accurrentmosfetmotoroperational-amplifier
I'm having troubles with a 3D printer. I need to change a motor for a more powerful one. For doing so, I 'll have to add an external small circuit to amplify the current but not the voltage given by the main board. After trying to understand the uses of MOSFET or OP amps, I feel a bit lost.
There's my configuration:
I can use an external 12v from the power supply
I need to get 1.7A from 0.9A (4V)
How do I amplify current for a stepper motor?
Is your project the stepper motor driver, or is this for another, larger project?
Unless you specifically want to learn about the intricacies of driving stepper motors, I would recommend just buying an off-the-shell stepper driver, either in IC form, or completely assembled.
I'm a big fan of the Allegro Micro stepper drivers.
If you want a simple, off-the-shelf solution, spark run offers a couple really inexpensive stepper-driver boards (seriously, they pretty much cost as much as the equivalent parts).
(Images are links)
They would be quite simple to interface with your USB DAQ system too. It would just take two DIO lines. One would set the direction, and the other would cause the motor to step - one step per cycle.
The 12 V 3 A supply sounds appropriate, but the 5 V supply is not. If you want to maximize the holding torque, you have to allow the maximum current thru the windings. This can't be achieved at only 5 V according to the specs you quoted.
Now that you have supplied a link to the datasheet, we know the specs for sure. Your motor is rated for 1 A, 3.8 Ω DC resistance, 3.8 mH inductance, and 75 V max. I have no idea where you got the 12 V ratings from you mentioned in your question.
Since the maximum current is 1 A and the coil has a resistance of 3.8 Ω, you can only apply 3.8 V to a winding continuously. The 75 V rating is the maximum you must not exceed under any circumstances.
Since the motor coils have significant inductance, it takes time to build up current when a fixed voltage is applied. However, torque is proportional to current. This is why stepper motors are often driven with a roughly current controlled supply. The voltage will be high initially as the current in the inductor builds up, then drop to the sustaining level. If you know you are stepping the motor fast, then you can use a higher fixed voltage keeping in mind the time it takes for the current to build to the maximum level given the coil inductance and resistance. Since these parameters are reasonably well known, you can do this open loop. The usual way nowadays is to have a micro do the calculations and control the coil drive with PWM from a fixed high voltage. The duty cycle is changed such that the coil sees a apparent high voltage right after switching, which then decays down to the sustaining voltage as the current builds up. You can also use the same PWM technique but with some current feedback. This feedback is used to adjust the PWM duty cycle to whatever it takes to maintain the desired current.
Best Answer
I think the practical answer to your question is that you can't put anything between the controller and the stepper motor.
Most 3d printers, and probably yours too use a dedicated stepper driver for each motor. These can microstep and regulate current. It's not a simple on/off sort of signal. If you put some device between the controller and motor then it's probably not going to work correctly.
You may be able to get a new motor that has windings such that that it generates more torque at same current as your old motor. You may also be able to increase the current setting of the driver by adjusting a pot on the board (if you're lucky), or by changing some surface mount resistors.
If none of that works, then I would try to find the step and direction signals, and simply replace the motor AND driver with something more powerful.