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I have a 2N2222 transistor that I plan to use with an Arduino Uno. My question is how much current can this transistor handle at it's base?

Passerby
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    Have you tried reading the datasheet? – Connor Wolf Aug 10 '13 at 06:53
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    insufficient preliminary research – Nick Alexeev Aug 10 '13 at 07:02
  • Start here: http://electronics.stackexchange.com/q/16868/2028 – JYelton Aug 10 '13 at 07:19
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    @ConnorWolf: not all datasheets list a maximum base current. I checked OnSemi and ST and neither seems to have this information. The question seems legimite. – Johan.A Aug 10 '13 at 09:39
  • @Johan.A - You can extrapolate maximum base current from the maximum collector current and beta (\$H_{FE}\$) – Connor Wolf Aug 10 '13 at 10:15
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    @ConnorWolf: ok, but that would be very much on the safe side. I've seen transistors with Ic max 1A, HFE 200 and Ib max 100 mA. A higher current than the theoretical 5 mA will be needed in saturation. – Johan.A Aug 10 '13 at 10:19
  • @ConnorWolf you forgot about saturation. – markrages Aug 10 '13 at 15:07
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    Who marked this as off-topic? – markrages Aug 10 '13 at 15:08
  • @ConnorWolf that's it's practical limit (more than that is just wasted/unneeded). Not it's absolute limit (more than that breaks it). – Passerby Aug 10 '13 at 19:54
  • @markrages I have started the close-voting here with a custom message "insufficient preliminary research". In this relatively new SE setup, custom close reasons exist only in the "off-topic" category and not at the root of the selection tree. – Nick Alexeev Aug 10 '13 at 21:16
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    The problem with this question isn't the base current business, but that it lacks a schematic, or at least a high level description of **how** the transistor is to be used with the microcontroller board. Perhaps the maximum base current handling is not actually important. In many transistor applications, we don't even give this a second thought. – Kaz Aug 11 '13 at 00:55

1 Answers1

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The Fairchild PN2222 data sheet says the base-emitter saturation voltage is 2V with a collector current of 500mA and a base current of 50mA. This is for a small duty cycle of 2% with the pulse width less than 300us.

I guess this means the base could take 50mA but how long could it take it for is a bit of a guess really.

If we look at the wattages, 50mA x 2V = 100mW and with collector-emitter saturating at 1V for Ic = 500mA we get 500mW. That's 600mW in total and pretty close to the absolute max rating of 625mW.

You could argue that if the collector current were limited to somewhat less than 500mA the overall power disippation would be less and maybe the 50mA is sustainable indefinitely.

Multicomp's 2N2222 data sheet quotes min H\$_{FE}\$ of 30 at a collector current of 500mA and this implies a base current of 16.67mA. The same data sheet also quotes 50mA base current when specifying base-emitter saturation voltage. It also has the same caviats on duty cycle and pulse width as Fairchild.

It looks like ON semi's data sheet has a nice graph on page 4 (figure 4) that gives a bit more detail.

My personal conclusion - don't run the base current continuously at more than 20mA - this is based on gut feeling and the data I've seen.

Andy aka
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