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Arduino pin6 is in PWM mode, writing values from 0 to 255. This is connected to the base of a BJT NPN and then the emitter is connected to a led.

Arduino and bjt

I don't understand why if I use a multi meter to read the current between pin6 and the base I get so low amps, like 40 micro amps for nearly to 5v(221 value in analog write).

multimeter reading

Circuit in 123d

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    \$\begingroup\$ Could you explain why you expect a different value? Also, your multimeter is likely not able to accurately measure a rapidly changing current (which your PWM signal is), so you should not trust the value too much. \$\endgroup\$
    – uint128_t
    Commented Dec 11, 2016 at 20:24
  • \$\begingroup\$ Because I read that some people use a resistor between the base and the pin6 to avoid burn the transistor. Also, there should be some relationship between the current provided to the base and output in the emitter \$\endgroup\$
    – fpilee
    Commented Dec 11, 2016 at 20:27

2 Answers 2

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You are using the transistor as an "emiiter follower" so the base current is what it needs to be to satisfy the beta equation of the the transistor for the current through the emitter. I don't know what emitter resistor (or LED) you are using so I can't tell you what current to expect but 40 uA x beta (say 100) gives an emitter current of about 4 mA.

Given that the base can be as high as 5 V and it'slikely that the LED doesn't need more than 2 volts to operate, using an emitter follower (common collector) is a perfectly good way to deliver the current needed.

If your meter is set to read DC current then it should adequately cope with the superimposed PWM signal and read fairly accurately. Also, in this type of configuration you don't need a base resistor (just trying to allay what has been said in comments).

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  • \$\begingroup\$ Agreed. No base resistor needed unless there is some high frequency oscillation problem to resolve with some damping. But provision for a small resistor value there might be conservative design practice. \$\endgroup\$
    – jonk
    Commented Dec 11, 2016 at 20:58
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Not sure, but I think that the NPN transistor is meant to be used as a low-switch, so try connecting it to the GND side:
enter image description here

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