Location of back EMF protection diodes

[joe_1]

I want to run bipolar (4-wire) stepper motor. The motor needs 35V, and the PIC has only 5V, so I will use power MOSFET.
I got each of the 4 wires from PIC going as follows:

PIC -> 1K resistor -> Gate of power MOSFET
Then Source of MOSFET -> 35V
Then Drain of MOSFET -> Motor

I did run the PIC and checked the 4 wires from the PIC, and they are working fine.
But I did not test it with MOSFET and hi voltage motor yet, because I do not know where to put the back EMF protection diodes.
I looked at those diodes and found this: 1N5408 with rating of 3A and 1000V.

I have not done any project like this before; so if you have any suggestions about the above connections or about where to place the protection diodes, or even if you know of a better diode than the 1N5408, please let me know.
My stepper motor is rated 3.1A, 35V, and 58W.
Also, the MOSFET is rated 60V, 4A.
Thank You.

 

[crutschow]

You can not drive the gate of a MOSFET with the source at 35V from a PIC. Where did you get such a connection? For an N-MOSFET that means the gate would have to be at a higher voltage than the source to turn on.

You need an N-MOSFET. Connect the source to common, the gate through the 1k resistors to the PIC output, and the drain to the motor windings. The common connection of the motor windings goes to the +35V.

The MOSFETs need to be logic-level type to allow proper turn-on from a 5V gate signal.

The protection diodes go from the MOSFET drain (anode) to the +35V (cathode).

 

[joe_1]

The problem is that my bipolar stepper motor does not have a common or ground wires. It has only 4 signal wires for the 2 phases.
Now I am not sure where to connect the 35V to. Also, not sure what to connect the Source to.

 

[crutschow]

I missed that it was a 4-wire motor. So it would seem you have to apply a bipolar signal to the 2 phases and this can be done with two bridge circuits. Google "H bridge circuits" for ideas on how to do this.

 

[unclejed613]

MOSFETs generally have a "body diode", which means the diode is part of the transistor substrate. if you want you can add an external one with the cathode at the FET drain, and the anode at the FET source, but chances are that the internal diode is faster. the motor you have requires what's known as an H-switch. this requires 4 transistors per winding so that the polarity across the winding can be reversed in sequence.


for an H-switch, you won't be able to control the "upper" mosfets unless the drive pulses are level shifted so the gate can be driven several volts higher than the voltage required across the coil. in the picture you can also see where the diodes go.

also make sure your driving logic doesn't allow both transistors on an arm to be activated at once. this is called "fuse testing mode" for obvious reasons