Test Only

[Bob Scott]

I did several circuits like that one already and looked at the current spike and wow it can go high, so i like people to know about this..

I can see where, in a push-pull transformer 12V configuration, that while Fet #2 turns on, it forces Fet #1's drain to rise up to 24V. In FET #1 the gate to drain capacitance pulls the gate back on, causing the current spike.

The transistors will stay cooler too with some dead time built in, and less EMF and noise on the power supply lines to screw up logic
states.

In any case, I would run the logic on voltage regulated power but the output circuitry on unregulated power. (In a RC plane speed controller I also added a 2N3904/2N3906 push pull buffer between the logic and the multiple parallel MOSFETs.) For the stepper motor I might add some sort of current regulation to the outputs dependant on the motor speed. I think the stepper motors will lose torque at higher RPMs. Do you have any knowledge of this?

I know that the data sheet for 12V 1.8 Degrees/step surplus stepper motors I have says that they are capable of instantly locking to 1,000 steps/second. The data sheet is ambiguous about certian specs.

A 12V unipolar motor has center tapped windings. Does the voltage rating apply across the entire winding or from one end to the center tap? Anybody?

The current rating is 1.6 amp. Is this total amperage or for each winding?
(Never mind, I'll go check the winding resistance.)

Bob

[MrAl]

Hi Bob,

Well, as far as the #1 MOSFET turning back on, i would hope the
designer made the driver output impedance low enough to prevent
accidental turn on as you were talking about. If the Z is low enough
the gate drain cap can not turn the device back on.
If the device is already on (as is the case with no dead time) that's
another story, and the current shoots up until the device gate source
cap discharges and the device turns off. Again a low impedance path
to ground in the driver circuit helps, but there is still a faster turn on
time than turn off time so dead time is almost mandatory, unless you
are using very overrated transistors and there is a high enough external
circuit impedance to help minimize the current spike during this
brief but important period.

I've read that the stepper motors have decreased torque with speed
yes, but then again isnt that true of many different types of motors?
I think it would be worthwhile to check out the manufacturers spec
sheet to see what the speed/torque curve looks like.

I also found this:
http://www.ams2000.com/stepping101.html