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Topic: [SOLVED]Drive servo with BJT (Read 408 times) previous topic - next topic

surepic

#15
Apr 07, 2019, 06:51 pm Last Edit: Apr 07, 2019, 06:54 pm by surepic
Its not stopping it its behaving erratically. Really hard to explain the behaviour but will do my best:

When motor is fed with 2ms pulse shaft is rotating to some direction.
Now when fed pulse is reduced to 1ms motor still is turning to same direction as with 2ms pulse.

Rechecked same process without bjt i.e. servo hooked up directly to rails doesnt show that problem. 1ms pulse is turning shaft to opposite direction of 2ms pulse.

Hope this description helps. Or i can make a small video.

Small addition to description:
If base current is getting below 10ma then motor is starving for current its starting to make noise and shaft is not moving. Im feeding it with 20ma base current to be safe.

slipstick

I've never tried controlling a servo when it's ground connection is not connected to signal ground but to a voltage which is either floating at 5V or is the transistor Vce above the Arduino ground. It doesn't sound like a good idea and it's probably one reason why everyone recommends high-side power switching.

Steve

surepic

There was something interesting in your suggestion so tried the pnp config S8550 bjt. This time ground of servo is tied to ground.

But exact same problem. Absolutely no difference with previous npn config.

Tried 5.5v unregulated supply same problem if done through bjt. So my assumption of vce voltage drop was wrong and its not causing this issue.

Was thinking of buying 6v regulator hoping will fix the problem but it wont. Ran out of ideas.

surepic

#18
Apr 08, 2019, 04:47 am Last Edit: Apr 08, 2019, 04:48 am by surepic
Problem solved

By adding flyback diode across bjt .

Thanks everybody for support.

Paul__B

Its not stopping it its behaving erratically. Really hard to explain the behaviour but will do my best:
...
If base current is getting below 10ma then motor is starving for current its starting to make noise and shaft is not moving. Im feeding it with 20ma base current to be safe.
Explained in #3.

Problem solved
By adding flyback diode across bjt .
We will see ...

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