Hi all,
This is a buck driver powered by 3x li-ion to one LED. Do you guys see a sense resistor on it?
Maybe R7/R8, it’s bigger than the others
R7/R8 looks like sense resistor, they are 0.27ohm each. The driver cannot be very high current.
Thanks you guys. I will try it. HKJ. You are correct, its only 700ma at the moment.
The batteries ( 3X Li-ion ) are in series or in parallel ? I mean , the input voltage is 12.6 V or 4.2V ? The current you measured is the led current or the input current ?
I'm asking these because , if the input voltage is 12.6 V and the 700mA is the input current you should have around 2 A to the led ( being a buck driver ). I doubt that in this case , with 2 resistors of 0.27 ohms , in parallel ( with a result resistance of 0.135 ohms ) you could not have only 700mA to the led !
But , if the batteries are in parallel (4.2 V ) , 700mA to the led is possible.
Do not forget , if you want to increase the current over 1A , check the fly back diode ( D3) , if it is type SS14 ( 1A,40V) , you have to change it with SS24 or SS34 , depending of the value of the current to the led ...
Please , answer to the questions , so we can find the best solution...
Yes, its a 3S pack. 700 to the LED. I stacked an R120 on there and got 1483ma to the LED, but I think your right about that diode, I tried going higher and when I did it stopped working. The LED will flash for an instant but that is all. When I backed down to the 1483ma it works again.
O.k . , thanks for reply , now everything make sense ...( or should I say : "Resistor Sense"!..)
The diode has nothing to do with loosing the modes and the driver stopping to work . Is more a problem of most of the buck drivers , when you try to increase the current above a certain limit . You are somehow lucky that you could increase the current more than double , usually , an increase more than 50 % , will change dramatically the modes , or will make the driver not to work , anymore !
This is happening because , lowering the sense resistor too much , the comparator that is reading the drop voltage on this , and compare it with a reference voltage (dictated by the modes ) , is not triggering anymore . The output of this comparator suppose to trigger a second comparator , that has in his output , the gate of MosFet , and on the other input , the teeth saw wave . So , if these 2 comparators are not both triggered , you 'll have no signal in fet's gate , and no current to the led. Simple as that !
This problem could be fixed by changing some resistors in the comparators area , but this implies a schematic of driver , laborious calculations and testing.., is not worth...
I'll suggest you to get rid of that nasty white stuff , ( silicone ) that is covering some of the components , so you can read , and Google for specifications , the type of Fet , microcontroller, and see the traces...Just immerse the whole driver (after disconnecting all the wires !) in some , slightly warmed paint remover thinner , and in a hour or so you'll have a nice and clean board. If some residues persist , a tooth pick will do the rest of the job...
I'm glad that you sorted it out...
Thank you for the help everyone. 