Hello,
I have a supply voltage of 4.2 V and a diode with a forward voltage of 3.85 V and a forward current of 350 mA. What resistor would work best for this? Because I don't have 1Ω resistor.
Hm, what have you learned from this one here (your thread)?
https://forum.arduino.cc/t/how-many-leds-i-can-use-with-a-battery/1031035
With an LED that takes 350mA you can't use a resistor at all.
What you need is a constant current supply, or driver. You probably need an external power supply as well that will give you a few more volts overhead for your constant current driver to play with.
Do you think @Grumpy_Mike that a buck converter to limit my voltage to 3.85 V would work?
No, not in the slightest.
That is only a typical figure, it is not the figure you will always get. Even that figure will change with the temperature of the LED. Also it will change as the LED ages. You simply can't just supply a fixed voltage to the LED because the required fixed voltage changes, making it simply not fixed.
What a constant current driver does is that it measures the current and adjusts the output voltage so that the stays the same.
At low currents and relative high resistors the resistor acts as a sort of constant current supply. But once the resistor turns out to be less than 100 ohms this breaks down and you need a constant current supply.
No doubt you can find someone on line that does not hold this view. There is a special name for them. They are called idiots.
Under what load current? Would that happen to be a Lithium Ion battery? If so, do you expect it to remain at 4.2V for its entire discharge curve?
@anon57585045 Here are the specifications of the battery:
● Model: NCR18650B
● Nominal Capacity: 3400mAh
● Minimum Capacity: 3250mAh
● Nominal Voltage: 3.6V
● Standard Charge Current: 1625mA
● Discharge Cut-off Voltage: 2.5V
● Continuous Maximum Discharge Current: 4.875A
You said your LED has a Vf of 3.85V. How do you plan to deal with that?
Pretty simple: the led will be a 'battery almost full indicator light'.
It will also drain the battery pretty fast at 350mA...
The battery goes up to 4.2 maximum when it's full, and that is also the maximum forward voltage for the LED but it can also work with less then 3.85.
You are misinterpreting the chart. Min/typ/max don't apply to an individual LED. It is talking about all the LEDS. The one you have in your hand will have some fixed value in that range.
My question is, what resistor do I need to use? if any if let's say the LED uses 3.85 V (just to be safe).
The real answer is, you can't use a resistor. See reply #6.
Ok @aarg , then how can I safely use this LED with this battery?
I anyone else has any Ideea how can I use this LED (if a resistor and buck converter would not work) please let me know.
Please have a close read of reply #6.
Your second question is also answered there. If I had to guess, I would say you did read it, and just refuse to accept that it might be a little difficult.
If you are so convinced, why wait for someone to come along and vindicate you? Just do it. Calculate a value and go off to the races.
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Here is a popular LED driver. It appears to be out-of-stock at the moment and it may need more voltage than you have.
Those boards need 4.5volt minimum, and have ~1volt dropout.
Unsuitable for a single LiPo cell.
Driving a power LED from a single cell is not simple.
It needs a current controlled buck/boost setup.
Maybe have to look for boards that are used in LED flashlights.
Or use a linear chip made for this, like the AMC7135.
Leo..
