linear driver 12v in

[dethlore]

I feel stupid for asking... because I have been playing with my own linear designs and I can't find any information through searching here...

using an LM350 (or LM317) I can't get a solid 1500+ mA with 2 18650s, but I can with 3.

1. Is this just because the output capabilities of these batteries may not be high enough? Other higher output 18650s could do the job?

2. If I use 3 18650s (12v in) but have current set to 1500mA, is the voltage going to be too high for a 445? The reason I ask is because I know these have variable voltage when in regulate current mode... so will the voltage matter?

I guess I am really wondering this in general for this. does volts in matter when regulating current?

I know I've seen this posted somewhere.. I just can't find it again.. Sorry for making a new thread!

 

[Blord]

Two 18650 should be enough. If you use higher input voltage then the output current will be the same but all the excess voltage will turn into heat. You don't want that.

It could be that you have bad 18650 batteries.

 

[Sigurthr]

Yep, assuming your three 18650s are in series (you quoted 12V) the current output should not change from two, or even one battery.

Batteries in series output the same current as a single battery, only the voltage changes. Batteries in parallel output the same voltage, but each additional cell increases current capacity.

It is likely you have a bad cell with high Ri.

Note that if you are checking the current to the diode and not the current to the driver, it may be that the forward voltage of the diode is not fully satisfied, and thus not full current can flow. Increasing the voltage then meets the requirement and lets full current flow. This would only be an issue with 405nm or 445nm diodes. I generally figure 4.5V to be the forward voltage for them, even though this is often a slight overshoot. Doing so ensures that at reduced cell voltage the laser still operates at full power. 4.5V + 3V dropout = 7.5V input required. If your 18650s are less than 3.725V each your diode may not be getting enough voltage. Also if there are any stray resistances in the system, it would incur a voltage drop, which would cause the same symptoms.

 

[Ghostchrome]

Note that if you are checking the current to the diode and not the current to the driver, it may be that the forward voltage of the diode is not fully satisfied, and thus not full current can flow. Increasing the voltage then meets the requirement and lets full current flow. This would only be an issue with 405nm or 445nm diodes. I generally figure 4.5V to be the forward voltage for them, even though this is often a slight overshoot. Doing so ensures that at reduced cell voltage the laser still operates at full power. 4.5V + 3V dropout = 7.5V input required. If your 18650s are less than 3.725V each your diode may not be getting enough voltage. Also if there are any stray resistances in the system, it would incur a voltage drop, which would cause the same symptoms.

That would be my guess, as the dropout voltage of the regulator and the laser diode is right at the limit of your cells. As you increase the current to the diode, the forward voltage demands of the diode go up as well, and your cells just can't keep up.

 

[dethlore]

All makes sense. I must need to clean up design and get some good batteries. I have a bunch of free ones harvested from laptop batteries.

Thanks for the input guys!

 

[dethlore]

Alright, I cleaned things up and am using 4x 1N5402 diodes and 1Ohm Resistor for the dummy. With 2 18650 li ion, I can get the measured output at 1500mA only if I directly connect the Adj and Out pins on the LM350... no resistors. If I'm not mistaken, a resistor of any size is needed to regulate the current. So would I need to get a super low resistor? or would no resistors be ok for the circuit? (not wanting to fry any diodes)