Hello!
I’m using a large RC servo (Futaba S3306MG) which is not constantly moving, so I want to turn it out when not been used to save power and avoid overheating.
The servo is working fine when it is directly powered by a power source (+6V AC/DC adaptor) and controlled with the servo library (so no controller issue). However, when I use a optocoupler and a MOSFET to power the servo it flutters a lot. This is the circuit I used:
Any suggestion to solve the problem?
Yes use a FET that will switch on with the voltage you have, not one that needs 10V to be on.
Grumpy_Mike:
Yes use a FET that will switch on with the voltage you have, not one that needs 10V to be on.
Thanks for your comment. If I understand it properly the BUZ 22 need less than 5V at the gate to operate. It says Gate threshold voltage min. 2.1V max. 4.0V. I measured 4.78 V between the gate and GND.
If I understand it properly the BUZ 22 need less than 5V at the gate to operate.
No you do not understand it properly.
The gate threshold voltage is the voltage when the gate just starts to turn on not when it is fully on.
Grumpy_Mike:
If I understand it properly the BUZ 22 need less than 5V at the gate to operate.
No you do not understand it properly.
The gate threshold voltage is the voltage when the gate just starts to turn on not when it is fully on.
How to I know the "propper" voltage then? Does it has an specific name at the data sheet?
You find it in the Drain - Source ON resistance line as a parameter of the measurement VGS
Rds(on) is the on resistance. This is quoted for one or more specific values of Vgs (gate
voltage). If you provide less than the minimum Vgs listed you are failing
to turn it fully on.
Lose the optocoupler its doing nothing useful. Drive the gate of the MOSFET through
a 150 ohm resistor from an Arduino pin, connect a 10k between gate and source to ensure
its off when the Arduino is reset. Choose a MOSFET like this which is logic-level:
You probably don't need a flyback diode as the servo control circuit should be
handling that, but I'd add one just in case across the load.
Lose the optocoupler its doing nothing useful.
I beg to differ, if the supply is high enough it will act as a level shifter and allow the FET to be turned on fully. The problem is that the servo power supply is not high enough for this FET. If it were a 12V supply and a solenoid then it would work.
Controlling the servo power on the ground side is sketchy at best. I know that small servos will start to act erratic and heat up when the ground wire comes lose. The path to the power ground may become thru the servo signal wire.
Thank you much for the information guys!
What does the rest of the people think about this? Will it be better to use a P-channel MOSFET to controlle de +V path?
zoomkat:
Controlling the servo power on the ground side is sketchy at best. I know that small servos will start to act erratic and heat up when the ground wire comes lose. The path to the power ground may become thru the servo signal wire.
Can you explain this comment a bit more?
You probably don't need a flyback diode as the servo control circuit should be
handling that, but I'd add one just in case across the load.
Will it be better to use a P-channel MOSFET to controlle de +V path?
Yes.
