MC-E mod?

[sailor612]

I've got a few ideas floating through my head for a MC-E mag mod. I continue to get confused with drivers/constant current regulators...
The MC-E has a Vf of 3.7V and runs at 2.8A.
The part that is confusing me is some constant current regulators i've look at for the MC-E and P7(which runs at similar current and Vf) provide a current of 2.8A but what does it do for a powersource that is greater than 3.7V?
I understand that some cc regulators require a min Vin to operate, but if I run one on 5-D NiMH batteries will the voltage be dropped (or do I have to add another part to drop it to the proper 3.7V)?

The one that i'm looking at is: http://www.dealextreme.com/details.dx/sku.20330

Will this "drop" the voltage to the required 3.7V (as long as I don't have a Vin greater than 8.5V)

 

[steve6690]

With a P7 or parallel wired MC-E you're going to be using a "buck" type driver. Buck drivers always need to be "fed" an input voltage that is greater than the Vf of the led, plus the voltage that the driver drops across itself. So, if the driver drops (consumes) 1 volt and the led needs 3.7v to run at 2.8A, the input to the driver would need to be a minimum of 3.7 + 1 = 4.7v. for the driver to continue regulating. You then need to work out what your planned battery pack will provide when it's fully charged , but also when it's near fully discharged. So, 5 x Ni-Mh in series will give out around 7.2v fresh from the charger which is above the 4.7V you need so that's ok. That will drop to around 5v when the batteries are as low as I'd go really (1.0v per cell). 5v is still above the 4.7v you'd need to stay in regulation so that's fine too.
The figure of 1v for the driver voltage drop is an example. You'd need to find the real values for your actual driver, plus also the real Vf of the led. I've used a couple of MC-E "M" bins recently and they've been around 3.25 volts at 2.8A.
By the way, if the supply voltage drops below the 4.7v in the example above, all that happens is that the led will start to get less voltage than it needs and the light will start to dim.

 

[sailor612]

With a P7 or parallel wired MC-E you're going to be using a "buck" type driver. Buck drivers always need to be "fed" an input voltage that is greater than the Vf of the led, plus the voltage that the driver drops across itself. So, if the driver drops (consumes) 1 volt and the led needs 3.7v to run at 2.8A, the input to the driver would need to be a minimum of 3.7 + 1 = 4.7v. for the driver to continue regulating. You then need to work out what your planned battery pack will provide when it's fully charged , but also when it's near fully discharged. So, 5 x Ni-Mh in series will give out around 7.2v fresh from the charger which is above the 4.7V you need so that's ok. That will drop to around 5v when the batteries are as low as I'd go really (1.0v per cell). 5v is still above the 4.7v you'd need to stay in regulation so that's fine too.
The figure of 1v for the driver voltage drop is an example. You'd need to find the real values for your actual driver, plus also the real Vf of the led. I've used a couple of MC-E "M" bins recently and they've been around 3.25 volts at 2.8A.
By the way, if the supply voltage drops below the 4.7v in the example above, all that happens is that the led will start to get less voltage than it needs and the light will start to dim.

Excellent Steve!!! Thank you for that!
How do you typically know what the driver consumes? DX doesn't include much description about their products. I don't want to purchase a driver and realize it doesn't drop enough (or even worse...drops too much) voltage for the 3.7ish that the MC-E needs! I am assuming that if it is marketed as a P7 or MC-E with 2.8A then it will work correctly as long as I have enough power going into it....?
I also want to avoid having to add or remove components from the driver because LED is a somewhat new concept for me!

Since the driver I was looking at runs at a max of 8.5V, what happens to the "extra" voltage that is not consumed from the driver if I was to run the Vin at 8.5? Do I have to add components to limit the "extra" so I dont put more than 3.7ish into the MC-E?
By the way, I'm not planning on running it at 8.5V just wondering what you have to do to with the "extra" that isn't consumed by the driver if it is higher than the required volts for the LED....
Thanks again Steve that was all very helpful!!!!

 

[yellow]

the step-down driver does the work, no need for You to care for that "overvoltage"

but with 5 Ds, better use a SHARK from Sandwich Shoppe and a series wired Led.

much less current (= smaller parts),
guaranteed the same current flowing through each of the emitters
(instead of the hope that the current is the same when parallel wired)