campus189
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The voltage across them changes quickly as the current changes a small amount.
Of course it's a bad design. There are two main reasons for this. First, the voltage of typical alkaline cells goes from 1.5 when fresh to 0.8 when considered dead. This means that your supply will go from 4.5V to 2.4V as the batteries go through their discharge curve. LEDs need to be actively current regulated when powered by a variable and unregulated supply. A fixed resistance, no matter the value can only be correct for a moment as the supply voltage passes through the one correct value (like a clock that doesn't run being right twice a day). When the voltage is high for the given resistance, the current is excessive, shortening the life of or destroying the LEDs and when low, the LEDs become dim or even fail to light. Second, LEDs are current driven devices so connecting multiple LEDs in series is the normal approach to keeping the current equal and correct in each LED. When they're connected in parallel, the supply current can be divided unequally due to manufacturing variations resulting in excess current flow in some LEDs while depriving others of adequate current to fully illuminate. As you might expect, this leads to the premature failure of the excess current LEDs.Must be a poor design.
This tells me voltage must be too high which in turn, means the resistor must be wrong.
Am I correct in assuming this ?
I need to know the voltage across the LEDs while the current is 480mA, if possible. 2.6V is too low for white LEDs at full current. It should be at least 3.1V.
Another test you can do that may even solve your problem is to connect the constructed 2.55Ω resistor pair in series with the existing 1.5Ω resistor for a total of 4.05Ω and power it with the 5.2V DC-DC converter. Measure the total current and the voltage across the LEDs. Test circuit shown below.
View attachment 57184
By the way, don't worry about the minor setbacks. I have a pleasant design surprise waiting to spring on you when we get the real Vf at 20mA@ worked out. :grin:
Voltage across the LEDs was 3.09 volts
327.6 mA draw
Well, now you've blown my surprise. I was going to suggest a modification to the converter after we got the Vf well defined. The SMD resistor change may be a problem for Campus. She's said she has limited soldering skills. My datasheet says 1.5A but I think the thermal design of the board will be the limiting factor.
https://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00001232.pdf
You should try measuring the current and voltage as before, but with the 1.5Ω resistor bypassed so just the 2.55Ω is in circuit as shown in post #22. Set up to measure current first and just turn on briefly to get the current measurement, then if it's less than 500mA, measure the LED voltage as well.
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