Stepper motor tutorial #2
Coils in a stepper motor, like all inductive components, will produce a voltage spike when the current is abruptly switched off. A common protection against this potentially damagingly high voltage is the freewheeling diode.
A popular but bad circuit design.
This circuit works, and is often used to drive simple inductive devices like relays. However, using this configuration for a stepper motor is a bad idea.
Motion Braking1. When a motor turns it acts as a generator.
2. When a generator is short circuited, the generator acts as a brake.
3. The diodes act as short circuits when the motor is turning.
This simple fact is easily demonstrated by turning an open circuit a stepper motor by hand, then short circuiting all the windings and turning the motor again. The torque required increases dramatically. This is just one extra load your motor will have to drive if you use the above circuit.
Drive BrakingAnother braking action comes from the fact that the coils are center taped. When a drive signal is applied, a freewheeling diode prevents the rapid rise of the magnetic fields by short circuiting the opposite half of the coil. The diodes allow the induced EMF to become a induced current with a magnetic field of its own. This momentary magnetic field is opposite in polarity and therefore preventing the rise of the intended field. This once again reduces usable torque, increases power consumption and places a larger load on the drive electronics.
Having said all this, many people still use this flawed design.
A simple solution to the problem is shown above.
The voltage of the Zener diode is chosen to be at least as high as the supply voltage. C1 is there because zeners tend to be a little noisy.One zener, capacitor circuit can be used for coil 1 and coil 2. D1 and D2 are allowed to suppress voltages only when they exceed twice the supply voltage.
No Drive BrakingA applied drive current can only generate a maximum voltage, in the opposite half of the coil, equal to the supply voltage. Because the requirement for currents to flow through D1 or D2 is twice the supply voltage, no drive braking occurs.
No Motion BrakingThe generator effect of a motor turning at speed A, cannot exceed the voltage required to drive the same motor to speed A. Therefore motion braking can only occur if the Zener diode has a voltage of less than the supply voltage.
Good fly back protectionIn fly back circuits, voltages far exceeding the supply voltage can be generated. It is because of the ability of the fly back EMF to rise above the supply voltage, that the fly back protection is easily clamped by the zener diode.
One thing to remember is that the fly back current, however short lived is equal to the drive current. D1,D2 and most importantly ZD1 will have to dissipate some power.
Simpler Smart brakes
At first glance the above circuit would not work for fly back suppression.
A self induced EMF in Coil 1A generated when Q1 switches off, would result in a rapidly rising collector voltage in Q1. D1 is not forward biased so how does the circuit protect Q1 from the back EMF?
The answer is that D2 is the one protecting Q1. The stepper motor winding is center tapped and any rise in Q1 collector voltage is accompanied with a fall in Q2's collector voltage. When Q2's collector voltage falls below ground, D2 is forward biased. The maximum voltage across the transistors is slightly more than twice the supply(depending on the efficiency of the steppers magnetic circuitry. In practice some BJT transistors like the TIP102 family are produced with the diodes within, and almost all MOSFETs come with the diodes as standard.
The circuit behaves in a similar fashion to the previous one in that the diodes are only forward biased once a drive transistors collector voltage exceeds twice the supply voltage. One difference is that the EMF is clamped back into the opposite coil. This will result in a slower falling magnetic field.
A topic to be investigated. Watch this space.
Forward to stepper inductance effects