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Subject: LX200 power info
From: Bill Arnett
Reply To: mapug@shore.net
Date: Wed May 22 02:06:57 1996

Having spent the last three (perfectly clear) nights messing with a toolbox
to hold all my power accessories tonight was cloudy (as per Murphy's Law
:-) so I decided to do a few tests of power consumption in the various
modes between which I can so easily now switch.

Anyone rich enough to own an LX200 couldn't care less about the power
consumption when running on AC. But it did measure the power supply output
voltage under various conditions:

unloaded 21.7 volts
tracking 20.0
fast slew RA 19.8
fast slew Dec 19.9
fast slew both 19.9
CNTR slew both 19.9

More interesting is the current draw on the battery. My battery is a 20
amp-hour gell-cell; fully charged it puts out 13 volts (unloaded). The
following current flows were measured at the battery terminal for running
directly off the battery, using the Meade 18v DC converter and using the AC
powersupply running off of my inverter (the inverter unloaded runs 100 mA):

12v direct 12vDC-18vDC Inverter/AC ps
unloaded 0.02 amp (*) 0.05 amp (*) 0.36 amp
Land mode 0,40 1.08 1.05
AltAz tracking 0.05 (**) 1.05 1.4
fast slew RA 0.69 1.6 1.8
fast slew Dec 0.69 1.4 1.7
fast slew both 0.81 2.0 2.2
CNTR slew RA 0.75 1.2 1.4
CNTR slew Dec 0.70 1.3 1.5
CNTR slew both 0.75 1.5 1.8

(* mostly due to the pilot lights I installed)
(** suspect; probably should be 0.50)
(None of the above numbers are anywhere near as accurate as they appear.)

The slewing numbers varied slightly depending on the position of the OTA
and the direction of motion. The current spiked up briefly to as much as
twice the above values at the start AND END of the fast slewing tests. In
each case, turning off the keypad lighting reduced the load by 50 mA and
turning on the map light added 50 mA.

Conclusions:
- the 12vDC-18vDC converter is pretty inefficient; the current drain is
approximately double using it vs running on 12v. I noticed previously that
it runs pretty hot (noticably hot to the touch but not painfully so).
- I have enough battery capacity to run all night with no problems in all
modes; two consecutive winter nights without recharging might be pushing it
except on 12v; several nights are possible at 12v.
- I also noticed that it takes several seconds for my pilot light LED to go
off after the battery is disconnected (a capacitor on the LX200's
motherboard?). Ditto for the AC powersupply (filter capacitors?). In any
case one should be careful not to short anything even after its
disconnected.
- slewing is quieter on 12v than 18v; I suppose its slower, too. Perhaps
I'll test that someday.

Questions:
- How much power does a dew heater for a 12" LX200 corrector plate use?
- Does anyone know how to measure the energy content of a battery? (Laptop
computers seem to know how :-) One would think that measuring the voltage
would suffice but some sort of conversion table would be needed; probaby a
different one for each kind of battery.
- Do I need to worry about the heat generated by my power stuff? I'm
generating about 10-15 watts of heat. I keep the whole thing beneath the
tripod (very convenient; no wires to trip over). How does this compare
with the heat output of a human being? Can you see any difference if there
is someone standing just beneath the front of your OTA?

[OFF TOPIC: I also measured the battery drain when powering my Macintosh
Powerbook 540 thru its AC adapter and my inverter:

battery charging 3.03 amps
normal operation 1.75
HD off 1.60
screen off 1.24
sleeping 0.59

(these INCLUDE the 100mA from the inverter itself)]

---
Bill Arnett                 "Science is a way of trying
San Jose, CA  USA            not to fool yourself."      -- Feynman
billa@zNet.com                              <URL:http://www.seds.org/billa/>