# What exactly do tail amp measurments tell you??



## bullettproof (Jan 25, 2010)

I have taken tail readings before on regulated lights but I always see everyone raving about wow its 2.1A at the tail and so on.I thought if a light has a good driver in it the tail readings would be less meaning the driver is more efficient and not draining the batteries as fast.For instance almost every 2x18650 light Ive ever tested has been between 1.7-1.9A at the tail. Also would IMR batteries have any effect on a regulated light as far as amps current is concerned??

Thanks


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## fyrstormer (Jan 25, 2010)

I don't know what the short circuit amperage is for an 18650 battery, but an RCR123 can push 16 amps in a short-circuit -- which is to say, when the chemical reaction inside the battery is capable of generating a pressure of 4.2 volts, it is also capable of pushing electrons out the tail of the battery at a rate of 16 amps, provided nothing else is getting in the way of those electrons.

The flashlight's emitter provides enough resistance to slow down the rate of flow considerably -- but if the full 4.2v pressure were applied, it would probably still allow electrons to travel the complete circuit at a rate of 3-4 amps, which would damage the emitter in short order. (This is less of an issue with tiny lights like the Lummi Wee and Raw, because their small batteries can't maintain 4.2v for long enough to overheat the emitter.) So, it's the driver's job to reduce the pressure coming from the battery to prevent the battery from burning out the emitter.

I'll be honest, I still don't really understand how DC-to-DC voltage converters work, but the basic effect is the same as a transformer -- it outputs a lower voltage than it's given as an input, which causes the maximum current flow to substantially increase to compensate. (for the time being, disregard the fact that the converter would probably burn out if you short-circuited the outputs of the converter together -- let's just pretend we have a badass converter that can handle that much current flow.) So, now you have a circuit that will maintain 3.5v of pressure for as long as the battery can provide the necessary wattage, and the maximum current flow will decrease as the battery wears down.

But there's still the emitter to deal with. At the lower voltage coming from the driver, the emitter's resistance can slow down the current flow considerably more, so (according to your own numbers) the driver's output pressure of 3.5v can only push electrons through the emitter at a rate of 1.7-1.9 amps. But, occasionally you get an emitter with a substantially lower resistance than normal, and it allows electrons to move at a substantially higher rate, like the 2.1 amp reading you were wondering about.

The driver does burn up some of the wattage coming from the battery just to do its job of voltage-regulation, but that's a pretty predictable amount nowadays with the science of DC voltage regulation being pretty much cut-and-dried. The battery tube has a small amount of electrical resistance too, but a battery tube has about the same resistance as a car battery cable -- and those allow a current flow of several hundred amps with only 12v of pressure, so its effect on the circuit is insignificant. So the only other significant factor is the resistance of the emitter, and if you happen to get an emitter with substantially lower resistance, that means you can shove more current through it and get more light out of it. I think that's the answer you were looking for.


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## aingel2k1 (Mar 31, 2010)

Hello, not to change the subject, but how do I measure the amps?? I hear you say "tail" amps, and Ive heard how one light uses more "amps" how do I go about checking my lights??


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## Illum (Apr 1, 2010)

unless its a custom light, a value higher than 5 amps is usually a bad thing, especially if the light is off.:nana:

the current tells you how much the driver is eating, if the LED current is known the two values can be used to determine the drivers efficiency, If efficiency is known first than the combination can determine LED current.


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## dhingbat (Apr 1, 2010)

There's a guide posted on the board here about using a DMM for a variety of tests, including current measurements.

Coming from a digital test background, current measurements are very useful for identifying the boost/regulator circuit in a few flashlights that have said circuits in inconvenient or inaccessible parts. Not that I have the analog-fu to interpret them if I did have access, but you can infer a lot from the tailcap readings.

For instance, a 1xAA flashlight that has reasonably flat current draw across NiMh, Alkaline, and 3+ volt Li type batteries is fairly well regulated. If the current draw increases linearly or even exponentially as you increase up the chemistry range, it's unregulated and will probably go  at the higher voltage and currents.

Driver efficiency and losses through components such as the switch, body, etc notwithstanding, current * battery voltage gives you an upper bound on the power consumption of the system as a whole. Too much and you may be driving the led or the batteries or your heatsinking too hard. Too little and it may give you a clue to an underperforming component. It's a decent way to estimate runtime too.


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