ETX TECHNICAL TIPS Last updated: 31 August 2001

ETX-60AT, ETX-70AT Power Supply

After many hours of working with the ETX 60 and enjoying the viewing experience, it became obvious that the 6 AA batteries were not lasting as long as I had hoped. Rechargeable AA batteries, were a better option, but offer even fewer hours of viewing than alkaline batteries.

I wanted an external power supply solution that would not modify the ETX 60 and void the remaining months of warranty on the scope. Another objective was to maintain the portability of the scope and not be confined to an AC adapter. Looking through the current Radio Shack catalog I found a portable DC Power Station (#61-2801) capable of supplying 1.5V, 3.0V, 4.5V, 6.0V, 9.0V and 12VDC at 7 AH. Since it was capable of many output voltages, the Power Station could be used for many applications, including a larger scope.

There were several concerns with an external supply, the primary being not to stress the internal wires of the scope, which are short and if pulled could break the connections in the scope. Second, avoiding electrical short circuits resulting in damage to either the scope or the external power source.

The solution was to use a 4 AA battery holder (#270-383) with a 9V snap connector from Radio Shack. The 4 AA battery holder is smaller than the Meade supplied battery holder, but it connects to the terminals and fits into the battery compartment without stressing the internal scope connector. Soldering to the positive and negative terminals of the battery holder and routing the cable through two holes near the holder terminals. This allows easy routing of the cable out of the battery compartment.

To provide additional strain relief on the battery cable, I routed the cable through the center of the coiled cable of the Autostar. In addition to providing strain relief, routing the cable through the Autostar cable helps to prevent wrapping problems while the scope is slewing.

After several extended sessions using this modification, the scope performs very well with no concerns for running out of power during a session.

If you attempt this modification, please confirm polarity at the battery terminals. Incorrect wiring could result in damage to the scope.

Joe Walker
jwlk3@ix.netcom.com

Subject:	Meade ETX-60 and ETX-70 AC Adapter
Sent:	Saturday, June 9, 2001 15:45:10
From:	dhill@prairieinet.net (Dave Hill)
Many people ask about converting their Meade ETX-60 or ETX-70 to run off
of an AC adapter. Most of these conversations just discuss voltages. The
number of AMPS that a power supply provides is just as important,
possibly more so, than the voltage. Put too many amps through your scope
and kiss it goodbye.

To illustrate this point, consider the difference between a typical
9-volt battery and a 12-volt car battery. I would be perfectly happy
running my scope off of 12 volts, it's close enough to not worry about,
but I would be insane if I thought I could hook up a car battery to it.
The massive amperage in a car battery would leave nothing but a smoking
pile of plastic where my telescope had been.

Here are my observations and measurements of the amperages used by my
ETX-70 when run off of the battery pack supplied with it, as well as
measurements and observations from AC adapters.

Just in case you don't know, 1 amp = 1000 milliamps (1000 mA).

Moving telescope on either axis with battery takes 250 mA (0.25 amps)

Moving on both axes at same time with battery takes up to 450 mA (0.45
amps).

Battery pack with new batteries has output of 1.4 amps when the battery
back is short-circuited through the amp meter. The amperage drops very
quickly when you short circuit a battery like this, this value is only
meant to determine the maximum amperage that could possibly be supplied
so we know what the scope can take. This means that any power supply
that supplies less than this amount of amperage at the same voltage
should not damage the scope. 1.4 amps is quite a bit of amperage, most
common household adapters have a stated amperage that maxes out at
around 500 mA, but don't trust that value, the actual voltage and
amperage that these AC adapters supply is much higher!

Radio Shack 9V battery eliminator AC adaptor puts out 10.5 volts with no
load and 650 mA when amp meter is placed across terminals (short
circuiting the battery through the amp meter). You would think then that
it should easily supply the 450 mA to run the scope but motors either do
not run at all or if they do run it is instantly a motor fault. Need
more amperage to get the scope going.

A brand new nine volt battery has output of 3.5 amps for a fraction of a
second when amp meter is short circuited across terminals and then the
amperage drops quickly (batteries don't like to be short circuited,
don't do it for more than a few seconds). Despite the fact that the 9V
battery seemed capable of supplying more amperage than the battery pack,
I thought it was safe to try with the scope.  I found that the scope
will operate with a standard 9V battery for a while. I didn't put it to
the test to determine how long it would last before draining this little
battery, but one person on the web suggested that a 9V battery will work
for 10 minutes. The maximum measured amperage that the scope used was
470 mA when slewing on both axes at the same time.

WARNING!!! Most AC adapters supply much higher voltages and amps than
they are actually rated for. For instance, a typical 9V 500mA AC adapter
actually puts out 12-14v and delivers 2.5 Amps when the amp meter is
placed across the terminals. From 12 volt 500 mA adapters I got 17 to 19
volts with no load and got 3.5 amps with the amp meter short-circuited
across the terminals. NEVER SUBSTITUE A POWER SUPPLY FOR YOUR SCOPES
BATTERY PACK WITHOUT MEASURING THE ACTUAL VOLTAGE AND AMPERAGE.

If you find an AC adapter that supplies an appropriate amount of voltage
and amperage, don't worry about the type of connector it has on the end.
Just cut that off and replace it with a 9V battery connector available
at Radio Shack. Use electrical tape on each exposed wire to keep them
apart from each other and then wrap the whole splice in electrical tape
for neatness and strength. YOU MUST HAVE A VOLT METER TO VERIFY THE
POLARITY OF THE VOLTAGE IF YOU DO THIS. A standard 9V battery, and the
battery pack supplied by Meade, have the large connector supplying
negative and the small connector supplying positive. You must observe
the same polarity or you will probably smoke your scope.

I finally decided on using an AC adapter that was rated at 9V 500mA.  It
supplied 15V at no load and 1.35 amps shorted. I cut the end off of it
and spliced a 9V connector on. The scope worked perfectly. Just having
the electronics running brought the voltage down to 12 volts. Slewing
one motor brought the voltage down to 9v, slewing both motors took the
voltage down to 7.5 volts. Slewing one motor caused 260 mA of current to
flow and slewing both motors caused 460 mA of current. The amperages are
almost identical to battery use and the voltages are just a touch higher
than the battery pack.

If you want to see how a particular power supply will act without
endangering your scope, we can figure out the equivalent resistance
offered by the scope and you can test the AC adapter with that amount of
resistance applied to it. This will call for you to buy a resistor at
Radio Shack. We know that at maximum drain, the scope draws about 450 mA
and has voltage across the terminals of 7.5 volts. We divide volts by
amps and get 7.5 volts/0.45 amps = 16.7 ohms. If you get a resistor that
is close to this value you can test the AC adapter with this resistor
across the terminals. Measure the voltage across the resistor and
measure the current going into the resistor. These values should be
close to 7.5 volts and 450 mA.

Note that the effective resistance of the scope with one motor running,
9V across the terminals, and 250 mA running through it is 9/0.25 = 36
ohms. That means you could also perform this test with something close
to a 36-ohm resistor and see if you get 9V and 250 mA.

Now take a look at your battery cover, there is even a notch on one side
for the wires to come out so you can replace the battery cover. This is
great in that it also prevents accidental pulling of the wires if you
trip over the cord.

So then we get down to the question of why Meade just doesn't offer a
suitable AC adaptor? Who knows. They designed the battery cover to
accommodate an AC adaptor. Maybe I should start a business making AC
adapters designed to run the Meade ETX-60 and ETC-70 series telescopes.

And:

Subject:	ETX Power Supply - Incorrect Information 
Sent:	Tuesday, August 14, 2001 15:55:39
From:	englezul@ntlworld.com (Englezul)
I have picked up lots of useful tips for my ETX90EC from your
site....thanks. However, whilst looking for info on polarity of the
external power supply socket on the body of the ETX90, I came across the
posting above. I'm afraid that Dave Hill has made some basic incorrect
statements that will be misleading to some of your readers.

Firstly, sticking an Ammeter directly across a power source is not a
good idea. This will in effect short-circuit the power supply and could
damage it permanently. Try it on a car battery and there will be a big
bang.

The other big mistake he makes is to say that the current rating
(Amperage to use his term) of a power source is irrelevant. As long as
it will supply more than enough current, it is fine. A 12v car battery
will work fine connected directly to an ETX. Any load will only draw the
current required and no more. So if you found a battery which could
supply 10,000 amps (slight exageration) as long as it was 12v it would
be fine, as the ETX scope would just draw 0.5 amps (or thereabouts). As
a warning though, short circuiting a car battery is very dangerous and
would certainly result in a loud bang and lots of sparks and smoke, so
for that reason alone, you would need to be careful.

He then spoke about AC power supplies which supplied a higher voltage
than stated under no load. These are unregulated power supplies and the
voltage will change as the current drawn is changing. Not a good idea,
so I would advise the use of a 12v REGULATED power supply rated at at
least 1 amp. Higher than this is fine but more expensive.

Hope that clears up any confusion.

Dave Johnson
UK

Subject:	ETX70-AT Battery Power Pack
Sent:	Thursday, June 14, 2001 16:20:55
From:	neil@westerncanada.com (neil)
The "AA" batteries in the ETX-70AT drain fast. Even NiMH aren't that
good. I made up a battery pack  using 6 "D" size-1.5V rechargable
alkalines. The only drawback is that the battery pack is large and
cumbersome and it has a long cord...but who cares, I'll be looking at
the sky!

Clear skies!

Neil

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