I have two 5V 2A chargers. I want to get 5v 4A. Can I connect both in parallel like this?:
Can I do it? Because in parallel connection voltage doesn't change, so 2A +2A = 4A
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You may be able to parallel the chargers by adding what we call "ballast resistors" in series with each charger (before the parallel connection). This is assuming that the chargers are identical.
The problem is that even with identical chargers, the output voltage isn't exactly the same. The charger with the highest voltage wins and tries to supply all of the current. Because it can't, it goes into current limit. That forces the charger with the lower output voltage to try and take over the full load. But it can't.
Depending on the circuit design inside the chargers, any manner of weird things can happen. The output might oscillate, it might collapse either partially or completely - we don't know.
By adding a low-value resistor in series with each of the chargers, you allow a small amount of voltage drop. Now the chargers are better able to work together.
You usually don't need much of a voltage drop in the ballast resistors. I'd try for 100 mV and see what happens. 100 mV / 2A = 0.05 Ohms.