Milliamp Question

Hello all-

I am new to this forum...I have a general question. I am building a project that uses a 1 watt 7.2 volt 750 mA led. Problem is that when I use a 7.5 volt source and use the proper ohm resistor I get a less than required mA and what I do get drops over a period of time. Am I overlooking something here?

Any suggestions would be greatly appreciated.

Let me move this to flashlight electronics, you will get a better answer there.

to find your resistor,
R=V/I
V is the voltage you need to drop, i.e. Vsource-Vled
I is the current you want to run it at, in Amps (1000mA is 1A)

Your drooping current may be a result o the source voltage you are using. Batteries, esp alkalines, will sag under load, meaning you may test a battery to be 1.5V, but when it's actually supplying current, that voltage may drop.

You may also want to find out if the Vf you were given actually holds true for that LED. If you have access to a variable lab supply, set it to constant current of 750mA and hook it up to the LED. Then you can read the voltage across the LED to find the Vf. It could be that the Vf of that particular LED is higher than the average Vf of that series of LEDs.

I did the the equation properly, I guess than the variable lab supply might be the next step then..any idea where I can get one...I can totally understand the fact of setting it at 750 mA and then testing the requirment of the leds...

What exactly is your "7.5 volt source"? (Unless it happens to be a regulated laboratory power supply, the actual voltage is unlikely to be 7.5 volts.)

Mr Happy said:

What exactly is your "7.5 volt source"? (Unless it happens to be a regulated laboratory power supply, the actual voltage is unlikely to be 7.5 volts.)

Click to expand...

I'm stll kinda stuck on the 7.2 Volt 750mA "1 Watt" LED! :naughty:

Welcome to CPF, Aproct! :thumbsup:

You really need to supply a little bit more information, but I am supposing that your problem is twofold. First your power supply is most likely dropping in voltage when the circuit is completed. Second, it is true that if you are using a battery of some sort, that the voltage will drop some over time, regardless of the chemistry.

Dave

is there even a 7.5 V 750 mA led?
somehow sounds like a Lux V to me :thinking:
that additional "1 Watt" is crap, forget it.

way to go:
what is You power source?
what really is You led?
get a suitable driver for power source to led - that ensures the correct power to the led, You have same output till battery is depleted.

with that resistor, Your ouput looks like that RED line here,
bright at beginning, immediately and heavily declining, then some slower fading away:

[edit]
,doh, wrong picture liked and I dont find a direct drive alone.
so, here is:
above: regulated - shorter runtime, but same output. That would be with Your driver.
below: direct drive - Your resistor version - extreme reduction within the 1st mins
[/edit]