I am using atmega328p and PNP transistor FMMT720.

Goal is simple to switch on and off transistor with GPIO. For specification part: Datasheet says continuous collector current is 1.5 A and my requirement is 500 mA.
base current according to my calculation is 27 mA and it may change as my voltage changes but still within range of 20 to 40 mA, which is tolerable by atmega328p on individual pin.(Schematic shown below) Gain is pretty high, more than 100 so output current should not be problem.

Problem: For PNP transistor base needs to be pulled down to turn it on. What I noticed is base is not getting proper low voltage from the pin hence transistor stays off. Although I can see a change of 0.4 V during the 2 second time (transistor ON time for my application) but not enough to turn it on. In addition it does turn on sometimes and thing works perfectly but not always.

Is there any problem in calculation part? Is this really a problem with transistor? I am also using PCINT with other hardware can it affect anyhow?

Circuit I am using is quite large so giving necessary part. Ask if need more clear idea. Thank you.


simulate this circuit – Schematic created using CircuitLab

  • 1
    \$\begingroup\$ Does the boost converter have an enable pin? \$\endgroup\$
    – Justme
    Jun 28 '21 at 10:03

Not enough base current it seems OR, the GPIO pin is not falling to zero volts due to the high pull-up loading effect from the base current. I'd consider using a MOSFET for this by the way.

If your load is a boost converter then it probably has a filtering capacitor directly across its terminals and this will need charging up. The current needed to get it charged in a shortish time is probably much in excess of the 500 mA you have calculated so, maybe the voltage rises a bit and sometimes the converter works and sometimes it doesn't.

  • \$\begingroup\$ Thank you for response. I'd like to use mosfet but I cant for now due to some reasons. If for instance there is issue of current, is there any solution for that ? there is capacitor of 22uF across input terminals of boost converter. \$\endgroup\$
    – dharmikP
    Jun 28 '21 at 9:46
  • \$\begingroup\$ I'd suggest using a MOSFET and trying it out to see what the difference will be - I expect the MOSFET would appear very superior but, if it didn't then maybe it's because the sudden surge of current through your transistor to charge up the capacitor is causing a power supply dip. \$\endgroup\$
    – Andy aka
    Jun 28 '21 at 9:56
  • \$\begingroup\$ OK sure ill try mosfet. My concern with mosfet is 3.3v logic level mosfets are not available locally and price is high in comparison too, will see what I can do. I am still looking for circuit solution to this problem with transistor if there is any. Thank you for your time. \$\endgroup\$
    – dharmikP
    Jun 28 '21 at 10:10

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