ETX TECHNICAL TIPS
From: Chuck Dvorak (firstname.lastname@example.org)Great site.....I really appreciate the effort you put into this site-I recently directed a fellow club member to your site and he appropriately characterized it as "a goldmine of ETX information". We both felt that the site fills the void created by the Meade documentation and web site....keep up the good work.
I have enjoyed the many projects listed on this site and wanted to share a project I recently finished. As stated on numerous ocassions in the past, I was looking for a way to save some hard earned cash by replacing the batteries as a source of DC power for the ETX. I saw many advertisements for rechargeable battery packs and have seen similar adaptations using smaller dry cells and motorcyle batteries which led me to my project I decided to use a 12 volt battery from my DeWalt rechargeable hand drill. This proved to be an excellent solution since I already had the battery and charger. The system has a quick charge capability that allows me to fully charge the battery in less than an hour. I have provided a parts list, photo, diagram and instructions for this project. This power cord should cost less than $10 to fabricate in approximately 1 hour.
1- 12 volt DC battery from cordless power drill or tool
1 Full-Wave Bridge Rectifier Radio Shack Cat# 276-1146
1 Coaxial DC Power Plug 5.5mm OD/2.5mm ID Radio Shack Cat# 274-1573A
1 Plastic Mini Project Enclosure Radio Shack Cat# 270-288
8 feet of 20 or 24 gauge speaker type wire (preferably marked for polarity)
2 Aligator clips with insulated covers
A small amount od silicon caulk
I was concerned about the potential for damage to the ETX if the battery was connected backwards due to human error. I presented the problem to a co-worker with much more experience in electronics (special thanks to Nay Aung). His solution was simple and provided a fool-proof solution for dark site viewing sessions. I used a Full Wave Bridge rectifier to regulate the polarity and insure that it remained correct regardless of the connection at the battery. I connected the aligator clips to one end of the speaker wire using a small amount of solder. The other ends were soldered to the center two leads (not labelled) of the bridge rectifier. (Use heat shrink or insulation to prevent contact between the leads) It does not make a difference which lead will be designated as + or - for this connection to the battery side. I then soldered an additional length of wire to the outer leads of the rectifier. These leads are marked + (positive) and - (negative) and it is extremely important that you connect the + (positive ) lead with wire to the center connection of the DC power plug. The - (negative ) lead is then connected with wire to the outer lead of the DC Power plug. I used approximately 4 feet of wire on each side of the rectifier so that the battery can rest on the ground, while the rectifier is placed on the tripod lens tray. I then used the plastic enclosure and a sufficient amount of silicon caulk to enclose the rectifier and prevent any moisture damage. Be sure to double check all connections and test them with a volt meter to insure that you have the correct polarity at the power plug. The battery has a plastic enclosure which separates the two metal leads of the battery. Push the aligators down as far as possible for a firm connection. Use your volt meter to measure the output at the DC power plug....if you connected the power plug correctly, you will receive a 12-13 volt reading with the center post positive regardles of the connection at the battery. Try switching the battery connections-the polarity remains constant and correct. I have used this setup on 5-6 viewing sessions with no problems and a considerable amount of savings. On my last session, I spent approximately 5-6 hours of "slewin-n-viewin" with no problems and a battery reading of 92% at the end of the night.
Remember, I am not an electrical engineer, as with all modifications to the ETX, you will void your warranty and I cannot be responsible for any damage. I encourage any suggestions or improvements to this project.
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