Max safe base current for a 2N2222A transistor?

Question

i want to use a 2N2222A as a switch to rapidly turn ON/OFF a load of about 700mA current, at about 250 times a second.
I just wanted to know whats the maximum safe current i can give to the base of this transistor?
The datasheet only specifies Ic as 800mA, and hfe values as a range from 30-100.... right now in a simulation i have 35mA going into the base and the circuit works beautifully....will this be safe in real life?

Answer 1

in linear range, collector current is base current times hfe.

since Ic maximum is 800mA (some manufacturers make versions that only go to 500mA), and lowest hfe is 30, there is no point in driving base harder than with base current of



Ib=800/30=26.67mA.



Once you exceed that, every single 2N2222 will be in saturation so higher base current is just waste. It is safe to run base current 30-50mA, transistor will simply go into saturation.



but there are many companies making 2N2222 and they have specs that differ from brand to brand.

700mA is at the limit of best 2N2222 transistors and way over the limit for many others.



if you exceed 60-70^ of collector current it is recommended moving up to a more powerful transistor (BD135 or similar).

Answer 2

Yes, as some answers have implied, this is ignoring the effects of heating of parts of the die known as second order breakdown. This is a runaway condition, because the transistor has a negative temperature coefficient. A hot part of the die causes more current to move to that part, making it hotter still. So the 700mA current is too high for switching on and off as during switching on and off the power level is higher, unless the supply voltage is very low. That is why a rule of thumb like 60-70% of the rated collector current exists. This effect depends on switching time, voltage and current. Note that dissipation in the emitter base junction adds to the total dissipation.



Go for a larger transistor, maybe using a driver to get enough current gain, or sometimes a simple solution is to use a MOSFET instead. This effect is much less likely to be an issue with Mosfets which have a possitive tempco., and the current rating can be 10A for this case. Note there are logic level types which only need 5V to drive into full saturation. The driver needs to be low impedance as the gate capacitance is high, but should nit be much of an issue at this low switching rate.

Answer 3

2n2222a Transistor Datasheet

Answer 4

This may vary depending on manufacturer. Their particular data sheet is the best information.

Another source of good hands on electronics is Youtube, afrotechmods.

Answer 5

35 mA will be fine, but check the data sheet for the maximum values.

Answer 6

Thank you! Extremely informative and this gives me better insight on the subject

Answer 7

ARE YOU JOKING TO SAY 2N2222A CAN HANDLE 800mA. It becomes quite HOT at about 30mA and burns out at 100mA.

Answer 8

37mA