ETX TECHNICAL TIPS Last updated: 26 June 2002

Another ETX-90EC electrical current info

Subject:	ETX-90EC Operating Currents
Sent:	Tuesday, June 25, 2002 10:45:57
From:	arkotz@attbi.com
In this post, I'm just passing along the operating currents I measured
for the ETX-90EC, for the benefit of those who can and do make such
diagnostic measurements while troubleshooting their units. I got
significantly different results than those reported in an older post
(Dale Kretzer, I think) that quoted very small currents and said one
could get by with pretty "wimpy" supplies (my paraphrasing). Perhaps his
scope was VERY different from mine. Sherrod and Seymour mention currents
in line with what I measure, but I'll report them here since I recently
measured them, for the benefit of anyone interested in such diagnostics.
My scope is a Feb 1999 (early) vintage, but I'd expect any
similar-torque motor system to give about the same results, with some
small percent variation depending on the axes-loading (how tight the
trunions are, etc.).

The following are the currents I measured with an Autostar 497 running
Ver. 2.2E:

Upon startup, 145mA to 170mA, depending upon the state of the Autostar
during initialization.

Before Aligning, its quiescent current settled down to about 60mA after
the initializing.

A Right Ascention slew at top speed: 190mA to 220mA.

A Declination slew at top speed: 190mA to 200mA (this can vary depending
upon the "loading," of course -- about 175mA without the OTA load).

Both axes slewing at top speed during a "goto" slew: 300mA to 340mA.

While tracking at tracking speed: 70mA to 100mA.

These were all measured with the Meade AC adapter, whose output voltage
measured from 12.9V to 14.2V, depending upon the current load.

I NEVER saw currents down around the 20mA mentioned in the
earlier-referenced post (??) -- The Autostar alone results in loads
around 60mA, or so.

I had earlier used a 250mA laboratory regulated power supply while
trouble shooting. When both axes slew at top speed during a simulated
"goto" slew, the current was ABOVE the 250mA capability of the lab
supply, and the resultant drop in supply voltage (to 7 or 8V or so)
caused the dreaded "motor failure" message to be displayed. Apparently
Meade uses the drop in voltage as an indication of a "motor failure,"
which a shorted or overloaded motor would cause. I noticed, even when I
replaced the 250mA lab supply with the "beefier" Meade AC adapter (good
for 1.5A, supposedly), it kept reporting "motor failure" upon
"calibrating" with the Autostar, until I "reset" the Autostar, after
which I could calibrate the motors without the motor failure message.
Apparently Autostar always "remembers" those voltage drops until it gets
reset (?)

Anyway, the upshot of all this is that one requires a pretty ROBUST
power source, capable of at least 300 to 500mA or more -- as Sherrod,
Seymour et al have said, which is quite different from the
earlier-mentioned post that said a pretty low current supply would
suffice (my paraphrasing). A "wimpy" supply will result in the dreaded
"motor failure" message. I make this post simply to help some future
trouble shooter from making the same erroneous assumptions I made based
on an old post.

And those "little AA batteries" are always going to be a problem
delivering the 2-axes high speed slews during "goto" operation unless
really "fresh." Be prepared for lots of "motor failure" messages,
forcing starting over the align procedure, etc.

Thanks for the great site, Mike.

Art Kotz

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