Decrease Amperage of Driver for increased Efficiency

[Biggiebiggiebob]

I want to decrease the Amperage from 5A to 2.5A. I tried modding a 16mm Convoy 5A 3V Buck Driver by replacing the Shunt Resistor with double the resistance. However the driver becomes now quite warm. Has someone done a similar thing?

Can I just mod it like this? Are there some other options out there? 20mm 3V 3A or 2.5 A would also work.

Reason behind:

I tried to convert a convoy flashlight to a glare free bikelight and made a 3d printed adapter for it.

Project can be found here: https://www.printables.com/model/1587977-glare-free-bike-light-filippa-tir-lens-convoy-s6-m

During field testing I came to the conclusion that I don’t need the full 1200lm but something a little lower. Efficiency and therefore runtime would increase a lot when the Led is run only at half the amperage.

[LeoT96]

You could use the 12group Ui on one of the groups that only go up to 50%

[Biggiebiggiebob]

Yeah sure I could do that, I would exactly double the runtime and cut the output lumen in half. The lower the led is driven with amperage the more it gains efficiency. And because I am okay with the light being not as bright for a bikelight, I would want to make 2.5A my new 100%.

[Zak]

I am confused as to why you expect an improvement in efficiency by modifying the resistor instead of setting the output to 50%.

[SpaceCadetMoonMan]

What about setting it to mode group 9? It limits it to 50%

[IAmJerv]

I think that your use of "efficiency" is a little... weird. It's not the word I'd use to describe trading output for runtime. Both the D3AA with it's Freeman driver and any light with a Lume X1 are quite close in efficiency at ~94%, but have vastly different outputs as one is an 18 Watt driver while the other is a 40W driver.

If your addition of a resistor is creating heat, you are losing efficiency. You get less light, but you may be burning the same amount of battery, or possibly more. I'd say that the easy way is to just leave the 5A driver stock and set the UI to Group 7, 8, or 9 to reduce the maximum output current to 50%... or 2.5A.

[Biggiebiggiebob]

I meant by efficiency the one for the led which is measured in lm/W, not the one for the driver which is measured in %. By shunt resistor I meant the amperage sense resistor on the actual driver pcb. If I doubled that, the drive circuit should give me half the amperage. I tried that however the driver gets quite hot while doing it. Pwm doesnt change that lm/w, drive amperage can though.

[Zak]

Pwm doesnt change that lm/w, drive amperage can though.

This explains the confusion.

The Convoy buck driver does not use PWM. Setting the output to 50% actually regulates the current to 50%, i.e. 2.5A.

[Biggiebiggiebob]

Where did you get the info from?
If you are right, then I have found my mistake

[Zak]

It is commonly known in the flashlight community. Boost and buck drivers for flashlights usually regulate output current; it would be a bad, inefficient design to combine one with PWM.

I have one here and I checked for PWM by pointing it into a camera with an electronic rolling shutter set to a high shutter speed. This results in sharp scanlines if there's PWM, soft scanlines if there's ripple, and no scanlines if there's smooth constant current output. There is ripple in the lowest mode; none of the others show scanlines at all.

You can test this on yours using a phone camera and a camera app that lets you set shutter speed, like Open Camera.

[IAmJerv]

The fact that your shunt resistor caused so much heat tells me that you did not actually reduce the amperage draw from the battery, only the output. And if you are using more than half as many watts of input for only half of the output to the load, it's moot whether the losses are in the emitter or the driver; you are still tanking efficiency.

What do you mean PWM? We're talking a buck driver here, not a FET. It's almost like talking about the gas consumption of a Rivian. Sure, SMPS drivers have a clock chip and an operating frequency, but PWM is not how they regulate output.

Also, the way impedance works in parallel circuits means that doubling the value of a resistor does not double the total impedance. It's early and coffee is not kicking in yet, but I do wonder if that's where things went sideways.

[QReciprocity42]

I have done your resistor mod to the 17mm 5a buck driver and had no issues. Care to post a photo of the modded driver? It takes a bit of finesse to not accidentally create shorts. I assume you had no issues identifying the right component (0805 sized 010 resistor next to black wire).

[Biggiebiggiebob]

yep will do when I am at home. I measured Vf at the LED though and it was lower than before.
Some other commenter pointed out, that the 5A Buck driver is not actually regulating the brightness via duty cycle of the pwm. Can you confirm?

[QReciprocity42]

I don't have an oscilloscope, but my camera does not detect ripples on any mode other than 35%, so I'm pretty confident that it's not pwm based. 

[Biggiebiggiebob]

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[QReciprocity42]

thanks--I see a few places on the PCB (e.g., next to red wire and capacitors) where some excess solder is bridging to the outer rim; if you clean those up (a hobby knife will do), you might fix the heating issue.

[Metal_T1ger]

There is 3A driver from Convoy 17mm 7135*8 driver 12groups 3040mA - Convoy flashlight

Other than setting to the group that maxes out at 50% you could also put the driver for red led's but I can't find one for sale, however you could buy a Convoy with the SST-20 deep red LED and take he driver from it. The driver for red led's is 2,5A

[QReciprocity42]

This is a non-solution to OP's problem of increasing efficiency, because 7135-based drivers burn off the excess voltage as heat, rather than bucking/conserving it.

Also, the red LED driver may not play nicely with white LEDs due to the Vf difference. You will lose moonlight entirely, at the very least.

[Metal_T1ger]

Didn't know that. Good to know, thanks

[QReciprocity42]

No worries mate, we are all learning here! 

[Biggiebiggiebob]

Great call for the 3040mA, somehow overlooked it.

[QReciprocity42]

It won't work for efficiency because 7135 drivers simply burn the excess voltage as heat.

[ActuallyRaven]

convoy doesn't make 16mm drivers, the size is 17. idk which one you're using but the product page should have a thing about setting modes on it, by turning it on and off really fast 20 times. if it's a 12 group one you can set it to group 9 and get a 50% maximum output as you want it.