Originally Posted by genericuser Hey H500 & phil,
Here's where my newbieness come into play, I'm not sure why the voltage across the mosfets is 2X supply. Can you explain briefly so I can further investigate? I assume your making sure that if I increase supply then I must also ensure mosfet max volts is in safe limit.
generic |
When the chopper fet turns off, the current in the coil ceases instantly. Those who are familiar with inductors might ask how this can happen since it violates the fundamental characteristics. The answer is that the motor acts like a transformer rather than an inductor.
The + and - phase coils on a unipolar motor is wound bifilar, ie: two strands of wire are wound on the same core.
So, when the fet turns off, the current is actually transferred to the other coil, flowing back into the power supply. This transformer action is what doubles the voltage on the fet
If you are interested in the mathematical details, just read up on transformer theory. Otherwise, just remember to use fets with at least twice the voltage rating as the supply.