Stepper Motor Drive

[YAN-1]

Hello everyone. I am trying to build a circuit in order to drive a unipolar stepper motor that I have. The circuit I am building is shown below. Ok I understand that the upper diodes are to protect the MOSFETs from back EMFs, but what are the lower ones for? I asked a lab supervisor at the university and he told me to get fast diodes for the upper four diodes and schottky ones for the lower four diodes. The lower ones do not have to have current capabilities as large as the upper ones. The motor windings are rated at 2 amps so I got 4 diodes with 3.5 amps current ratings. The forward recovery time of the diode is 210 ns and the reverse recovery time is 50 ns. Is this considered fast enough?! This is the fastest diode that I found that has the required current ratings.

The MOSFET I got is the IRF530 and the schottky diodes are of the type 1N5818. Please tell me if this is a good design.

I also have a final question please. The rated voltage of the motor is 3 Volts. However, I only have one power supply at the moment and I need to operate the motor from a PIC (5 Volts). Normally, I set the supply at around 9 volts and use a regulator to get a steady 5 volts to power up my breadboard. What should I do now? Should I use a potentiometer to get the 3 volts for the motor?! I know this is bad design but until I get another supply, what can I do?

Thanks a lot.

Nichola V. Abdo

 

[Nigel Goodwin]

Hello everyone. I am trying to build a circuit in order to drive a unipolar stepper motor that I have. The circuit I am building is shown below. Ok I understand that the upper diodes are to protect the MOSFETs from back EMFs, but what are the lower ones for? I asked a lab supervisor at the university and he told me to get fast diodes for the upper four diodes and schottky ones for the lower four diodes. The lower ones do not have to have current capabilities as large as the upper ones. The motor windings are rated at 2 amps so I got 4 diodes with 3.5 amps current ratings. The forward recovery time of the diode is 210 ns and the reverse recovery time is 50 ns. Is this considered fast enough?! This is the fastest diode that I found that has the required current ratings.

I see no reason for the bottom diodes, but they won't do any harm. I also see no reason for using fast diodes?.

I also have a final question please. The rated voltage of the motor is 3 Volts. However, I only have one power supply at the moment and I need to operate the motor from a PIC (5 Volts). Normally, I set the supply at around 9 volts and use a regulator to get a steady 5 volts to power up my breadboard. What should I do now? Should I use a potentiometer to get the 3 volts for the motor?! I know this is bad design but until I get another supply, what can I do?

3V sounds exceptionally low for a stepper?, is it particularly small?, if so why are you looking at powerful transistors and diodes?.

It's VERY advantageous to feed steppers from a higher voltage via resistors - it increases speed and power. If you google for 'stepper motor tutorial' you will find plenty of info about it!.

 

[YAN-1]

Yeah I read about that. I can make its time response faster and make sure the current doesn't exceed the ratings through resistors and current sensing so I can chop the voltage when Imax is reached. Anyways, it's actually a linear actuator; a stepper motor with a threaded screw for a shaft. It's rated at 2 amps and it's supposed to give 85 N of thrust, so I don't think it's that small! A Microship tutorial about stepper motors said the lower 4 diodes are neccessary because of the center taps. They make sure the voltage across the MOSFETs doesn't go below ground when one end of the winding is high and the other is grounded. Guess I'll give it a try.

So I can either use a potentiometer to get 3 volts (ineffitient), or I can add resistors in series with each winding and apply, say 5 volts. This will help me since I already have a 5-volt supply and will make the response faster, right? But I have to make sure I use high-wattage resistors that can pass the 2 amps, right? Not the 0.25 Watt ones used in breadboards for LEDs and simple systems.

 

[Roff]

You may have seen this, but here is a tutorial that explains the diodes and has a section on current limiting.
According to the datasheet, the IRF530 has integral diodes that appear to be adequate to limit the flyback due to autotransformer action.

 

[hjl4]

Don't get too concerned with the 3V rating.

Stepper motors are often, if not always overdriven. Go with your 5V supply and you will see very little negative effects. If anything, you will get better response. I have two stepper motors one rated at 24V and the other at 36V,

My power supply is good to 30V. I run the 24V one at 30V.
The motor gets slightly warmer (3-6)degree difference compared to the 36V one.
It has been used for a year with no problems as a linear actuator.

 

[YAN-1]

I see. Thanks. I just want to ask concerning the auto transformer effect of the center tapped windings. Will the lower 4 diodes be required to carry the complete load of the motor (2 amps) or would my 1-amp schottky diode suffice? Thanks.

 

[hjl4]

Your lower diodes rated at 1 amp should be fine.
Like Nigel and Ron H said, there use is somewhat trivial.


Try it and let us know.
Good luck.