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13493Re: [ETXASTRO] ETX-90RA dc power supply mod??

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  • EricTv
    Apr 13, 2014
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      Thanks for this info.  What I ended up doing was using a spare wall wart multi voltage unit with selectable voltages from 1.5-12 VDC.  To get finer regulation, I built a discrete LM317T based regulator and fed say 9-12 V in from the wall wart, then I can adjust voltage of the '317 potentiometer for finest voltage.  The '317 circuit was being built for another purpose anyway (to help regulate my barn door tracker's 3 VDC motor), but it easily serves the ETX need should I decide to try piggyback with the ETX.  

      For the circuit you describe, is this already a part of the ETX circuitry, or something you added?  I know there's a small board already present within the ETX as I had to replace one of the transistors on the board a couple of years ago. 

      4.5 volt wall warts were common accessories with portable CD players. I have a large heavy box of assorted wall warts from dead appliances. I can get you a 4.5 for the postage. Probaly the best way to do this is to run some wires to the battery holder from a coaxial jack mounted in the wall of tha base. Jacks and plugs are available from Radio Shack or vendors on eBay.


      The motor control of the RA is a very simple op amp based pulse width modulated generator.


      It consists of one capacitor, one resistor, the op amp (LM385) and a 2222 switching transistor. There is no provision for adjusting the pulse width, which will vary with input voltage and the accuracy of the cap and resistor. Most op amp circuits of this type will have a trimmer resistor to adjust the timing. The Celeston Logic Drive and similar drives sold by Meade are fashioned this way. I've copied the Logic drive's circuit and have been able to build it on a square inch of perfboard. If you want a diagram let me know.

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