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I have a 3 volt microcontroller that I want to control the power to a 5v microcontroller. What transistor can use a 3 volt base and allow 5 volts through the emitter/collectors. What type of NPN transistor will work for this application? (a MPS222A doesnt work :/ )

schematic

simulate this circuit – Schematic created using CircuitLab

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You need a PNP transistor ideally controlled from an NPN transistor.

schematic

simulate this circuit – Schematic created using CircuitLab

The reason why a single NPN emitter follower doesn't work is because the emitter output HAS to be lower than the base (3V) to get any current passing from collector to emitter. So the emitter might be 2.5 V when base is at 3V and collector is 5V (or greater).

You might be able to use one PNP transistor with appropriate potential divider values or maybe a small zener diode and resistor on the base to the GPIO.

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    \$\begingroup\$ FETs can also be used, as that will help a little in reducing quiscent current.. \$\endgroup\$ – Umar Jul 7 '15 at 14:14
  • \$\begingroup\$ @Umar very true. \$\endgroup\$ – Andy aka Jul 7 '15 at 14:14
  • \$\begingroup\$ @Umar What I would call quiescent current (consumption when off) would be greatly improved by adding 100K on Q1 E-B. But greatly is relative, it's typically b*gger all (maybe tens of nA) at reasonable temperatures. \$\endgroup\$ – Spehro Pefhany Jul 7 '15 at 14:45
  • \$\begingroup\$ This is the same application mine but mine doesn't work youtu.be/63sPXfRx9VM \$\endgroup\$ – GisMofx Jul 7 '15 at 16:38
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    \$\begingroup\$ THAT makes all the difference - the emitter tied to ground and the base slightly higher - then the collector will drop to nearly 0V when activated. Look at thye PNP in my circuit and imagine it was an NPN with emitter connected to ground. My circuit is just a mirror image of the video circuit but, because the GPIO is gnd referenced there has to be another transistor. \$\endgroup\$ – Andy aka Jul 7 '15 at 21:14

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