I need to power a 5V device that needs more or less 1.2A in order to work correctly. I'm using a 13.8V power source with two diodes in series (1N4007) to limit the voltage to roughly 12V. I tried using the voltage regulator AZ1084T, since it has 5.0A of maximum output current, but even with a large heat sink attached it was heating up a lot, like 117°C (and the maximum admissible temperature on the datasheet is 125°C).
So, just to test, I replaced the AZ1084T by the old and good LM7805CV, which has 1.5A of maximum output current. It worked just the same, heating up just the same (and that's a good thing, because this one is much cheaper). But how can I avoid the heat? I'm pretty sure it's the power dissipation from 12V to 5V that's causing it, but this voltage regulator was supposed to work with that input voltage.
I tried this scheme, using TIP127 to "split" the current, but my device won't even turn on (the measured output current was showing peaks of 2A~3A, which kept turning off the power source which limit current is 1.7A).
Anyway, any suggestions on how to split the current while maintaining the output voltage or on how to decrease the power dissipation are welcome.
Best Answer
Just because a regulator is able to output 5 A of current, does not mean it can do so over it's entire input range - especially not without a good amount of heatsinking (potentially with fan). The reason both of the devices heat up the same is because they have to get rid of the same amount of power, given by
$$P = (V_{in}-V_{out}) \cdot I$$ which in your case is about \$(12\ V-5\ V)\cdot1.5\ A = 10.5\ W \$, which is a lot of heat! (there is a reason computers have big heat sinks with fans on top).
You have a few options. You can look at going to switch-mode powersupplies, which are far more efficient (because they don't lower the voltage by turning the excess into heat). Alternatively, you can look at using multiple regulators in series - Use a regulator to go to 9V, then to 7V, then to 5V. This way, the load is spread out over multiple regulators and each has less power to dissipate (of course the total dissipated power is the same!). Finally, you can look at using external components like you already were doing. You mentioned having issues - I would suggest trying that topology with lower loads and a good amount of protection on the powersupply end to see what is going on - could be that you are getting some kind of instability.