Re: Electronics gurus please help

Scott,

I took a quick look at your circuit and here are my comments based on
some quick calculations of how I think your circuit behaves.

When any MOSFET turns "on" current will build up in the coil at the
rate of V/L where V is your power supply voltage and L is the coil
inductance. For a 2.4 mH inductance and a 24VDC supply, V/L would be
10,000 amps per second, so it would take 200 uS for the current to
reach 2A.

Once this current reaches your reference threshold sensed by the
comparator, the MOSFET is turned "off". This is where trouble begins.

A property of inductance is it resists abrupt current changes. The
coil will generate a voltage that will avelanche (zener) the MOSFET
drain to source in order to keep the current thru it flowing. This
will be about 100 to 120VDC if the parts are IRF530s. The V/L would
now be 120V / 2.4 mH or 50,000 amps per second. It would take 40 uS
to have the current drop to zero.

This presents two problems.

The first is the decay current is not recirculated so it is
dissipated as heat. The amount of heat is 2A * 120V * 40uS / (200uS
+40uS) or 40W, so I'll bet your MOSFETs get real hot.

The second problem is your current is a saw-tooth that goes from 0 to
2A. Step motors respond to the average current which would be 1A, so
I'll bet your holding torque is going to be 50% of rated.

I'm not saying this to tear apart your circuit, it is just to point
out where the problem areas are. I can e-mail you an output topology
that recirculates the current and thus would have a much lower
dissipation.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "scottdbtnet" <scott@d...> wrote:
> I apologize for the quality, but I uploaded a JPEG of this
schematic
> to the files section as "Unipolar Stepper Driver.jpg" with
> description of "PIC Driven Unipolar Stepper Driver"
>
> Hope you can read it.
>
> It uses the PWM output of the PIC to set the comparator threshold
for
> the current limit. When the voltage across the sense resistor
> exceeds this threshold the comparator signals an interrupt on the
PIC
> at which point the interrupt code turns off the outputs for about
> 25uS. It then turns the ouputs back on and the cycle repeats. The
> proper output state is stored in a variable to make this easy.
>
> If anyone is interested I can provide more details. I am only
saying
> this circuit is simple and it does work. As I recall I can only
get
> about 300 RPM out of a Vexta PH264 using this circuit, but that was
> adequate for my needs at the time.
>
> Scott
>
> --- In CAD_CAM_EDM_DRO@y..., "Ian W. Wright" <Ian@i...> wrote:
> > Hi Scott,
> >
> > This sounds like an interesting way of doing things - how about
> putting your
> > circuit in the files section?
> >
> > Best wishes,
> > Ian
> > --
> >
> > Ian W. Wright
> > Sheffield, UK
> >
> >
> > The following added for automatic email harvesters!
> > abuse@w... abuse@p... abuse@b... abuse@y...
> > UCE@F... abuse@t... abuse@f...
> >
> > ----- Original Message -----
> > From: "Scott M. Thomas" <scott@d...>
> > To: <CAD_CAM_EDM_DRO@y...>
> > Sent: Wednesday, September 11, 2002 8:54 PM
> > Subject: RE: [CAD_CAM_EDM_DRO] Re: Electronics gurus please help
> >
> >
> > > Hmmm. I suppose you could use 6 if you want to. I built one
> using a PIC
> > > and chopped the input to the 4 "end" transistors, worked very
> well. I
> > also
> > > used a common current sense resistor, so this simplified it
even
> further.
> > >
> > > I'll have to check out the voltage doubling effect, hadn't
> thought of
> > that.
> > > But this shouldn't have a huge effect on things depending upon
> the power
> > > supply voltage.
> > >
> > > Thanks
> > >
> > > Scott
> > >
> > >
> > > -----Original Message-----
> > > From: mariss92705 [mailto:mariss92705@y...]
> > > Sent: Wednesday, September 11, 2002 3:38 PM
> > > To: CAD_CAM_EDM_DRO@y...
> > > Subject: [CAD_CAM_EDM_DRO] Re: Electronics gurus please help
> > >
> > >
> > > Scott,
> > >
> > > If it were an R/L drive you would be correct. The center-tap of
> the
> > > motor coil would go to the + supply thru a large current limit
> > > resistor and the winding ends would have the transistors.
> > >
> > > Since a unipolar chopping drive was mentioned, 6 transistors are
> > > required; the current limit resistor is replaced with a
> transistor to
> > > which pulse width modulation (chopping) is applied. The end
> > > transistors still would behave the same.
> > >
> > > A center-tapped (6-wire motor) winding is a 2:1 step-up
> transformer.
> > > because the center tap goes to the + supply, pulling one end
wire
> to
> > > ground causes 2 times the supply voltage to appear on the other
> end.
> > >
> > > Mariss
> > >
> > > --- In CAD_CAM_EDM_DRO@y..., "Scott M. Thomas" <scott@d...>
wrote:
> > > > I count 4 mosfets not 6 for a unipolar drive, did I miss
> > > something? And,
> > > > because there are half the parts and no high side drivers
> involved,
> > > I would
> > > > say that a unipolar drive is much simpler. I also don't
> understand
> > > why the
> > > > voltages would be different.
> > > >
> > > > Scott
> > > >
> > > > -----Original Message-----
> > > > From: mariss92705 [mailto:mariss92705@y...]
> > > > Sent: Wednesday, September 11, 2002 12:59 PM
> > > > To: CAD_CAM_EDM_DRO@y...
> > > > Subject: [CAD_CAM_EDM_DRO] Re: Electronics gurus please help
> > > >
> > > >
> > > > Hi,
> > > >
> > > > It takes 6 power transistors and 6 high speed rectifiers to
> build a
> > > > unipolar "chopper", it takes 8 power transistors and 8 high
> speed
> > > > rectifiers to make a bipolar (full bridge) one.
> > > >
> > > > The power rectifiers are not across the power transistors in
the
> > > > unipolar drive. If MOSFETs are used in a bipolar drive, the
> > > > rectifiers are included in the transistors.
> > > >
> > > > The power transistors "see" twice the power supply voltage in
a
> > > > unipolar drive. In a bipolar drive, the maximum
voltage "seen"
> by
> > > the
> > > > power transistors is equal to the supply voltage.
> > > >
> > > > The performance of a unipolar drive is about equal to a
bipolar
> > > drive
> > > > running a motor in a half-winding connection.
> > > >
> > > > A bipolar drive is prefered because it is simpler (when
MOSFETs
> are
> > > > used) and the circuit voltages are lower.
> > > >
> > > > Mariss
> > > >
> > > > --- In CAD_CAM_EDM_DRO@y..., Vajk Fekete <vajk.fekete@o...>
> wrote:
> > > > >
> > > > > question1: is a chopper unipolar driver much worse than a
> chopper
> > > > bipolar drive?
> > > > >
> > > >
> > > >
> > > >
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