messing with the natural volt and amp

i've rigged a step up board to a constant current charger curcuit, and wired to a 7.2v maglight krypton.

(the charger curcuit sets volts and amps seperatly).

24 volts open curcuit and the upped the amps untill the bulb switched on.(as in it was off untill then).

@ 790mah the bulb pinged on nicely.(gets brighter if the amps or volts are upped).

accross the bulb is 7 volts.

accross the output of the charger part of the curcuit measures 7v..

the question, why?

surely there should have been 17 volts accross the bulb and not 7v?

and why didnt it switch on before 790mah?

hoffmyster86 said:

i've rigged a step up board to a constant current charger curcuit, and wired to a 7.2v maglight krypton.

(the charger curcuit sets volts and amps seperatly).

24 volts open curcuit and the upped the amps untill the bulb switched on.(as in it was off untill then).

@ 790mah the bulb pinged on nicely.(gets brighter if the amps or volts are upped).

accross the bulb is 7 volts.

accross the output of the charger part of the curcuit measures 7v..

the question, why?

surely there should have been 17 volts accross the bulb and not 7v?

and why didnt it switch on before 790mah?

Click to expand...

Uh well, voltage across the bulb = voltage that bulb 'sees'. 17 volts would blow 7.2V-rated bulb.
And it didn't lighted up when current was less than 790mA (it's mA, not mAh by the way) because filament didn't reached temperature high enough to emit visible light. It was probably emitting quite a lot of heat at this point (when current was close to 790mA but it still didn't light up).

A filament is a resistor. The voltage drop across a resistor is set by V=IR. The bulb has a given R, and if we pretend that its R is constant while you measure voltage, then the R is set. By setting the I to 0.79A, you get a fixed V.

V=IR
7=0.79*R
R=8.9 ohms

That '7v' krypton bulb is actually rated at one of the following voltages:

1.2, 2.4, 3.6, 4.8, 6, 7.2; being 1 through 6-cell mag lite bulb ratings.

AnAppleSnail said:

A filament is a resistor. The voltage drop across a resistor is set by V=IR. The bulb has a given R, and if we pretend that its R is constant while you measure voltage, then the R is set.

Click to expand...

Do not assume constant resistance from a cool to a hot bulb, there is a significant increase in resistance (Can be more than 10 times). I.e. if you feed the bulb from a constant current source, you might have a current as high as the normal running current but without any light in the bulb.

HKJ said:

if you feed the bulb from a constant current source, you might have a current as high as the normal running current but without any light in the bulb.

Click to expand...

No, it's impossible. If current flows, filament heats up, resistance increases, voltage increases until an equilibrium is reached.

Datman said:

No, it's impossible. If current flows, filament heats up, resistance increases, voltage increases until an equilibrium is reached.

Click to expand...

Some bulbs have "cold" resistance so low that it's basically comparable to short circuit, in this case power (which depends on voltage) will be very low even with high current, so it might not heat up filament quicker than heat gets dissipated.

HKJ said:

Do not assume constant resistance from a cool to a hot bulb, there is a significant increase in resistance (Can be more than 10 times). I.e. if you feed the bulb from a constant current source, you might have a current as high as the normal running current but without any light in the bulb.

Click to expand...

I'm assuming that the voltmeter doesn't change the resistance of the circuit appreciably. I guess 'Constant while you measure voltage' should more clearly state what you've noted.

Filaments should be driven at constant voltage for regulated brightness. LEDs should be driven at constant current. For a filament lamp design, a constant-current design will have interesting effects. High-power incandescents need a soft-start to account for lowered resistance of a cold filament. I think a regulated voltage OR regulated current might have similar effects to each other, and regulated current might even provide a sort of soft-start compared to direct-drive. I don't know if it's voltage or current that burns out filaments, since they are so related to each other.

AnAppleSnail said:

I don't know if it's voltage or current that burns out filaments, since they are so related to each other.

Click to expand...

OC voltage is irrelevant (within reason) when the current is limited. If voltage is too low the wattage will never reach optimal.
If voltage is too high then the current must be limited which in effect actually drops the available voltage in a closed circuit.

When current is not limited externally the filament controls the current but can only do so within a specified voltage range.
The little cheapie unregulated power supplies that say for example: 12v @1000ma will test a lot more than 12 volts without a load and less than 12 volts when the load is too high.

This brings me back to my question that so far no one has tried to answer.
If a led is driven at 2.5 amps , what is the wattage?
Everyone makes it sound like it does not matter if the voltage is 3 volts or 500 volts as long as its 2.5 amps.
What is the standard?

I have a flashlight. It draws 1.3 amps with 2 18650 cells. The same flashlight draws 2.6 amps with 1 18650 cell.
What I've been trying to say is that tail cap amperage means nothing without a voltage reference.
When ol Joe says " My flashlight draws 1.7 amps" is that on a 1.2 volt cell or a 12 volt battery?
The difference is 2.04 watts VS 20.4 watts.

lwknight said:

This brings me back to my question that so far no one has tried to answer.
If a led is driven at 2.5 amps , what is the wattage?
Everyone makes it sound like it does not matter if the voltage is 3 volts or 500 volts as long as its 2.5 amps.
What is the standard?

I have a flashlight. It draws 1.3 amps with 2 18650 cells. The same flashlight draws 2.6 amps with 1 18650 cell.
What I've been trying to say is that tail cap amperage means nothing without a voltage reference.
When ol Joe says " My flashlight draws 1.7 amps" is that on a 1.2 volt cell or a 12 volt battery?
The difference is 2.04 watts VS 20.4 watts.

Click to expand...

Of course you need the voltage to calculate watt, but in many cases you can make an estimate (Tolerance might be 20% or even 50%).

Remember that many lights uses power converters, i.e. power in is equal to power out, except for a small loss. I.e. if the light uses 10 watt from the batteries it is probably putting 9 watt into the led.

HKJ said:

Remember that many lights uses power converters, i.e. power in is equal to power out, except for a small loss. I.e. if the light uses 10 watt from the batteries it is probably putting 9 watt into the led.

Click to expand...

Exactly my point. When ol Joe says his light draws XX amps , its totally meaningless without stating the supply voltage or at least what battery configuration. They use amperage like a standard when in fact its totally meaningless by itself.

So back to the original question:
What voltage is ol Joe Whoever talking about?

lwknight said:

This brings me back to my question that so far no one has tried to answer.
If a led is driven at 2.5 amps , what is the wattage?
Everyone makes it sound like it does not matter if the voltage is 3 volts or 500 volts as long as its 2.5 amps.
What is the standard?

Click to expand...

You have to know the forward voltage of the specific led used. Some have a very low vf some have a high vf. Example of vf of a xp-g R5 vs current applied.

lwknight said:

I have a flashlight. It draws 1.3 amps with 2 18650 cells. The same flashlight draws 2.6 amps with 1 18650 cell.
What I've been trying to say is that tail cap amperage means nothing without a voltage reference.
When ol Joe says " My flashlight draws 1.7 amps" is that on a 1.2 volt cell or a 12 volt battery?
The difference is 2.04 watts VS 20.4 watts.

Click to expand...

You can not figure the power without the voltage. P=VxI ol Joe should have told you the batteries used and the state of their charge. Or better yet the actual measured voltage while measuring the current. Their is also voltage sag to factor in. Like HKJ said tail cap measurement is just a wide estimate. If you now most of the voltage losses in all the components of the light then you can get a close estimate.

lwknight said:

This brings me back to my question that so far no one has tried to answer.
If a led is driven at 2.5 amps , what is the wattage?
Everyone makes it sound like it does not matter if the voltage is 3 volts or 500 volts as long as its 2.5 amps.
What is the standard?

I have a flashlight. It draws 1.3 amps with 2 18650 cells. The same flashlight draws 2.6 amps with 1 18650 cell.
What I've been trying to say is that tail cap amperage means nothing without a voltage reference.
When ol Joe says " My flashlight draws 1.7 amps" is that on a 1.2 volt cell or a 12 volt battery?
The difference is 2.04 watts VS 20.4 watts.

Click to expand...

some things are voltage run and some things are current run..

i have a torch that with a 1.2v supply is hell of a bright, if i put the other advised 3.6v in its place its a dweeby hand light, but for 10 hours instead of the 20 minutes with the 1.2v battery..
so i was asuming the higher volts made it draw less amperage to keep the watts the same to avoid over heating.(up the volts the amps have to reduce to keep the same answer in the equasion..change the amps the volts have to match to get the same wattage).

with a few LED's i've played with they all basically throw the extra voltage over the bulb (as in pos and neg from the meter before and after the bulb), so if is a 3.2v diode, and i put 8 volt into the curcuit about 5 appears over the diode, put 12v in and about 8 apears over the diode.. mess with the amps and it goes brighter or dimmer or blows up with too much.(get the unatural relasionship right they dont seem to blow).

with proper bulbs they seemed voltage dependent, hense the up the volts and hold the amps exsperiment. with these narmaly if the volts go up the amps goes up as they should do but they pop really quick after a very bright short life.


the volt and resistance seems feezable above ty..and its definatly the amps that burns stuff lol. 64000 voltes jumping in a spark.. doesnt blow the wire or the rod tip, well not notisabley, but theres singulat millamps flowing there, up to thousands and theres a burn effect more noticablly.(hold the amps and the volts can go way up high before the inevatable, up the amps and it'll burn..)

more info to confuse issues, the other krypton i blew in the past with overvolting, the switch on of was the kiss of death, if i left them on they lasted longer, that and they threw the extra voltage over the bulb. (not consumed 17 out of 24).
i'm assuming the two curcuit boards and bulb along with the battery impedence or miss match in impedences is the culpret, does that make sense?


edit in..some bulbs have a little curcuit of some sort, so theres an unknown variable there posabley...