Driving stepper motor using TIP122

If your stepper motor has a high current rating then its better to use TIP120/TIP121/TIP122 for driving your stepper motor. (TIP122 is most commonly available in Ritchie street).

The TIP122 is silicon epitaxial-Base NPN power transistor in monolithic Darlington configuration mounted in TO-220 plastic package. It intended for use in power linear and switching applications.

About Darlington pair:

Darlington transistors or Darlington pair is two transistors connected together so that the current amplified by the first is amplified further by the second transistor. The overall current gain is equal to the two individual gains multiplied together.

A darlington pair behaves like a single transistor with a very hihg current gain. It has three leads (B,C and E) which is equivalent to the leads of a standard individual transistor. To turn on there must be 0.7V across both the base-emitter junctions which are connected in series inside the Darlington pair, therefore it requires 1.4V to turn on.

Circuit Description :

The driver circuit must withstand the voltage and current required by the stepper motor. The stepper motor which i used required 12volts and 1.5A to provide good torque, so i selected using TIP122. Driver for each wire include a TIP122, a 1k ohm resistor and a diode. The resistors are used for limiting the current and the diodes are used to avoid back EMF. The circuit is shown below,

The common terminal of both the winding are shorted and connected to motor supply. When logic 0 input is provided to the base of TIP122, the corresponding motor will remain floating as the impedance between collector and emitter of TIP122 is very high. So no current flows through that motor winding. When logic 1 input is provided to the base of the TIP122, its collector and emitter get shorted as a result the motor wire will be grounded resulting in current flow through the corresponding coil.

I used the PCB displayed above to drive two stepper motors. While driving the stepper motor having high current rating, large amount of heat will be generated hence to dissipate the generated heat you must use an Heat sink. Only by providing heat sink the TIP can drive maximum current.

Driving stepper motor using ULN2003

The simplest way to drive stepper motor having lower current rating is using ULN2003. The ULN2003 contains seven darlington transistors. The ULN2003 can pass upto 500mA per channel and has an internal voltage drop of about 1V when on. It also contains internal clamp diodes to dissipate voltage spikes when driving inductive loads. The circuit for driving stepper motor using ULN2003 is shown below.

For higher current torque motors, you can use TIP120. The advantage is that the TIP120 can pass more current along with heat sink. The disadvantages are that the more wiring is required and four TIP120 is used to control the motor.

IC's and transistors used for driving stepper motor

The IC's and transistors which are used for driving stepper motor is as follows, the selection of the IC's or transistors is based on the current requirement of the stepper motor. For the stepper motor which requires low current less than 500mA ULN2003 could be used. For the stepper motor which requires high current greater than 500mA transistors like TIP120, TIP 121 and TIP122 can be used.

• ULN2003 - used to drive stepper motor which has current rating less than 500mA. current rating can be improved by piggybacking two ULN2003.

• TIP120, TIP121, TIP 122 - used to drive stepper motor which has higher current rating greater than 500mA. Diodes must be used to avoid back emf from affecting the control circuit. Heat sink is required for obtaining the maximum current. More economic and easy to use for driving stepper motors.

• SLA7032M, SLA7033M - two phase stepper motor unipolar driver IC's which provides output current of about 1.5A and 3A.
  
• SLA7070M, SLA7071M, SLA7072M and SLA7073M - motor driver IC's for unipolar devices which provides output current of 1A, 1.5A, 2A and 3A.
• L297/A/D - stepper motor controller IC generates four phase drive signals for two phase bipolar and four phase unipolar stepper motors in microcomputer-controlled applications. The motor can be driven in Half step, normal and wave drive modes. The direction of rotation can also be switched.
• SLA7024M, SLA7026M and SMA7029M - High-current PWM, Unipolar stepper motor controllers/drivers. They provide an output current of about 1.5A, 3.0A and 1.5A.
• You can even use mosfet transistors and some other power transistors for driving the stepper motor.
• Mostly i prefer using ULN2003 and TIP122 for driving the stepper motors because they are easily available in Ritchie street.
  

Identifying the leads of Unipolar stepper motor

Unipolar stepper motors have four coils and either five, six or eight wires. No matter how many wires, unipolar stepper motors are interfaced in the same way. Five wires will go to the controller circuit. Four wires will be excited by pulses from the control circuit and the remaining wire(common) is given to the power supply. To identify the wires, you need a resistance meter.

First, identify the common lead. For the six and eight wire versions, some wires are twisted together to form the common lead and reduce the lead count to five. The common lead is connected to the positive of your battery or power supply.

• Eight wire motors: Find the four pair of coils. These will be the wire pairs that have the coil resistance between them. There will be infinite resistance between pairs. One wire from each pair will be all joined together to form the common lead. Without a wiring diagram, you will have to do a lot of experimentation to determine which lead of the pair is for the common.

• Six wire motors: Find the two wires whose resistance to two other wires reads the lowest of the measured resistances between wire pairs. Join these two wires together to form the common.

• Five wire motors:Measure the resistance between all pairs of wires. One wire will read about one half the resistance to all other wires when compared to the resistance between other pairs. This wire is the common wire. No wires are joined in this case.

Now, proceed to identify individual coils in order of sequence.

• Connect the common lead to the positive of your battery or power supply.
• Connect any one of the other four leads to ground. This will be coil 4.
• With coil4 still grounded, connect another lead to ground. If the shaft does not move, you have coil2. If the shaft rotates clockwise, you have coil3. If the shaft rotates counter-clockwise, you have coil1.
• Repeat until you have identified all four coils.
  

Identifying stepper motors

While buying a stepper motor you must note the following aspects

First, check for the number of wires coming out. If 5 or 6 or 8, that's good because you have a unipolar stepper. If 4, that's bad beacuse you have a bipolar stepper which requires two phase to drive. Confirm you have a stepper motor by turning the shaft. You should feel the little detents indicating each step. Most probably in ritchie street you will get unipolar motors with 5 or 6 wires.

Second, check the label on the side. The label will have the voltage and step size printed. Look for 12V steppers. If you have a 5V stepper, and it is large, the currents required will be large. Small 5V steppers are no problem to drive. If your stepper motor is new then it will have their voltage rating marked by its side, the problem arises when you are buying stepper motor in second sales. New stepper motor will cost about Rs.1500(for 1.8deg) whereas the second sales stepper motor with the same step angle would be just Rs.250.

Third, get out your digital ohmmeter (probably your multimeter) and start measuring the resistances between the leads. you will get different values depending on which pair of leads you measure. The lowest resistance you find is the coil resistance. Use I=V/R to compute the coil current. If 250mA or less, you are in good shape. Most stepper motors in Ritche street will have very low coil resistance and requires current above 1A.

Fourth, look at the output shaft and determine if it is something you can handle. Check whether the shaft has suitable height so you can use it. Common steppers have plain shafts with 0.125, 0.196 or 0.250 diameter. Gears press fit onto the shaft may be useful to you or can be removed. Don't try to remove the gears by yourself, you will damage the shaft if you try to remove it by yourself. Lathe with chuck must be used to remove the gears to avoid damage to the shaft.

very large or very heavy steppers will mostly require more current than you can control.

Where to find stepper motors

Stepper motors can be found in almost any piece of electro-mechanical equipment. It can be pulled from old electronic equipment such as floppy drives, dot-matrix printers. Most of the stepper motors in Ritchie street is pulled from printers and the most common step angle you could find is 1.8deg and 7.5deg which is of permanent magnet type. Look for "unipolar" or "4-coil" or "4-phase" motor (they all mean the same thing) with 5 or 6 or 8 wires. Avoid "bipolar" motors because they require an entirely different control scheme. The require two phases to control them and have four wires.

Operating voltage of 12V is convenient for robotics projects. Look for motors with a coil current of 250mA or less (coil resistance of 48ohms or more for a 12V motor <-> V=IR). Higher currents do give higher torque, but will also drain your battery faster. Another key specification is the holding torque which is how much torque the motor can resist when energized.

stepper motor - introduction

A stepper motor is a electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence. Stepper motors enable precise positioning without needing sensors to measure motor position.

Advantages

• The rotation angle of the motor is proportional to the input pulse.
• The motor has full torque at standstill(if the winding are energized).
• Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3-5% of a step and this error is non cumulative from one step to the next.
• Excellent response to starting/stopping/reversing.
• Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependent on the life of the bearing.
• The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.
• It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft.
• A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.
  

Disadvantages

• Resonances can occur if not properly controlled.
• Not easy to operate at extremely high speeds.
• More complex to control than DC motors.
• Consume precious battery current even when not moving.