dc motor swithing problems

[kraete]

i just treid to post this and i think it bugged out so sorry if this is a double post.
i have 2 motors running off a 9v battery, the battery is connected to 2 PNP transistors darlington style so my 3v microcontroller signal will switch them
the setup works perfectly fine except for when i switch off the transistors the motors still spin very very slowly. i added a 10k trimpot to the base to try and tweak it which helps but they still spin, if i leave the transistor off and connect the 9v after they dont spin, only after running and having the transistor switched off do they continue to move


oh and heres a terrible picture
oh nevermind its too large

[Soeren]

Hi,

i have 2 motors running off a 9v battery, the battery is connected to 2 PNP transistors darlington style so my 3v microcontroller signal will switch them
the setup works perfectly fine except for when i switch off the transistors the motors still spin very very slowly. i added a 10k trimpot to the base to try and tweak it which helps but they still spin, if i leave the transistor off and connect the 9v after they dont spin, only after running and having the transistor switched off do they continue to move

That's not surprising, as the controller will be 3V max and the PNP Darlington has to see an emitter-base potential of less than ~1.4V, which with a 9V supply means it has to be over 7.6V.
You may actually harm the controller this way.

Change it to NPN-PNP instead and it will work.
Need a schematic?

[kraete]

hmmm im cobbling this together with parts from broken electronics, it seems i only have PNP's. i dropped the supply down to 5v and it seems to be fine. anyway i could make it work with the 9v? i want it to go a bit faster than the 5v will allow.

[Soeren]

Hi,

hmmm im cobbling this together with parts from broken electronics, it seems i only have PNP's.

That sounds unlikely. Almost any piece of hardware will have more NPN's tha PNP's, as the latter are slightly more expensive.
If you have stripped at least 3 different electronic devices, you're almost bound to have an NPN somewhere.

i dropped the supply down to 5v and it seems to be fine. anyway i could make it work with the 9v? i want it to go a bit faster than the 5v will allow.

Find an NPN and try the attached circuit, assuming a power PNP for Q2.