RE: [CAD_CAM_EDM_DRO] Re: stepper driver help

We're all saying the same thing, just not very well :-)
The driver stage of a motor controller heats as a function of the average
current through it and the average voltage drop across the DRIVER
TRANSISTORS. This voltage drop is higher for bipolar transistors (L298,
L6203) than it is for field effect transistors (Gecko, allegro) but still
not too bad in the 2-4A range. The low on resistance quoted in the FET
datasheet is misleading as it takes longer/more for a FET to switch on than
a bipolar transistor, while switching it's dissapating quite a lot. Some FET
driver circuits deliver over 1A to the gate..

If you reduce the current at idle then you can get away with a very small
heatsink and a 298 or 6203. This is because a stepper draws a small fraction
of it's rated current when spinning at any speed and chops (switches the
coils on and off very fast) at low speeds and idle. If you set a stepper
motor and drive up on the bench and run the motor flat out, the drive will
stay cool. If you leave it stationary but energised, both the drive and the
motor will get hot.
I find it confusing to think of a stepper as a motor. I prefer to think of a
stepper motor as a rotary solenoid.

One thing worth mentioning is that bipolar transistors are a LOT harder to
blow up than FETS.. I've done stuff to 298 drivers that would have destroyed
a fet driver instantly (like unplug a running motor).

Gecko's and Xylotex drivers are fantastic products and have changed the
hobby CNC world, if you are going to build your first controller from
components, go with a 298/6203 or simillar. You'll have a much higher chance
of success.

PK



------Original Mail------
From: "John Dammeyer" <johnd@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Thu, 27 Sep 2007 20:24:10 -0700
Subject: RE: [CAD_CAM_EDM_DRO] Re: stepper driver help

Hi David,

Yes. My nose has already been rubbed in this tiny tiny tiny little
technical error <GRIN>. I do know better. Really I do.....

John

> >with 50W dissipation. Smell that magic smoke!
>
> John, technical correction:
>
> 50 Volts across 1 Ohm is 50 Amperes or 50V X 50A = 2,500 Watts. Much
> better magic smoke!
>
>
> David G. LeVine
> Nashua, NH 03060
>
>
>
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If you wish to post on unlimited OT subjects goto:
aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if
you have trouble.
http://www.metalworking.com/news_servers.html

http://groups.yahoo.com/group/jobshophomeshop I consider this to be a
sister site to the CCED group, as many of the same members are there, for OT
subjects, that are not allowed on the CCED list.

NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM.
DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
bill
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