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Is there a simple circuit solution for this?
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ballen:
I want to feed a small laser diode, 3.5VDC @ 25mA.  My power source is a single 18650 Li-Ion cell, whose nominal voltage range is 3.7 - 4.2V.  Is there a simple circuit which will provide either fixed voltage @ 3.5V or fixed current @25mA, dissipating the extra (0.2-0.7)V x 25mA as heat?  My usual ways of doing a constant current source would be with a 3-terminal device, or a transistor, which always creates at least one junction of voltage drop.  I can't afford that here, since if the battery is nearly discharged, I only have 0.2V of headroom.

 So, given that the load is fixed, is there a simple way to create a constant current or constant voltage source which will work well over the dV range of 0.2 to 0.7V?

PS: if this is the wrong forum, please tell me to go and I'll move the post.
wraper:

--- Quote ---My power source is a single 18650 Li-Ion cell, whose nominal voltage range is 3.7 - 4.2V.
--- End quote ---
It's not. More like 3.0V-4.2V, unless you want to use only a small part of it's charge. Basically you need a buck-boost converter with constant current regulation. Or boost converter + resistor to drop excessive voltage/limit current (less efficient). You should not connect laser diode directly to the voltage source without current limiting, the same as LED.
SilverSolder:


It probably isn't going to be possible with the simplest circuits, it seems to me.

It might be possible to design something around a pass transistor, low value current sensing resistor, and a low voltage op-amp?  -  that is still pretty simple, just not quite as simple!  :D



ballen:
Wraper: I see your point -- here is a charge/discharge curve:



Almost all of the capacity is in the 4.2 - 3.5V range.

A buck upconverter would be a reasonable solution, given the small amount of power needed.  I suppose that I could use one of the tiny 5V output ones, in series with a 1.5v zener.  Is a simpler solution?
wraper:

--- Quote from: ballen on August 08, 2021, 04:42:54 pm ---I suppose that I could use one of the tiny 5V output ones, in series with a 1.5v zener.  Is a simpler solution?

--- End quote ---
You can use 5V boost converter + current limiting resistor. Zener should not be used since laser diode is a current controlled device. Powering from 3.5V will not result in stable 25mA current. It can vary a lot depending on particular laser diode and temperature.

--- Quote ---Almost all of the capacity is in the 4.2 - 3.5V range.
--- End quote ---
According to the curve you provided in 3.5-3.7V range. But it also depends on a cell, discharge current and ambient temperature.
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