Adjustable LED Strobe Array--Please Review

[SolderPoint]

Hello,

I'm not an engineer by any means, but I would appreciate it if I could get some feedback on this circuit and any advice. I've been reading here and there when I can get time, and this was the best I could come up with. Apologies for the crudity of the diagram--a quick GIMP image.



This is for a 96-LED array in a series of 16 strings.

Forward voltage on the LEDs is 2.3 volts, with each pulling 350mA.

Adjustable voltage 12-15V Pyramid 60 52 Amp power supply that will be set at 13.8 V.

I have two potentiometers put in so I can adjust the duty cycle of the strobe. I've already been playing with a 555 timer calculator, but this is my first 555 circuit and I didn't know what a MOSFET was until a couple of days ago, so that about sums up my level of knowledge.

Should I use this one?
https://www.digikey.com/product-detail/en/IRFZ44PBF/IRFZ44PBF-ND/812810
Or one of these?
https://www.digikey.com/product-detail/en/IRFZ48PBF/IRFZ48PBF-ND/812825
https://www.digikey.com/product-detail/en/IRFP048PBF/IRFP048PBF-ND/812764
https://www.digikey.com/product-detail/en/AOT462/785-1147-5-ND/1855919

Thanks in advance for any assistance.

SolderPoint

 

[MikeMl]

Do you really think that you can run parallel strings of LEDs without doing anything to equalize the current between the strings?
Do you really think that you can run high-current LEDs from a constant voltage supply?

Please read this thread

 

[alec_t]

The combined forward voltage of one string of LEDs is ~6 x 2.3 = 13.8V. You will also need a respective current-limiting resistor in series with each string to allow for LED-to-LED Vf variation and that will drop voltage. So your supply voltage should be at least 15V.

 

[audioguru]

Post the datasheet for your LEDs. Cheap LEDs on E-Bay do not have a detailed datasheet.
LEDs do not have a certain voltage like a simple light bulb. LEDs have a minimum, typical and maximum forward voltage because they cannot make them all the same like simple light bulbs.

If the actual forward voltage is lower than you think then without having a resistor to limit the current then the LEDs will burn out.
If the actual forward voltage is higher than you think then without having a resistor to equalize the voltage then the LEDs might be very dim or not light.

 

[SolderPoint]

Here is the data sheet.
View attachment right datasheet3.PDF

I tried to post another response--it may not have been approved yet.

Thank you for the replies.

I asked if I could increase the supply voltage to 15V and if I put a 287 ohm resistor in place for each series, would that work?

Now I'm wondering if I won't have to go through and measure each diode's voltage drop and wire them accordingly.

Thanks again for any assistance.

 

[audioguru]

Detailed datasheets show graphs. Your Chinese datasheet has no graph but says the forward voltage is from 2.0V to 2.1V plus or minus 0.1V which is actually 1.9V to 2.2V when the current is at the absolute maximum allowed current of 350mA. Why do they recommend operating at the maximum allowed current?

The forward voltage reduces when the LED warms up as the graph on the datasheet should show.
The current increases very much when the LED has a lower forward voltage and it decreases very much when the forward voltage increases as the graph on the datasheet should show.

Use 5 LEDs in each string. If their forward voltages are all 1.9V then their total is 5 x 1.9V= 9.5V. The resistor has a voltage across it that is 13.8V - 9.5V= 4.3V. For 350mA the resistor is 4.3V/350mA= 12.3 ohms. Use 13 ohms at 5W then the current will be 331mA.

If all the LEDs are actually 2.2V then five will have 11V. The current with a 13 ohm resistor is 215mA.

If all the LEDs are half-way then their current will be 273mA.

If you use strings with 6 LEDs in series then the supply voltage must be increased or the difference in the currents will be huge.


96 LEDs in strings of six make 16 strings. You want the current in each string to be a maximum of 350mA then the total current is only 5.6A, not 33.6A.
How did you calculate 287 ohms? For a current of 350mA then the voltage across the resistor must be 100.5V!

 

[SolderPoint]

*Looks at ground and shrugs shoulders: "Misplaced a couple decimal points when calculating."


I did just speak with the company rep from whom I bought the LEDs (that was just one page of the data sheet). Long story short, the data sheet provided differed from the label which was stamped across the LED packaging. The rep told me to use the label provided, which gives a VF of 2.2-2.4, and I assume the variation is still +/- .1, so that would be 2.1-2.5V in that batch.

Thank you for your patience, by the way.

Would it be advisable to measure the voltage drop across the individual LEDs in order to arrange them to produce an even light output?

I'd like to keep them in strings of 6, so I'd probably increase my supply voltage to 15.

Thanks very much.

 

[audioguru]

Use an LM317 to make a constant current source (look at the schematic on the datasheet) and a 3.6 ohm 5% 1W resistor. It can be used to power each LED one-at-a-time where you will measure the forward voltage. Group them together with six so that the totals are nearly the same then calculate a series current-limiting resistor for 350mA or a little less.

15V is not enough voltage if all six LEDs are 2.4V or 2.5V. Six 2.5V LEDs are 15V so a current-limiting resistor value will be so small that it will cause the current to increase as the LEDs warm up. Then they might be damaged.

 

[SolderPoint]

Ok. Thank you!

I will probably be back with more questions.