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http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/General/RelayBoard-Large-v16.pdf

enter image description here

I'm trying to determine the base current of the NPN transistor but keep getting numbers an order of magnitude higher than the requirement of the 2N3904.

Any suggestions are greatly appreciated.

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  • \$\begingroup\$ The schematic in the area of JP2 doesn't make any sense. Specifically, it looks as if D13 might be able to be tied HI or LO as well as driving the transistor. It really doesn't make sense. \$\endgroup\$ – Dwayne Reid Mar 2 '15 at 4:48
  • \$\begingroup\$ @DwayneReid Look at the schematic he linked to at the top of the question. Everything to the right of (and including) JP2 on his schematic is on a board he bought from SparkFun. The three pin connector is used to route ground, +5, and the base drive of the transistor to the board from the Arduino. It is not an option header. It would have been better if the OP had drawn a dotted line around the SparkFun circuitry or something. \$\endgroup\$ – tcrosley Mar 2 '15 at 9:01
  • \$\begingroup\$ Better picture added per your suggestion! \$\endgroup\$ – user68950 Mar 2 '15 at 14:02
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Assuming a 0.7 drop for Vbe and a 5v Arduino system the base current should be about:

(5v-0.7v) / 1000 ohm = 4.3ma.

That should be well within the transistor's normal operating range for base current.

(If you have a 3.3v Arduino system the base current is about 2.6ma, with the same assumptions.)

There doesn't seem to be any problem.

(If you needed any additional accuracy here the 10k would actually reduce the base current very slightly, by about 0.07ma. - just in case anyone wants to complain.)

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The collector current with an 83mA/12V relay (T9AS1D22-12, if I got that right) and an LED is in the 100mA range. An appropriate base current is in the 5-10mA range to keep the transistor well saturated.

Note that the saturation voltage is specified at Ic/Ib = 10 (from your datasheet):

enter image description here

The 2N3904 is not particularly well suited for this application- the beta is falling off quite rapidly at 100mA, but chances are it will work okay, at least with 5V drive. A 2N4401 is better, but costs marginally more. You can shunt R2 with something like 470R to get more base drive.

Note that there is no serious problem with having a bit extra base current, but having too little base current could lead to the transistor getting hot and/or the relay not pulling in.

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