I'm sure it is discussed before somewhere, but i can't find it.
I'm going to drive a IR led with a Vf between 1.5 and 2V (only 1A) with a AK47 driver on a LiFePo battery. The current chips will burn off quite a lot of excess voltage, I am not concerned about that, but does the controller chip actually run on only 2.5-3.6V ?
ai, that could spoil it all. hmm, I will have a go at it, and if it does not work well enough (shut off early because 3V is reached, I may go for a bare 7135 driver without controller, or a boost driver, with a AA NiMh (eneloop).
Actually should flash to warn you of the low voltage at around 3V.
I have not tried to see if the 7135 can output as lows as 1.5V-2.0V. I hooked one up the power supply (just a chip) and under 2.42V input will not work anymore with a XM-L1
looked up some specs from the 7135 chip, minimal supply voltage is 2.7V, so that one will also drop out before the battery is drained.
I think I will try a AA boost driver then, any thought on if it (say the nanjg 110) will be happy boosting to 1.8V instead of 3+ V? I'd guess it will only be easier on the driver?
I am not that into driver electronics, so I would not know which is the battery voltage divider on the AK47 and how to mod it. Enlighten me please :-)
I am also on another track: would a simple AA boost driver like is present stock on any cheap AA flashlight (like 1 mode sk68 clones) just do the trick: boost to whatever voltage is needed between 1.4 and 1.8V to get around 1000mA. Or will it supply too much voltage?. Or will it just not put out that low output voltage and stop working?
The polarity protection diode consumes .6 volts. So bypassing it will increase voltage to the MCU and, I guess, the PWM signal going out to the 7135's by that amount.
Removing the 470 ohm resistor is supposed to eliminate the low voltage protection.
I don’t know how much power output you need, but FWIW I ran a cheap chinese IR diode with just a NIMH @ 1.2-1.4V. The Vf was supposedly 1.8 as well, but I tried it for fun and it worked. More info in my post. The IR68.
Removing the diode also raises the low voltage trigger point - the MCU thinks the voltage is higher than what it's programmed for and calibrated for via the resistor.
edit: Wow, the way I put that is about as clear as mud. Low voltage function will not trip until the cell voltage is lower than it would be with the diode in place... er, that may not be much better. I think I need a nap! :Sp
I remember your thread well, I will probably put it in a sk68-clone host too . Well, I don't know either how much power I need because it is just for fun :-) (apart from my daily edc-light I do not need any of my lights, it is all hobby), it is a Osram Oslon black 850nm led 600mW output (90deg. Output angle), it might put out a bit more light than your chinese led (so I will not point it at my son ;-) ) . The aim is still is a suitable 1A driver, but if all else fails I will go for direct drive on AA's, thanks for the suggestion :-)
hmm, I did some testing on the led on copper tonight, I may at least want to go to 2A :evil:
Here's the underside of the led, the solder pads footprint is smaller than an xpg but I reckoned it would fit a xpg Sinkpad. I just reflowed it:
And yes, it worked (50mA from led-tester), a very faint red glow in reality but my phone camera (even though it will have an IR-filter) picks up a bit more of the IR radiation:
The central heat sink pad of this led is unfortunately not electrically neutral (it is connected to led+) so it will be an extra challenge to fit this board into a flashlight while keeping a well enough heat path from led to the outside.
The test set-up, with the luxmeter clamped-in close to the top of the led to pick up enough radiation to measure anything at this insensitive wavelength. Ledboard on huge chunk of alu, directly connected to power supply.
2.8 A :evil: :
And the data neatly put into a graph. The measurements are quick and dirty, as usual because of lack of time and motivation to do it well, but it all seems to make sense:
Of course the led had less than a minute (about 30 seconds) on the highest currents, but I reckon it should at least survive 2A for a while, even inside a flashlight :-) . The current really runs away with minime voltage rise, so I really want a good current regulated driver now.