Hi everyone. I have been struggling with this problem for several days. I need your help. There are somethings that I cannot understand. I am trying to build a battery driven LED panel driver with constant current. In order to obtain constant current, I can use DIODES Inc.'s ICs. They are basically a current limiting and sensing transistors in package. The problem with LEDs is there is no constant and standart design. There may be 3 string in parallel each have 10-40 leds on them or completely different configuration. So current sense resistor can be a potantiometer so that I can adjust the required current accourding to my needs. here is the problem. For example if I use 3 18650 batteries in series, my battery voltage will be 11,1 V. If I design a boost converter that can output up to 120 V and connect that to current limiter IC. What happens ? The current will remain constant in %10 range mostly and the load will vary from LED panel to LED panel. This would result change in voltage. But will the boost converter will change its output accordingly or I will try to output 120V ? If it continuously give 120V, LEDs will bum! Am I wrong in any of this or something is missing ? Dont worry about battery side of the circuit I will manage that part with over temp,current,charge protections. Have a nice day

 

You're problem is how much Constant Current are you going to set in the leds, most white leds take 15mA and use 3.3V drop, so you could put 3 leds in series across the battery with a lm317 regulator , or if you want to boost the voltage up, by X11, and use a different regulator , then your current will be more from the battery, and therefore won't last as long.

 

You're problem is how much Constant Current are you going to set in the leds, most white leds take 15mA and use 3.3V drop, so you could put 3 leds in series across the battery with a lm317 regulator , or if you want to boost the voltage up, by X11, and use a different regulator , then your current will be more from the battery, and therefore won't last as long.

Let's say I will draw current between 20-350 mA. And this will be determined by a potentiometer. The problem is what happens to voltage while a regulator gives constant voltage. I will build a boost converter so that the battery voltage can be converted to higher voltages. Let's say I have a boost converter input is 10 V and the output is 100 V. If I connect 5 white LEDs in parallel. And apply constant 20 mA current. What will be the voltage and how will it be. Which mechanism sets the voltage according to the load. How will boost converter know the output voltage accourding to load ? I have worked with constant voltage sources a lot but CC sources are new to me and im having hard time to understand them.

 

You're not interested in voltages just the Constant Current, the regulator maintains the current until the battery voltage drops and can't take any more. Leds are current driven, so your first priority is to work out how much current your leds are going to take, this will then depend on how long the battery lasts,.

A 18650 goes from 4.2V to 3.6V thats only 0.6V drop until it cuts out, so depending on it's capacity in mAh will give you the time scale.

 

Time scale is approximately 1 hour more or less. I have 3 Led panels rated 40,50 and 60 watts. Here is the LED arrangements;

40 watts - 20 Leds series - 2 parallel string --- > 20*3.3 = 66 V and 40/66 = 606.06 mA. Since I want only %50 illumination, I will limit the current to 300 mA for this one.

50 watts - 25 Leds series - 2 parallel string --- > 25*3.3 = 82,5 V and 50/82,5 = 606.06 mA. %50 illumination means I need 300 mA current for this too.

60 watts - 20 Leds series - 3 parallel string --- > 20*3.3 = 66 V and 60/66 = 909,09 mA . %50 illumination means I need about 450 mA current for this one. I dont want to make different designs for each one of this Led panels. I will use a potentiometer to set the current.

In this situation, what do you suggest about boost converter ? What will be at the input side and the output side of the converter. For 40 watts lets say. I dont understand how will boost converter adjust the voltage acording to load.

 

I have an idea. Boost converters are used for increase the output voltage by chamging the duty cycle of the mosfet. If i can build a circuit that can convert current to voltage and use this voltage as a refference to a regulator with switch output, can alter the voltage accourding to current. Is this a good approach?

 

No help ?

 

What you seem to be missing is that using a DC-DC converter to boost the voltage reduces the available current. For example, let us say you have a DC source that can provide 3 Amperes @ +12VDC. The product of those two numbers gives 36 watts of power. Now let us say you want to use a boost converter to raise the voltage by a factor of 10 to to +120VDC. What do you think happens to the available current? If we assume the conversion process is 85% efficient, then 36 watts of input power will allow for 30.6 watts of output power. That amount of output power will be +120 VDC @ 0.255 Amperes. You just cannot get something for nothing. It would be better to arrange your DC sources to provide the needed power without using DC-DC converters. Were you expecting the situation to be different than physics allows?

 

Im aware of the conversion. As you know battery voltage decrease in time thus i need a dc dc converter. Lets assume your 3 A 12V power supply connected to 2 1 watts leds. What happens if there is no current limiting resistor ? The amps will go up. This is what i do not want. I want to drop 12 V to 2 leds level around 6.6 V and limit the current. To be able to do this i need to measure the current and set a maximum level to it. When the current reaches the limit, voltage will go up and down not the current. To be able to do this, i need to change duty cycle of the transistor with respect to current with different voltages.

 

People here either does not want to help or they are lack of advanced information. You have prpposed the simplest things that i cannot use. I will do it eventually with or without your help. If you cannot get help from forum like this, remember that you did not help and you wont get it too. Thank you for not helping.