total water output = watts

Sure talking in water speed may be easier for some to understand but it is also less correct and doesn’t translate as easily to what is actually going on in an electrical circuit. For example you can have water pressure in a hose that is capped off but you can not have any water speed.

A good video example of your giant 1.5v battery is this :

Fun with ultracapacitors!!

That is only a 2.2v “battery” in the video but with buttloads of current capability.

[Texas_Ace]

@DB Custom

“…and both would have the same brightness.”

Um, not necessarily so.

The Luminus SBT-70 is a good example of that, at 17Amps and 3.7V, it’s still only making 1500 lumens. A Cree XM-L2 U4 3C at 3.6V and 4.9A can do 1800 lumens. Equal Wattage is not necessarily equal output.

Again, 2 lights built in the last few days. Same host, same Samsung 20R cells. Both utilize 6V emitters. One pulls 10.45A for 5451 lumens, one pulls 9.4A for 4126 lumens. It’s not all about V*A=W.

All the talk is well and good guys, now go build some flashlights and lets see what you come up with. There will almost certainly be some surprises in store.

I personally have only taken the XHP-35 to 2.89A at 2200 lumens. Other’s have tried higher amperage and have thrown fried emitters in the can. YMMV

Like I said before, I copy pasted the explanation because these people needed a basic understanding on electrical circuits 101.

Trying to explain the forward voltage, LED efficiency, coefficient of heat ect is most certainty electronics 102.

That said in the quote they are talking about incandescent light bulbs, NOT LED’s.

With an incandescent light bulb watts is king since they are virtually unaffected by other factors. So yes, the light output would be the same with an incandescent light bulb in the above examples.

In a perfect world so would LED’s but this is not a perfect world and people need to learn how things are supposed to work before they can grasp how they actually work.

The 101 course always comes before the 102 course.

[staticx57]

Thanks for making this Miller.

[brjones]

Good job, Miller.

@Texas_Ace

A good video example of your giant 1.5v battery is this :
https://www.youtube.com/watch?v=EoWMF3VkI6U
That is only a 2.2v “battery” in the video but with buttloads of current capability.

Yes. A valuable clarification I could have noted originally is that a car-sized single-cell 1.5V alkaline battery still would not light up a Cree LED. Despite having tons of power. Nor would one of these supercapacitors, in theory (depending the voltage they provide).

So to me, in a way I see voltage as ‘height of the water flowing downhill’. I know we all hate the water analogy, but the fact is, if that analogy had never been used in high school, I never would have gotten it.

So to me, it’s back to the old ‘water wheel’. To make a water wheel go, you need “head” (height or drop of the water), and “flow” (speed of the water). With a water wheel, it’s not necessarily about the sheer amount of water.

The more “head” you have, the more distance the water drops, so it’s about height or steepness of decline. If you have a pond up on the hill, and it drains to a pond down at the bottom, the steepness of a hill is a factor… that’s like voltage. (Like producing “pressure” of the water.)

I say this because, if you have one pond that is lower than the other, the water in the lower pond will not flow into the water of the bigger pond, even if there is much more water in the lower pond.

Water still always flows downhill. It is very difficult to get water to flow uphill.

So, if you had a car-sized battery at 1.5V, it still would not light up a Cree LED that needs 3 or so volts to light up. That would be like water flowing uphill, I guess.

So the higher and steeper that hill, the higher the voltage. (And at some point, the height and steepness of the hill can become dangerous.)

So, you have a chemistry of a battery, and that voltage will always be the same (in a certain range). So, the “height” of the hill is already set. The only thing you can modify is the width of the water opening, to a degree. But, in our world, you get to stack multiple “hills” on top of each other, so they form one great big hill . This is called batteries being in Series.

Or you can have multiple hills side by side, draining off their water at the pond on the top of their hill, all the same height draining into the same pool. This is called batteries in Parallel. Can be the same amount of water in the end, but the way the water flows is different. Obviously, hills stacked on top of each other is more dramatic, and has more potential… to a point.
^These were further thoughts since writing the above .

---

Crazy video, BTW, Texas Ace. That was a good thing you pasted… and it spurred me on to more thoughts, which apparently have been helpful, gratified to hear, and you’re welcome, BrianK and any lurkers who felt the same… you know who you are.
I hope we get past chemical battery technologies soon… this is what’s holding back most electrical innovation, particularly with vehicles and flashlights.

---

The sad thing: beyond my explanations above, I have little grasp of electrical “electricity 102”, to borrow a phrase from above. For instance, I just can’t wrap my mind around Power Factor; I’ve tried. Perhaps when the analogies to our 3D macro world run out, I don’t have much aptitude. There’s probably a youtube video that could help me on that, though, if I weren’t too busy researching other more-pressing stuff.

And I guess it’s better to say it here than in the original thread: I thought it was funny that the begging to stop writing about this subject only came after it was clear that there wasn’t going to be more discussion on it (there, anyway). I even pre-said it in my last post there (“[resolved, so] complaints are belated”), and it didn’t do any good. I’ve seen that a lot on forums, and elsewhere, and haven’t figured that out. Like, an argument can break out (even though that’s not what this was), then it ends, and THEN people rush in to beg the people who were fighting to “stop!”. Weird.

[pinkpanda3310 (Pinky)]

Can I make things even more complicated?

I skimmed through and didn’t see any mention to cell internal resistance. Please excuse me if there was a reference.

The highest capacity cell (of it’s size) is not always the one to deliver the most amps output. In fact with 18650’s there is a bit of a push me/pull me effect for mid capacity high current and high capacity mid current.

For regular edc’s with average output then a high capacity cell is best. A wow light is best with high current cell which is a combination of capacity and ‘C rating’, these cells have low internal resistance. Generally speaking the lower the better. Having lower internal resistance will mean the cell will hold a higher voltage when under load, thereby giving more watts to the driver/led.

Larger capacity cells like the D size l-ion may not have very high ‘C ratings’ but that is offset by the high capacity eg -

18650 cell, 2400mah, 10 c rating
2.4ah * 10 = 24 peak amps (dead short) it will quickly reduce as the cell heats up and internal resistance changes

32650 cell, 6000mah, 5 c rating
6.0ah * 5 = 30 highest possible amps (more than the 18650 even with lower c rating)

Hope I confused things a little more. :disguised_face:

Note - the above numbers were pulled out of my head, happy to be corrected

[Rufusbduck]

Keep it simple. Internal cell resistance is just like having another resistor in the curcuit.

[Texas_Ace]

@Rufusbduck

Keep it simple. Internal cell resistance is just like having another resistor in the curcuit.

Ditto^, for the people that this discussion was aimed at the internal resistance is way over what they need to know. They simply want / need to know how batteries work together in series vs parallel since that is all that 95% of them will ever do with electronics.

That said once you grasp how electricity works and interfaces with your daily life, lots of things start making a lot more sense.

[pinkpanda3310 (Pinky)]

Sorry. I missed the target

[Texas_Ace]

@pinkpanda3310

Sorry. I missed the target

Not to worry, you didn’t do anything wrong. The info is mostly correct as well but to hash out those small differences would seriously derail the thread for the target audience and honestly the vast majority of them will never care about all the things that would be discussed after going down that road.

[ImA4Wheelr]

DB Custom wrote:

. . .

A few rare lights have the ability to run one cell or two cells to the same 3V emitter, and this is what’s causing the confusion. When fed 2 cells this driver goes into Buck drive with constant current, halving the voltage to the emitter so it doesn’t blow. These types of drivers are usually only around 86% efficient, making heat in the conversion process to eliminate the excess voltage. So there isn’t a doubling effect due to the heat losses. This kind of driver is what’s causing the confusion, no doubt.

. . .

DBC's post (most not included above) does a nice job of explaining some of the key concepts we deal with in this hobby.

To the best of my understanding, the bold text is slightly off though. For the type of buck drivers we typically use, heat is a by-product of the work being done by the components used in the bucking process. The higher the current, the greater the amount of heat. At moderate currents, it is difficult to detect any heat in some buck drivers.

These buck drivers reduce voltage to the emitter is by using an inductor to absorb excess voltage. The emitter takes the voltage that it needs and the remainder is absorbed by this inductor. The inductor stores the excess voltage by building a magnetic field. A chip ("buck converter") monitors the process and when the inductor is getting saturated, it cuts battery power to the LED. The inductor then releases it's stored energy to the LED. The magnetic field in the inductor begins to collapse as the stored energy leaves. This triggers the buck converter to reconnect the battery to the LED and the cycle repeats.

I too am an electronics novice. The above is just my gleaning of what appears to be happening by looking as circuits and working with them as a hobbyist.

[kiriba-ru]

@kiriba-ru

I think defenitions of constant voltage and constant current should be added to the first post.

Once again. I don`t know english language enougth to make this myself.
Somebody please add defenitions - what is constant voltage, what is constant current, what is current–voltage characteristic and how does it looks for leds.
Without this first post looks like some explanation that starts in the middle and ends not far away from it.

[Rufusbduck]

In linear drivers using the 7135 chip(nanjg 105C) the heat waste is a design feature needed to drop the supply voltage to led voltage, it’s supposed to do that. For a buck or boost driver any heat waste is not intended or desired but comes as a penalty for driver inefficiency. Better parts means more energy for use by the led. Space is the biggest handicap for boost/buck as better usually means bigger.

Linear just means same current into the driver as out(less what the driver itself uses).

A boost driver takes low voltage and high current from the cell and changes it to higher voltage lower current to the led. Input voltage needs to be below led voltage in lowest mode or it goes direct drive.

A buck driver takes higher voltage lower current from the cell and alters it to lower voltage higher current to the led. Input voltage needs to be ~1V above led voltage in highest mode or it falls out of regulation.

All 3 are most efficient when the change necessary is minimal.

[scs]

I have a question I’ve been meaning to ask, but before that I need to preface it with a statement that needs confirmation: the current corresponding to a particular forward voltage is the MAXIMUM current that the LED will take @ that voltage, right? In other words, at that forward voltage, the LED can be powered by anything equal to or less than that max current.

If so, is it possible to design a driver for 1x18650 and 3V LEDs, that instead of bucking the voltage, feeds the full cell voltage to the LED (minus any voltage losses), BUT varies the current to provide constant WATTAGE to the LED, thereby providing constant output? At any output level, as the cell voltage drops, the driver draws more current to keep wattage constant, until the cell can no longer supply the required forward voltage needed at that output level. Then the driver drops to the next lower output level, which requires a lower forward voltage, and the cycle continues.

Would this kind of driver be more efficient than a buck driver?
How would the current variation be achieved?
Would the means of varying the current introduce additional losses that would negate any gains, assuming there are any to begin with?

[Rufusbduck]

Forward voltage and forward current are tied together. It’s more accurate to say that to get a given current through the led you need to supply the forward voltage for that current. Current flows as a result of having enough voltage. This is why as the battery voltage drops, the current through the led drops, at least in direct drive applications. With 7135 drivers the battery usually has a surplus of voltage which the 7135’s burn off as waste in order to maintain constant 350 or 380 mA output. When voltage falls below the emitter Vf for the number of chips used then current starts to drop.

[kiriba-ru]

@kiriba-ru

@kiriba-ru

I think defenitions of constant voltage and constant current should be added to the first post.

Once again. I don`t know english language enougth to make this myself.
Somebody please add defenitions - what is constant voltage, what is constant current, what is current–voltage characteristic and how does it looks for leds.
Without this first post looks like some explanation that starts in the middle and ends not far away from it.

If nobody will do this, there will be tons of such questions (#15) in this thread.

[scs]

@Rufusbduck

Forward voltage and forward current are tied together. It’s more accurate to say that to get a given current through the led you need to supply the forward voltage for that current. Current flows as a result of having enough voltage. This is why as the battery voltage drops, the current through the led drops, at least in direct drive applications. With 7135 drivers the battery usually has a surplus of voltage which the 7135’s burn off as waste in order to maintain constant 350 or 380 mA output. When voltage falls below the emitter Vf for the number of chips used then current starts to drop.

Thanks, Rufusbduck. I understand that. What are you thoughts regarding my statement and questions?

[Rufusbduck]

You’re describing the way a buck regulator works but there’s not enough difference in the cell voltage and the led voltage for the driver to operate, it’s called headroom. At low currents with low Vf it would work for awhile but not for the full battery cycle. There are buck/boost drivers that do both as needed but they are proprietary to certain brands and have yet to be either reverse engineered or designed from scratch to accommodate our penchant for custom modes. They are also limited to fairly low output levels by BLF standards.

Kiriba-ru, I would think constant voltage and constant current are definitions in and of themselves, or are you referring to that as opposed to pwm control? We’re not university profs with experience in clear, textbook analysis, just a bunch of hacks doing our best to share what we know. Since your not in such great command of the language maybe you could be less critical of that lack in others, eh? We each understand and learn different ways, it makes sense that different explanations will make sense to different people. If all else fails, the curious reader can poke around the forum and do a little searching on their own. It’s all here several times over. I find for myself I tend to learn more this way if not always what I was anticipating.

[scs]

@Rufusbduck

You’re describing the way a buck regulator works but there’s not enough difference in the cell voltage and the led voltage for the driver to operate, it’s called headroom.

I don’t think I am. This is what I’m talking about: Say the LED requires 2W at a Vf of 2V. Instead of taking 4V and 0.5A from the cell and bucking it down to 2V and 1A to feed to the LED, the driver feeds 4V and 0.5A directly to the LED. Then when the cell is down to 3V, it feeds 3V and 0.67A, and so on.

I know about voltage overhead.