Q: What's the deal with LEDs wired in series or parallel? What about "current hogging" and "thermal runaway"?
A: If your input voltage and current are acceptable, LEDs can be run in series or parallel. The issue is how well the setup will work, and how reliably. LEDs experience "Vf shift" when they heat up, with Vf dropping as temperature rises. With a constant-current (CC) source, this is no problem, as the only effects will be less light (because of the heat) and less power consumed (a lower Vf affects the V*A=W formula). Otherwise, there won't be much to worry about. With a constant-voltage (CV) source, however, the Vf shift comes into play. If Vf for a particular current goes down, but you're only keeping the VOLTAGE constant, the current at that voltage will go up. This is a problem because it will cause more heat and further Vf shift, drawing more current, leading into a feedback loop where the LED(s) eventually pop from the stress. This is what is called "thermal runaway." As you might guess, this is an issue when deciding between a series or parallel LED setup. Recall that devices in series will all draw the same current (at whatever voltage each device needs for it), while devices in parallel all see the same voltage while having potentially different current levels going through each one. If you have a CC driver running a series LED string (a "string," or "leg," is a group of devices, like LEDs, connected in series), you won't have thermal runaway problems. However, LEDs in parallel, even driven by a CC source, can experience "current hogging." As the devices are in parallel, they won't necessarily have the same current. Each string in a parallel array driven by a CV source can experience thermal runaway. If driven by a CC source, one string can still undergo thermal runaway, but another string sharing the CC source will simply get less of the total current - the current was "hogged' by the other string. If your CC source in such a setup is set to drive LEDs near their limits, then having one string bear the load intended for two or more strings can be problematic. Remember that when LEDs pop, the circuit breaks, leaving the other strings in a parallel setup with a kind of surplus of current, which leads to even more rapid thermal runaway, which will eventually leave you in the dark. This is why it's generally recommended to wire LEDs in series, as well as using a CC source if possible. If you don't have a CC source, use an adequate resistor to limit current.
