I am trying to get into electronics. I'm planning on getting several general-use transistors for arduino projects, I know that a 2N5551 will do what I need. I just want to know what the assosiated numbers mean. For a 2N5551, the Collector − Emitter Voltage is 160 V, the Collector − Base Voltage is 180 V and the Emitter − Base Voltage is 6 V. Are these the max voltages that may pass through? The Collector Current − Continuous is 600 mA, it has a gain of 80. Wouldn't that mean that I'd need a current of 7.5 mA (600/80) going through the Base to turn the transistor all the way (I'd get 600 mA going from the collector to the emitter)?
The datasheet of 2N5551

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    \$\begingroup\$ How did you decide that the 2N5551 will do what you need if you don't know what the numbers mean? \$\endgroup\$
    – Dave Tweed
    Nov 24, 2013 at 5:24
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    \$\begingroup\$ This question is good, but the title is awful. \$\endgroup\$
    – JYelton
    Nov 24, 2013 at 7:03

1 Answer 1


A voltage doesn't pass through anything. But semantic details aside, yes, I suspect these numbers are the absolute maximum ratings for your transistor. You do not want to design with those alone.

Yes, at least approximately. The gain is usually used for small signals, so getting to the max. might require a (very) slightly different base current. Do read the fine print; the maximum continuous current likely requires keeping the ambient temperature below some value.

  • \$\begingroup\$ I assume that the tolerance directly applies to the gain? (±5% of 80) \$\endgroup\$
    – Cameron346
    Nov 24, 2013 at 5:31
  • \$\begingroup\$ The tolerance applies to whatever it is stated with. How about linking to your transistor's datasheet or whatever your source is? \$\endgroup\$
    – user30985
    Nov 24, 2013 at 5:36
  • \$\begingroup\$ Sure. The datasheet in question \$\endgroup\$
    – Cameron346
    Nov 24, 2013 at 6:29
  • \$\begingroup\$ Also, the gain will vary widely with the device and with temperature. If you need a smaller base current for a larger load current, consider a darlington array of two transistors. \$\endgroup\$ Nov 24, 2013 at 9:28
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    \$\begingroup\$ You'll need to learn how to read datasheets (and calculate things they don't state directly). The gain is for low currents only, and its value is guaranteed to be at least 80, with a typical value of 250 at 10 mA, with a lower minimum value for higher currents. The max. current is an absolute maximum value for 25 degC ambient temperature and not meant to ever be reached in normal operation. \$\endgroup\$
    – user30985
    Nov 24, 2013 at 10:10

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