Homebrew PCBs

Robert Hedan wrote:
> "maybe this has been discussed but why not use mosfets instead of the NPNs?
> If you can turn them fully on they will run a lot cooler with little voltage
> drop."

Mosfets will work just fine.  You'll need to assure
sufficient drive voltage in the case of an H-bridge
for the upper N channel mosfets.  This means needing
somewhere between 2-10V greater for the drive than
the mosfet is switching.  Same issue exists for NPN
bipolar drivers though not as severe.  For unipolar
steppers this issue doesn't exist.

BTW the discussion of external suppression diodes for
mosfets is likely moot for devices where an internal
parasitic diode is present.  The main reason to add
an external schottky in SMPS designs is that of
efficiency as the voltage drop across the internal
diode is typically greater than available via the
external schottky.

> "Also, how are you limiting current?"
>
> See, this is something else I need more information.  I have no idea what
> you mean by limitting current.

If you want to drive a stepper at maximum possible
velocity it requires a high drive voltage to the
coils.  This voltage is only needed initially in
the cycle and in fact if allowed to be imposed
statically on the windings will cause excessive
current consumption and quickly overheat the works.
One of the techniques to achieve this voltage profile
is a constant current scheme which limits the steady
state winding current to a nominal value but will
allow the highest available drive voltage to be
applied initially in the step cycle.

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uhmgawa@...        www.gnu.org