Stepper motor 101 was Re: Lini-stepper

Re: "Uh..not quite." Not quite.:-)

1) The transistor (NPN or MOSFET) is operated as a 1A current sink.
2) You have a 1A, 5 Ohm motor coil from +supply to collector/drain.
3) The coil develops a 5VDC drop at 1A.
4) Your supply is 30VDC.
5) The voltage collector to emitter / drain to source is 25V (30 - 5)
6) The transistor (NPN or MOSFET) dissipates 25W (1A @ 25VDC).
7) Makes no difference in linear operation.

Mariss



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "jzmuda2000" <hwhacker@...>
wrote:

>
> UH...not quite.
>
> When the BiPolar transistor is in a fully conducting state...the
> resistance from Collector to emitter is low. And the transistor
> effectively conducts as if it were a wire. Sending 1 amp of

current

> through at 30 volts does NOT dissipate 30 Watts in the transistor
> body. (Do the thought experiment. Conducting 30volts/1Amp through
> a wire doesn't dissiparte 30 Watts in the wire, does it?) The GREAT
> BULK of that power is dissipated in the LOAD...that is being driven.
>
> And the closer the transistors "turned on " resistance is to being
> zero...the more close to true it is that the entire 30 Watts are
> being disipated in the LOAD and not the transistor.
>
> The reasons for choosing FETs over BiPolar (if you can find big
> enough MOSFETs for the current you want to carry)[See Note 1] is
> that they MIGHT have lower resistance (source to drain) than an
> equivalent BiPolar transistor. That results in LESS power being
> dissipated in the drive transistor..in its "fully on" state.
>
> * * *
>
> But really, the place to woory about power dissipation is NOT when
> either sort of transistor is fully switched on (or off for that
> matter) but when it is IN TRANSISTION. You will usually have two
> transistors in a "totem pole" arrangement. And the idea is to
> insure that you never have BOTH of them transistioning at the same
> time...because that will cause a current surge (it is as if your
> supplies plus was shorted to minus through the body of your
> transistor.) So, what you do to avoid that is introduce "dead time"
> where you switch the one that is on off before you switch the one
> that is off on.
>
> I have, of coruse, the picture of an H-bridge in my head as I am
> writing this...so it is really a pair of totem poles. But each one
> has the problem in itself.
>
>
> Jim
>
> [Note 1]: On some of my "projects"...I switch 312 volts at 200
> Amps. No single MOSFET can handle that. (I drive an electric car.)
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
> <mariss92705@> wrote:
> >
> > I'm assuming the output transistors operate as current sinks.

This

> > means they are operating in the linear region (simultaneous
> voltage
> > across and current thru the transistor).
> >
> > For linear operation it makes no difference if bipolar NPN or n-
> > channel MOSFET transistors are used. The power dissipation in the
> > device will be exactly the same. 1A of current with 30V across

the

> > device is 30 Watts either way.
> >
> > Mariss
> >
> >
> >
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "turbulatordude"
> > <dave_mucha@> wrote:
> > >
> > >
> > > > And the PIC makes an interesting and flexible way to drive
> it.
> > > >
> > > > But OK, not linear, but an approximation of a sine wave,
> similar
> > to
> > > micro
> > > > stepping.
> > > >
> > > > Alan KM6VV
> > > >
> > >
> > > > > IME Linisteppers work fine in their "designed-for"
> > applications...
> > > > > IMO they're great for learning about--and messing with--
> > > > > PIC "stepper" code, and while the heatsink and power supply
> > > > > requirements are bigger than for chopper drives; they are
> still
> > > > > inexpensive and user repair-able.
> > > > >
> > > > > ALSO, the Lini- is NOT linear stepping; but linear
> > amperage/winding
> > > >
> > >
> > > This is where the polishing of the program and circuit needs to
> be
> > > done.
> > >
> > > Switch to FET's to lower heat. a no brainer.
> > >
> > > Linear amperage does not equal linear steps, but then there has
> > always
> > > been the discussion of exactly where the step/armature is on a
> > > microstep. is it exactly at 1/7th of a full step, or is the
> power
> > > ramping between steps smoother that offers better power
> delivery ?
> > > and it is 'somewhere' between 1/2 steps, but not exactly

linear ?

> > >
> > > Also, there is that how close is good enough bit. does a screw
> > with
> > > 26,000 steps per inch suffer if the step if off by 1/10th of a
> > step ?
> > >
> > > Dave
> > >
> >
>