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By Tom Campbell
Custom Made Replacement For
The Meade 1812 12v to 18v Converter
Overview Meade no
longer produces the #1812 12v to 18v DC converter for use with the
LX200 telescopes. Its design was insufficient and its main problem
was failure of its inductor coil due to heat dissipation that
exceeded its design rating. The result was voltage at the LX200
input power panel being anywhere between 12v and 18v DC and very
sluggish operation of the Meade LX200 series telescopes. Other
issues with the design were poor heat dissipation due to its plastic
case, wire size too small in the DC power cable and RFI noise
emissions.
This circuit has a conservative design power margin and is capable
of maintaining +18VDC with a continuous load current of 3 amps
indefinitely. However its actual steady output current is limited by
a 1.5 amp slow blow fuse (F1) to protect the LX200. When fast
slewing while tracking, the pulsed stepping motors draw peak
currents that are greater, but the fuse only senses the average DC
current level. I recommend a modification to the LX200 power panel
(REF.,
http://www.covingtoninnovations.com/astro/powerinlet.html ),
but use my inputs that simplifies this modification process: The jumper wire soldered
across the 0.1 ohm power panel resistor (R1) is OK as shown. This
will disable the power panel bar graph current meter, but that is a
small price that will nearly eliminate its ground loop noise
problem. There is no need to reroute the + lead of the 2,200 micro
farad electrolytic filter capacitor, as the diagram shows. That is
because the external 12 to 18V converter’s fuse (F1) provides
reverse polarity short circuit protection that will prevent this
large LX200 electrolytic 2,200UF capacitor from blowing or literally
exploding. Cut a rectangular hole in the vector board that will enable IC1 to be bolted to the bottom of the case in a later step. Drill 1/8th inch holes near the 4 corners of the board that will be later used for the insulated solder posts (E1 – E4). Use the board as a template to mark 1/8th inch hole locations in the bottom of the case & drill those 4 holes in the case bottom. Put the metal case aside for now and proceed to assemble the component board. Use ¼ inch long 4-40 screws to temporarily hold terminals E1 – E4 in place on the board during component assembly. Dip the torodial core inductor (L1) in clear polyurethane varnish, dry at 160F (home oven) for 20 min & repeat two more times. This will assure sealing of any possible winding cracks in the coil wire varnish insulation that maybe there when factory wound. If you choose to use the swage mounted component solder terminals, enlarge the 0.041” vector board hole to 0.062” at each terminal location, push the bottom of each terminal through their respective enlarged holes and strike the bottom with a center punch swaging the bottom that will permanently hold each terminal in place. Use a small anvil or vice on the top side to keep the terminal seated while swaging. An alternate way is to use a copper clad proto board with jumpers between the copper pads, but do not use the copper foil for the + or – voltage rails. Use 14 gauge tinned copper buss wire for the ground rail between its respective corner insulated solder posts. Mount and solder all components and make all the interconnections with jumper wire except for IC1 which will be mounted later to the case. Put the component board aside for now and proceed to drill holes in both ends of the metal enclosure for the panel-mounted parts. Drill a 3/8" hole near the bottom center of each end of the case close to the bottom for the power cables that will be installed later. Drill a hole in one end that will enable you to mount the fuse holder (XF1) directly above the power cord hole. Allow enough clearance so the fuse holder’s mounting nut does not interfere with closing of the top cover. On the other end of the case, drill the appropriate sized holes to mount the power switch (S1) and the LED power indicator (DS1). Unmount the panel hardware for now and remove the 4 temporary holding screws from the bottom of the E1 – E4 insulated corner terminals. Push the four 3/8 inch long 4-40 component board mounting screws through the 4 holes in the bottom of the case, and use 3 or more flat washers on each screw as board spacers on the inside of the case and tighten each screw in the bottom of each insulated post until the component assembly board is securely mounted to the inside bottom of the case. Use care not to over tighten the screws that would cause the posts to rotate. Make sure the board is totally electrically isolated from the metal case. Trim protruding leads and/or add more washers as necessary. Lay IC1 inside the rectangular opening in the board and use the hole in its tab to locate its mounting hole in the bottom of the case. Drill the appropriate sized hole for the chip’s T220 tab hole, de burr the hole and use thermal compound on both sides of the optional heat sink and bottom of the regulator chip tab. Bolt the regulator chip directly to the bottom of the case, no mica insulating washer needed because the tab is at ground potential. Bend the 5 leads accordingly so they are above the top of the vector board. Solder the 5 regulator leads using short pieces of hook up wire to as in accordance to the schematic drawing. Important note: The metal case is only connected to the ground rail potential at one point and that is via the tab of IC1 which is electrically the same as pin 3 and only pin 3 of IC1 is connected to the ground rail. This eliminates any ground loop currents from flowing around the case that would otherwise be a possible source of RFI noise maintaining the integrity of the die cast aluminum case’s RFI shielding properties.
The next step is fabricating the 12VDC input and
18VDC power output cables: For the 18VDC power output cable, if less than 4 foot in length, use an off the shelf Radio Shack 5 amp rated “adapt-a-plug” 18 gauge DC power cable (#2701588). The store will give you a free “type N” coaxial adapt-a-plug (5.5 X 2.5 mm) with the purchase of this cable that will plug directly into the LX200’s power input panel. Cut off the length that has the cigarette lighter plug. If the output cable length is longer than 4 feet, cut the RS #18 gauge cord a little shorter and splice 14/2 gauge wire to it to get your desired length. Dress up the splice with heat shrink tubing. Push the pig tail of the power input cord through its hole in the case on the side where the power switch will be mounted. Push the pig tail of the power output cable through its hole under the fuse holder hole. Leave about 4 inches of the power cable wires inside the box so you will have enough length to work with. Use a 3/8th inch grommet & ½ inch heat shrink tubing over each cable where the cable passes thru their respective holes in order to protect the wire insulation from chafing. Important to make sure those holes are deburred prior to inserting the power cables. Split (separate) the +/- leads of each cable to the point where they enter their hole and securely fasten each lead with an 1/8th inch wire clamp as shown in figure 1. You will have to determine where to locate and drill the wire clamp mounting holes because I failed to measure all hole locations before I sent the prototype off for evaluation. Use the ½ inch long 6-32 stainless screws and nuts to clamp the wires to the bulkhead that immobilizes the cables where they pass through the holes. An alternative is to use a bulkhead mounted plastic wire strain relief, but I could not find one large enough to fit the power cables. Mount the switch, LED indicator and fuse holder to the die cast enclosure. You can use dry transfer letters to label the bulkhead parts if you wish prior to mounting the bulkhead hardware. Cut to length, strip, tin, and solder the (+) power leads to their appropriate places of the circuit. Be sure to leave enough wire for service loops. Cut to length, strip, and tin the (-) power leads. Solder these leads together at the same point on the ground rail as shown in figure 7. This enables the ground current to flow nearly unimpeded from the LX200 to the ground return battery post resulting in better performance and less noise from the telescope. Solder the other end of the fuse holder and the other terminal of the switch to the component board as indicated by the schematic and wire the indicator lamp with its series ballast resistor (Rb) to the +18VDC and its other lead to ground. Fasten the cover with its 4 screws, insert F1 into its panel mounted fuse holder, insert F2 into its +12VDC in line fuse holder and you are ready to go.
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Corrected Version??
This site was last updated 08/06/17
© Copyright, Derek C Breit. All rights reserved.
