Last updated: 29 July 1999

ETX Electronics

From: Dale N. Kretzer (dkretzer@inreach.com)

For those ETX-90/EC users who enjoy tinkering with electronics, some of the following information may be of use in making their own AC supplies, battery cables and other 12-volt power sources for our great little scopes. Some of this is not new, but I'm repeating it to keep the information together.

The power connector on the ETX requires a coaxial DC power plug of 5.5mm O.D. and 2.5mm I.D. dimensions. Radio Shack sells these in a solder-type connector, Part #274-1573A. The center conductor takes the positive lead, the most common arrangement.

With a digital ammeter, we took test readings to get an idea of the current load created by the drives and AutoStar to determine the ratings for power source wiring, etc. With both motors driving and the AutoStar operational, we got starting peaks under 200 ma. (milliamperes) and an average current draw of 30 ma. The AutoStar idling by itself draws nearly 20 ma., much of that power drain probably attributable to the display panel.

What this means in practical terms is that a 12-volt D.C. power supply rated at 500 ma. (half an ampere) will always run cool with this light load, and even smaller supplies down to 300 ma. should do just fine. Wiring for such a supply, and also for running between a vehicle lighter socket and 25 feet or so to the ETX, can be 18-gauge stranded or even smaller.

I made up both types of power sources from materials found in my ham radio "junkbox" and they operate just fine. On the other hand, I don't consider the Meade equivalents of these items to be particularly overpriced and recommend them for those who don't wish to make their own.

In either case, I strongly urge everyone to utilize these alternate power sources and save your batteries for those times when anything else is inconvenient. Having the on-board power is great, but can become expensive over time and worrisome if you don't have spare batteries on hand.

By the way, the amount of power drain by the ETX is so minimal for a typical automotive battery in good shape that you should be able to star gaze much of the night without fear of drawing down the battery too far to restart the vehicle.

I've enjoyed reading the exploits of other tinkerers on this excellent Web site and have followed numerous of them. I particularly like the adaptation of the Daisy Red-Dot Sight, and find it more useful and precise than even the 90-degree Meade viewfinder I installed before trying the red-dot sight.

Thanks for your even hand in maintaining this excellent Web site, Mike.

-Dale Kretzer (K6PJV) in Lodi, CA.

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