Re: [CAD_CAM_EDM_DRO] current limiting resistors??????????

Thanks for the very interesting info, now I know why "Chopper" drives are so
popular!

My question was more on .... can one limit current, without reducing
voltage? Let me explain, I have a +5 voltage supply for my opto's on my
stepper drives, I know they only require a current of around 150 ma, but the
supply is rated at 5 amps! I don't want to damage them, by supplying too
much amps, or should I even be concern?

Thank,
RC

----- Original Message -----
From: <JanRwl@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Saturday, April 20, 2002 4:25 PM
Subject: Re: [CAD_CAM_EDM_DRO] current limiting resistors??????????


> RC: I assume you are talking about the "current-limit resistors" used in
> series with the "positive common center-tap" of each CT'ed winding of
> "six-wire" unipolar stepper motors? The reactance of the motor-winding
> increases as the pulse-rate ("steps/second") increases to the motor. So,
if
> you have a power-supply voltage say ten times the "normal, steady-state,
DC"
> coil-voltage, you have to somehow REDUCE it so you don't smoke the motor.
> So, you insert this big, energy-wasteful series I-limt R. Calculate R so
> that the "DC voltage" is the max. for which the motor is specified. NOW,
as
> you "ramp up" the stepping-rate, the reactance increases, but the
> VOLTAGE-available is ten times as high as the rated winding-voltage, so
the
> voltage-drop begins to "share" between the increasing coil-reactance and
the
> I-limit R, keeping the current more-nearly "what it should be" than it
could
> possibly be, were the DC voltage LOW, with NO I-limit R. An old-fashioned
> compromise which "kinda works", particularly for summa us po' ol' boys!
>
> HOWEVER! The new "[bipolar!] chopper-drive", also using a higher-than
> steady-DC voltage, "chops" the DC into pulses that total a "percent" of
> constant-on-time, so that the AVERAGE DC-current is the max. rating of the
> coil. BUT, the voltage is WAY above the coil's "max. DC rating". This
way,
> when reactance increases when pulse-rate goes up, the "duty-cycle" % of
> time-ON increases to maintain that average DC. NO wasted power in I-limit
> R's in series. MUCH more efficient!
>
> Hey! This is a very difficult concept for an ignernt old troll to convey,
> and Mariss is the genius EE "in here", and I am SURE he can explain it
much
> better! Wot, Mariss?????
>
> HOPE I have been, however, of SOME use! Jan Rowland, Ignernt old
> troll
>
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