I've got a basic project which lights 32 LEDs which are set up in series of 4 all in parallel as shown below. As each LED is 3V / 20ma, I didn't add in resistors or anything to each series.
Unfortunately, I've twice now run into an issue (thermal runaway?) where the LEDs are only running at about 25% brightness when set at 100%. The first time this occurred, I found with the multimeter that each series is getting 3V instead of 12V. Sometimes a series flickers, too. When I replace the series, everything goes back to normal for a brief period.
How can I remedy this to where it won't be an issue in the future after the next repair?
Supply is available. GND is also available everywhere where necessary, but the current limit for the LEDs is missing, so the power supply switches off as soon as it goes to 100% (maybe even a little earlier). He apparently built a PWM control, which is why it also works in the lower area without the resistors, because that reduces the current flow through the LEDs.
@bzowk Your LEDs are not the high power type that @GoForSmoke mentioned. They may be sold as "ultra high brightness" or some such, but they are just normal 5mm, 20mA LEDs. Their temperature won't change enough to affect their forward voltage significantly. @GoForSmoke is talking about the type of LEDs that are 1W or 3W for example and can take 100s of milliamps and require heatsinks to keep them cool.
I would not describe COB LEDs as "high power LEDs', because they are in reality arrays of low power LEDs in a single package. Having many, maybe many 10s, of LEDs spread over the package also spreads out the heat, so it's not so intense in any single spot, and the package includes a metal, usually aluminium, heatsink. So the individual leds don't get as hot as a "true" high power led, and their forward voltage does not change as much.