I'm new, so please go easy!

I'm trying to use a TIP120 transistor as a switch in conjunction with an Adafruit Trinket Pro 3.3v. According to the documentation for the Trinket, the GPIO pins should be able to source 20mA of current, which, if I'm understanding the transistor documentation correctly, should be enough to bias the TIP120.

Fabulous, except that I'm only ever measuring 10mA from the GPIO pin when I have the circuit hooked up.

The TIP120 is being used to switch on power from a 3.7V 18650 battery, and I do actually get some bias from the 10mA - I read about 2v - but I need the full 3.7v.

Could someone please let me know if it sounds like I'm misunderstanding anything here? Why can't I get the Trinket to source 20mA to fully switch the TIP120?

Thanks for your time and help!


1 Answer 1


The TIP120 is a Darlington transistor pair, which consists of two transistors with the Emitter of the first one connected to the Base of the second one. This provides very high current gain, but doubles the required Base bias voltage and increases the Collector saturation voltage.

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10mA Base current is plenty enough to 'fully switch' the TIP120, but it has a minimum Collector-Emitter saturation voltage of about 0.75V (rising to 1V at 2A). You will never get the full voltage out of your battery using this transistor.

If you are switching the positive battery lead with an NPN transistor then it has to be wired in Emitter Follower configuration, and the output voltage will be less than the control voltage (3.3V in your case). To get full output voltage you must use a PNP transistor or a P-channel MOSFET, with level shifting to ensure that it is correctly biased. A MOSFET has the advantages of negligible bias current and lower saturation voltage than a typical bipolar transistor.

The level shifting circuit below uses a general purpose NPN transistor to drive either a PNP power transistor or a P-Channel MOSFET. With R1 at 150Ω it delivers ~20mA bias current to the PNP transistor. The MOSFET requires no bias current, but its Gate threshold voltage must be 1V or less to fully turn on with 3.7V.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ That's one of the best, more thorough answers I've gotten on StackExchange - thank you!! Makes perfect sense! \$\endgroup\$ Sep 7, 2015 at 18:47
  • \$\begingroup\$ Bruce - I'm currently trying to build this circuit - but I think perhaps I'm misunderstanding. In the diagram where is "G" on the NDP6020P going? And are "D" on the NDP6020P and the collector of the TIP42 both receiving the bias current? \$\endgroup\$ Sep 13, 2015 at 17:48
  • \$\begingroup\$ I tried to show how to connect a bipolar transistor or a FET. If you use an NDP6020P then connect its Gate to the junction of R3 and R1 (and don't install the TIP42!). \$\endgroup\$ Sep 13, 2015 at 17:58
  • \$\begingroup\$ Gotcha - ok, trying that now! \$\endgroup\$ Sep 13, 2015 at 18:01
  • \$\begingroup\$ I appreciate your continued help! I must still be doing something incorrectly. I stuck a push button switch in place of V1 with a 3.7V supply. I read about 500mV between R2 and the base of Q2. But only read about 40mV on the collector of Q2. Any idea what I'm doing incorrectly? \$\endgroup\$ Sep 13, 2015 at 19:14

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