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'[EE]: 5Amp 2 terminal constant current source'
2000\12\16@165952 by Lawrence Glaister

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I have an l/r stepper driver system that uses a 200w 8ohm resistor in each
coil of the drive cct (8 total).... needless to say, these are quite the
toaster elements. I would like to replace each of them with a 2 terminal
switching cct that would permit 2 selectable current levels (a 5a run
current and 2a hold current for example). I envision using  high current fet
and some type of regulator IC used like a current limited supply. An opto
interface to switch in an alternate current setting resistor shouldn't be
too hard.
Anybody have any suggestions? With all the switching power supply IC's out
there, someone must have used them for this purpose.
=======================================================
Lawrence Glaister VE7IT             email: spam_OUTlgTakeThisOuTspamjfm.bc.ca
1462 Madrona Drive                  http://jfm.bc.ca
Nanoose Bay BC Canada
V9P 9C9
=======================================================

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2000\12\16@172039 by David VanHorn

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>Anybody have any suggestions? With all the switching power supply IC's out
>there, someone must have used them for this purpose.

Do you need separate sources?
Is this a unipolar or a bipolar motor?

Current chopping isn't real hard, you can let the motor itself play a role,
and just use a simple transistor circuit for unipolar drive.

There probably exist 5A chopped driver ICs as well, like a beefy version of
the PBL3717, but I don't know part numbers.

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2000\12\16@232210 by Roman Black

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Lawrence Glaister wrote:
>
> I have an l/r stepper driver system that uses a 200w 8ohm resistor in each
> coil of the drive cct (8 total).... needless to say, these are quite the
> toaster elements. I would like to replace each of them with a 2 terminal
> switching cct that would permit 2 selectable current levels (a 5a run
> current and 2a hold current for example). I envision using  high current fet
> and some type of regulator IC used like a current limited supply. An opto
> interface to switch in an alternate current setting resistor shouldn't be
> too hard.
> Anybody have any suggestions? With all the switching power supply IC's out
> there, someone must have used them for this purpose.


We need more information?? Are there two unpolar driven
4-coil motors? That's my guess. You can just use ONE
constant current supply for each motor, this will work
fine. I would hate to find out you built 8 switchmode
supplies when you only needed 2!

If you want to drive the motors with half stepping to
give 400 halfsteps/rev you need dual current levels,
at 100% and 141%. One coil = 100% current, two coils
=141% current for same torque at both full and half
step positions. You might also want another lower
(usually about 40%) current level for "holding"
and allowing the motor to cool a bit when you don't
need full torque.

Of course things get different if you want to use
quarter or finer microstepping then you need the
extra complication of controlling different current
for each of the two phases in the motor. :o)

I am curious re the per coil current and voltage
of your motor, and what speeds you need.
-Roman

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2000\12\16@235618 by Lawrence Glaister

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Hi Roman,
the motors are 6 wire unipolar 4.7amps/1.7v coils size 34. (Slo Syn
M091-FD-440).
With the recent cold weather, its kind of nice having the heat in the shop,
but it does
waste a LOT of power. I have about 35-40volts feeding the system (depends on
line voltage) with a resistor feeding each center tapped coil and a fet
switch to ground on each of the 4 phase wires. I have 4 motors driving a cnc
router table (2 on x axis).
A change to 1/2 stepping would be quite useful as I am indirectly driving a
rack system and my resolution could do with improvement. I may change to a
lead screw drive which would require more top end speed to keep the system
speed up. I am looking at rebuilding some or all of the electrical system,
so I am open to suggestions. I understand that I could use a system where
1/2 of each coil is driven bipolar with something like one of Mariss's
drives... what I'm not sure of is what current setting I would use in the
bipolar drive case. 4.7Amps? or do I have to drop to 1/2 this because in the
unipolar drive, only 1/2 the coil is energized at a time (50% duty cycle on
coil)?
It seems if one holds on one phase, it should be able to handle full current
from centertap to outside of a coil. I have been trying to find a datasheet
on these motors but they dont seem to be in the current catalogs.... does
anyone have an old slo syn catalog with the above model specifications?

=======================================================
Lawrence Glaister VE7IT             email: EraseMElgspam_OUTspamTakeThisOuTjfm.bc.ca
1462 Madrona Drive                  http://jfm.bc.ca
Nanoose Bay BC Canada
V9P 9C9
=======================================================
{Original Message removed}

2000\12\17@004035 by Bill Westfield
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I thought that this general configuration (big power resistor, high voltage
supply for stepper motors) was used to get FASTER behavior out of a given
motor, where the motor is generally rated at a significantly lower voltage
(and similar current.)  By having the HV supply available, current can ramp
up faster (into the inductor) and increase step rate/etc.  If that's the
case, and you don't need that sort of speed, you might be able to simply
decrease the stepper's power supply voltage...

BillW

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