Author Topic: Uni polar steeper motor drive design doubt  (Read 771 times)

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Offline NASKTopic starter

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Uni polar steeper motor drive design doubt
« on: July 10, 2019, 06:15:16 am »
 I have a doubt about design stepper motor drive , therefore i refer this website http://www.hteck.ca/electronics/stepper-motor-drv/sm-driver.html

but I cannot understand why use diodes series with Nchanal Mosfet     

« Last Edit: July 10, 2019, 06:17:57 am by NASK »
 

Offline MosherIV

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Re: Uni polar steeper motor drive design doubt
« Reply #1 on: July 10, 2019, 06:50:45 am »
It is normal/standard to put 'flyback' (at least I think that is the technical term) diode to provide a path for current to flow when you turn off a coil.
De energising a coil, the collapsing magnetic field will create a voltage oppisite to the one applied. In a circuit, the current will flow through any path available. In transistor switch circuits, the transistor needs to be capable of withstanding the high voltage.
So it is normal to provide a path for the induced current to flow back.

In the circuit shown, it is not well drawn for beginners to understand. There is a connection to positive voltage near the coils. The diode in series is the current return path only.
 

Offline NASKTopic starter

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Re: Uni polar steeper motor drive design doubt
« Reply #2 on: July 10, 2019, 07:10:38 am »
thank you for the support !  Is it same reason for this bipolar mode ?

I think you telling about back emf current. Normally  diode is used for protect transistor form the back emf .
« Last Edit: July 10, 2019, 07:15:54 am by NASK »
 

Offline pwlps

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Re: Uni polar steeper motor drive design doubt
« Reply #3 on: July 10, 2019, 09:12:17 am »
I think you telling about back emf current. Normally  diode is used for protect transistor form the back emf .

Beware that in motors there are two phenomenons often both called "back EMF":
https://en.wikipedia.org/wiki/Counter-electromotive_force
but the naming is a bit misleading because they are actually of opposite polarities. 

The diode protection is for the EMF induced by a decreasing current in the motor inductance when the switch opens.
 
De energising a coil, the collapsing magnetic field will create a voltage oppisite to the one applied.

Not opposite (at least as far as the diode function is concerned).  The voltage current relation for a coil is  V=-LdI/dt  so when the switch is opening there is a large negative dI/dt causing a large positive V, i.e. the induced voltage wants to sustain the current.  A flyback diode will limit the induced voltage to the diode threshold voltage so that stopping the current will take more time. The energy of the stored magnetic field is then dumped into the diode.
 


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