Not using a base resistor sometimes works, but it's a terrible practice and it's just asking for trouble. The base resistor needs to be large enough to prevent damage to the transistor, but should still allow sufficient current to ensure the transistor switches on and off as per the base voltages. The BJT is a voltage controlled device and hence current is not the driving factor for switching. The main purpose of the base resistor is to limit excessive current to the base. Or, I suppose, alternatively, am I completely off track? It seems like 8k2 ohm would keep the base voltage below the saturation max voltage and the 6k8 ohm would keep the base voltage above the max base emitter saturation voltage. Given that 7k4 ohm is not a common resistance, should I drop to a 6k8 ohm resistor or go up to 8k2 ohm resistor? So, if that's correct, what am I trying to achieve with this resistor? Am I trying to prevent energy loss or heat production? Or, am I trying not to exceed the max base saturation voltage? I computed the collector current as $$I_ = 7,400\Omega$$ Here're my likely incorrect calculations. Simulate this circuit – Schematic created using CircuitLab one of the ones with the breadboard and springs). I don't have a datasheet for the relay (it's part of an old hobby kit. This is the datasheet for my transistor: Is the purpose of the base resistor just to lower the current to prevent wasting energy? Or to prevent generating heat? I don't fully understand why there's a need for a base resistor, and if there is, why the relay in my circuit worked fine without the resistor and also with a smaller resistor. I've been trying to learn more about base resistors and I feel like I computed that I should have a larger resistor. Then I noticed people tend to add a base resistor so I stuck a 1K ohm resistor in there and it still worked fine. At first, I just connected the base to the output pin and it worked fine. I have an NPN transistor to switch 5 volts through the relay coil that's 125 ohm. I'm using an Arduino digital out pin to power a relay. I'm not strong with electrical engineering and I have prototyped a circuit which works however, to my chagrin, I don't understand why.
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