It depends on the voltage regulation of your power source.
Four strings of leds are going to draw more current than three strings of leds. The increased in current load may further drop your incoming supply voltage, or that may not be an issue.
Best to just try it and see.
The increase in current will mean the LM350 will get hotter so just be prepared for that.
And I agree with Brian, each string should have a small series resistor to equalize the current through each string.
I suspect you also use the fan to cool the LED heatsinks which may be drawing much more than rate 0.7Amp for each string.
caution
mismatched LEDs can burn out fast in parallel without say 0.1 equalization series resistors or wire resistance on each parallel string. Its called thermal runaway.
Do you have a multimeter? LED p/n's? Wire guage? AWG and length?
What is Vdc at source? At load?
What heatsink?
From the Voltage, I assume these are RED and that you aren't driving these at 16A
Do you have a multimeter? LED p/n's? Wire guage? AWG and length?
What is Vdc at source? At load?
What heatsink?
There are a number of issues here but they can all be resolved easily.
1. The LEDs are not wired optimaly. A LED is a constant voltage device, beyond it's Vf it draws more current if you let it. This means you are effectively limiting the voltage regardless of what the LM350 is trying to do. Rewire the LEDs so there are only two in series and add a resistor of say 1 Ohm in series with each pair. That leaves one LED left over so if possible add another so you have 10 wired as 5 x (2 LEDs + resistor) in parallel. This will overcome the top voltage limit you are seeing.
Something is wrong. If the clips are connected to the DVM it tells me 4.98V is dropped across the 1 Ohm resistor so the LEDS must be passing (I=V/R) 4.98/1 Amps. Almost 5 Amps each and the board is drawing 20A total! Also the resistors would be dissipating 5W each and they look like 0.25W or 0.5W types so there would be lots of smoke!
The voltage across the LEDs should be fairly constant at 2 x Vf whatever the current is because they are constant voltage devices. You don't want to put 7V across them as that would instantly destroy them. With a conventional filament lamp the brightness is set by the voltage you supply across them, with LEDs the voltage stays steady and you increase the brightness by letting them pass more current. The 4.87 volts is somewhere close to what I would expect, it's how you get 9.85 without fillling the room with smoke that eludes me.
Brian.
I'm guessing a single dedicated Al clad PCB is probably too expensive and difficult to solder, based on the present style of construction.
You could try it like this:
Start with a sheet of single or double sided copper clad board, the kind that PCBs can be made from. Using a needle file, carefully make four, 1mm wide cuts through the copper horizontally and three cuts vertically so it has 12 isolated copper squares on it. Place a dot of heat sink compound under each LED and position them flat to the PCB so the cut is *JUST* inside the LED solder pad. The intention is to keep the LEDs as close to the center of the copper squares as possible while still being able to solder one pad to the next square. Across the 4th cut, wire the 1 Ohm resistors. That will keep your 3 x 3 LED matrix and also mount the resistors. You can then link the first and last pads of each column to make the power connections.
PWM is easiest with an MCU, can you program a PIC, for example the PIC12F683?
Brian.
From matching your picture and description , these LEDs are now rated only at 1Watt which means 1 Ohm +\-50% for ESR
Pls confirm!,
https://www.lumex.com/content/files/ProductAttachment/SML-LXL705UWC.pdf
If you measured 700mA per string before, they must be either bluish UV or extremely hot or very well heat sunk with fans.
Dont underestimate your thermal resistance from case to sink. Consider an old surplus CPU heatsink.
All power SMT LEDs must be thermally attached very well to substrate. below shows using many thermal feedtrus in centre with very flat surface! otherwise thermal resistance is very poor.
View attachment 119151
Reconsider doing it right the first time.
A better regulator than the lossy LM350 is called a Buck regulator is common or attach a thermistor to LED sink and regulate case temp with PWM using a MOSFET and hysteretic schmitt inverter clock with thermistor biasing duty cycle or shunting a current sense R that provides the minimum current limit. Adding an inductor smoothens the current.
Sounds hard but easy for some bright volunteer here with more time.
but the simplest method is what I did .. 12.0V to 4S strings with feed wire <0.1V drop.
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