I have a npn transistor configured as a switch which is normally on and at times I have it switch off and on at 7Hz-8Hz, with R1 5.10k and R2 100k and 0 V to 5 V MCU pin and max \$I_C\$ current from 50-60 mA. With these values the \$I_B\$ come to be around 0.81-0.83 mA. Will this configuration values will okay in the long run or is a fail?, with this type of NPN transistor transistor datasheet. The circuit seems to operate okay, but should have lowered the Rth resistance to lower value to secure complete saturation like 2k and having a 5.10k not enough over time. Thank you. enter image description here


2 Answers 2


The rule of thumb for transistor switches is to make the base current 1/10th of the collector current to force it into saturation. This would put you at around $$ R1 = \frac{V_{in}-V_{BE}}{I_B} = \frac{5.0V-0.7V}{6mA}= 716.\overline{666}\Omega $$

so 720\$\Omega\$ should do it.


Will this configuration values will okay in the long run or is a fail?


max \$I_C\$ current from 50-60 mA.

It comes down to how low you want the saturation voltage to be. Consider this data sheet graph: -

enter image description here

If you want the saturation voltage to be less that 100 mV then you should adopt a value of \$I_C/I_B\$ of ten. If you can live with maybe 200 mV you can probably get away with \$I_C/I_B\$ being 20.

  • \$\begingroup\$ Andy & GodJihyo. The 10 ratio needs to be for complete saturation. Now the circuit I have will fail over time/stop switching and hot, if it is on the active region and not in complete saturation? If the worst case is right the transistor max power mentioned is about 250mw@25*C prob low at high temp, with Ib=0.83, Vbe =0.7, ic=60ma, Vce = 900mv. Total power dissipation is about 58.6mw. I went and measured the real value for Vce Im getting about 160mv which will result in less dissipation of about 10mw. And Vcb reaches 0.211mv when Ic reaches up to 80ma. Thank you for your knowledge. \$\endgroup\$
    – Citi
    Commented Apr 1 at 7:59
  • \$\begingroup\$ @Citi If we are done here, please take note of this: What should I do when someone answers my question. If you are still confused about something then leave a comment to request further clarification. \$\endgroup\$
    – Andy aka
    Commented Apr 1 at 8:19

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