I'm Confused RE: Amps vs Watts vs Volts

[Lampbeam]

I'm confused about how people describe their lights "horse power". I mean when you're talking about a car you say, "My car has 455 horse power and will eat your little Honda's lunch!" Well fine, I'm sure it would, but when people on CPF talk about their lights they usually talk about how many Amps it draws or how many Watts it has. I grew up using volts in the good old Incan Days. A twelve Volt light was a monster. A six Volt light would get you by. But today I'm not so sure I know what Volts mean anymore when it comes to these new LED lights. I know compact fluorescents are more efficient than incandescents and that LEDs are more efficient than compact fluorescents. I can see that the Watt can describe a lights power as long as you take the lights source of emission into consideration. But what I don't understand is why people use Amps so much to describe their lights. Especially in reference to IMR. I mean, why would you want a battery to deliver ten Amps? Wouldn't that just exhaust the battery too fast considering that those cells have lower mAh? Is it something you have to use judiciously keeping the light on low power most of the time? What reference is best used to measure a light?

 

[StarHalo]

We don't use any one metric to judge a light, hence lumens and lux and amp draw and runtime, etc. etc. And we don't use the insane-output light as a long-term emergency power outage light; damn-the-batteries antics are fun, but a proper moonlight mode means more in an EDC.

The reference by which you judge a light is how well it fits your application, nothing more. First define the application, then find the tool.

(and if you're interested in cars and horsepower, do peruse the cars thread.)

 

[Chippy_boy]

How bright something goes is down to how many watts it consumes, not volts or amps. Watts is the measure of energy given out or consumed per second and this is directly related to the brightness.

But watts is a function of both voltage and amps. The more amps that flow, the higher the power (i.e. more watts). If you have a given emitter, the more amps you drive through it, the brighter it will be. So its easy to say the a Cree xyz led driven at say 3 amps will be much brighter than the same emitter driven at 2 amps.

i guess this is where the confusion comes from. People know that if they can put more amps through their xyz led, then it will be brighter and they start talking about 3 amps vs 2 amps or whatever.

But amps does not mean power. Amps is only useful for comparing power and brightness with the same flashlight with the same led. Different led's operate at different voltages and need different amounts of amps in order to consume a given number of watts. So you cannot say the one led driven at 3 amps will be brighter than a different led driven at 2 amps. So it is lazy and stupid and probably wrong to talk about amps if you want to describe one led's brightness compared to another.

Same with volts. Volts does not tell you how much power because the volts needed for any given amount of power (brightness) varies from one emitter to another. Volts and amps are only useful measures when talking about the same emitter. For a given emitter, more volts = more amps = more watts = brighter.

incidentally, for the reasons above, Volts is a stupid way of trying to measure the power of two different electric drills or lights or anything for that matter and I have no idea how the industry got sucked in to thinking otherwise. One manufacturer's 12v drill could be twice as powerful as another's 18v drill. Its a meaningless measurement if you want to know the relative powers. Same with flashlights.

 

[Lampbeam]

With your replies in mind, I just looked up Cree's Specs for their XM-L2. It is rated 3 Amps and 10 Watts max. So the Voltage is the variable (I think) due to LED lights being current regulated. Therefore, with that or any other emitter, all you have to do is consider the power source to get an idea of what that light is capable of, not withstanding the optics. I think I kind of get it now, although it does require knowing the light and the math behind its Amps x Volts = Watts equation.

 

[SilverFox]

Hello Lampbeam,

In addition to the wonderful comments presented let me add a battery perspective to this.

Watts gives you an idea of power. You can achieve the same watts by varying the amps at different voltages. For example 10 amps at 6 volts gives you 60 watts, but so does 5 amps and 12 volts.

At this point you need to review the design. It may be more difficult to deal with a 10 amp current draw where 5 amps is acceptable. This means that if you are looking for 60 watts you need to increase the voltage.

Moving on to the power supply we find that portable devices run on batteries. If you put 3 Li-Ion cells in series you come up with a little over 12 volts open circuit voltage, but if you draw 5 amps from that battery pack what voltage do you actually end up with?

Historically flashlights were used to provide a flash of light. If your light only has a run time of a few minutes but can light up an acre of land in total darkness in a flash, it may not be "useful" but think of the awesome "wow" factor.

Tom

 

[hiuintahs]

Here is as simple of an explanation I can make of it without getting into too much theory. I know some of you are EE's but there are a lot of folks that visit CPF that aren't.

Voltage is the electromotive force that pushes current. Look at it like a garden hose as the wire, the water is the current and the pressure is the voltage. Power is voltage x current.....or in this example, pressure x water.

In the old days of incadescent bulbs with a filament, you just put a voltage across the filament (which is basically a low resistance resistor such that it glows). Thus voltage was a better way of determining a vague expression of how bright the light would be. And the light would get dimmer the lower the battery voltage dropped. But in reality its the amount of current that is passing through the filament that determines how bright it is. And its the amount of voltage that is pushing the current through the load (the filament) that determines how bright it is. But in the end its how much current is being passed through it. Current = V/R

The same thing with an LED. In the end its how much current is being passed through the LED that determines how bright it is. However, it's too hard to just put a voltage on a diode without limiting the current for the reasons I explain below.

An LED (light emitting diode) is not a passive component like a resistor, but is a diode which will pass current in only one direction from anode to cathode. But in order for current to begin to flow, you have to over come a voltage threshold known as the forward voltage drop of the LED (Vf). This Vf varies from diode to diode or from LED to LED. With the high power LED's its around 3.0 volts. With the 5mm LED's its under 2.0 volts.

And not only that its a very tight area where the LED is either fully off or fully on almost like a switch. It's only a couple of tenths of a volt and is thus hard to determine exactly the brightness you will get..............if you were trying to control an LED based only off of the voltage across it.

So all well controlled LED's are current limited with some type of feedback mechanism that senses current with an accompanying electronic circuit in the form of a boost or buck type of power supply that will regulate the voltage across the LED so as to maintain constant current.

Since it takes an electronic circuit to regulate the LED, then it doesn't matter if you have a single AA battery or a multiple cell battery configuration because the manufacturer will design the circuit to either boost or drop the voltage to whatever is necessary to maintain regulated current.

And this is where "power" comes in. With the law of conservation of power, power out to the LED will equal power in from the battery + the power being consumed by the electronic driver circuit. Typically if the electronic driver circuit consumes less than 20% of the power, that is considered good and efficient.

Power is volts x current. So if the battery voltage is higher, less current will be extracted from the battery and if the battery voltage is lower like a single AA, then a higher amount of current has to be extracted. There are limitations and thus an AA cell cannot drive a high power LED like a 18650 cell can.

Hope this helps .

 

[Lampbeam]

Historically flashlights were used to provide a flash of light. If your light only has a run time of a few minutes but can light up an acre of land in total darkness in a flash, it may not be "useful" but think of the awesome "wow" factor.

Tom

I looked up the history of flashlights last night before I read your reply and was surprised to learn that early flashlights could only sustain a momentary "flash" of light due to their batteries and bulb filaments. So today's IMR people are just carrying on the tradition. The IMR light is just sort of an old Lionel train on steroids. I've found a new appreciation for the IMR crowd.

Hiuintahs, thank you for your reply. It seems I was wrong about Voltage being variable. Apparently, it's the current that gets varied to maintain a precise voltage. Boost and buck I don't understand but I've searched and think it means that if a battery is too weak a locomotive (to extend the train analogy) for a big LED you "boost" and if it's already an adequate locomotive you "buck".

 

[inetdog]

Maybe a better analogy is a car engine.
If you want to go a certain speed and the engine directly driving the drive shaft will not turn at a high enough RPM, then you need an overdrive ratio or boost convertor.
If you need to go slower but still run the engine at a high RPM, you need a lower transmission ratio or buck convertor.
But unlike transmissions, most LED drivers can only do one or the other.

 

[Chodes]

You don't need analogies. Watts = Power. Literally.
P=EI. Power (Watts) = Electromotive Force multiplied by Current. (or in other words, Power = Volts x Current.)

Use common LEDs or automotive examples to confirm and practice this theory.

A SSCP7 LED often referred to as 10W LED.
Power (10W) = Volts (3.7) x Current (2.8) 3.7 x 2.8 = 10.36

55w Automotive Headlight - 55W = 12 x ? Now you know 55W headlights draw around 4.5Amps.
Interior auto lights - 8 Watts is common. ie draws around 0.7 Amps.

So a common 50Ah auto battery should last around 3 days with the interior light left on. (50Ah divided by 0.7A = 71.4 hours - ie 3 days)

 

[Lampbeam]

You don't need analogies. Watts = Power. Literally.
P=EI. Power (Watts) = Electromotive Force multiplied by Current. (or in other words, Power = Volts x Current.)

Use common LEDs or automotive examples to confirm and practice this theory.

A SSCP7 LED often referred to as 10W LED.
Power (10W) = Volts (3.7) x Current (2.8) 3.7 x 2.8 = 10.36

55w Automotive Headlight - 55W = 12 x ? Now you know 55W headlights draw around 4.5Amps.
Interior auto lights - 8 Watts is common. ie draws around 0.7 Amps.

So a common 50Ah auto battery should last around 3 days with the interior light left on. (50Ah divided by 0.7A = 71.4 hours - ie 3 days)

Assorted LEDs

Cree XP-E: 3.5 W = lower power light

Cree XP-G: 5 W = medium power light

Cree XM-Ls: 10 W = higher power light

Luminus SST-90: 30 W = higher power light

I will make it a point to know which LED is in a light. No more analogies, promise.