12v - 18v Converter

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 Portable 12vdc to 18vdc Converter

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.
 
The custom designed replacement circuit is totally enclosed in a die cast aluminum case that provides both superior heat dissipation and RFI noise shielding with the aluminum case as well as additional decoupling capacitors. The LT1170 regulator is bolted directly to the metal case that also serves as an excellent heat sink (the tab of this chip is at ground potential). The biggest design improvement is its efficient toroidal core inductor of the same 330 micro henry value, but larger coil wire gauge designed to handle sustained DC current levels when slewing the LX200. Both the new inductor and the LT1170 regulator chip no longer overheat, only lukewarm to the touch and this power supply is very reliable.
 
Theory of operation
 
This switching regulator (IC1) is wired in its DC voltage boost configuration. It switches (shorts) L1 to ground repeatedly at the rate of 100,000 times per second. The switching rate is set by the time constant of the series wired Rc and Cc. When IC1 internally shorts pins 4 and 3, L1 at the junction of D1 gets shorted to ground, begins to draw current and magnetic lines of force begin to expand around L1’s coil. Magnetic energy gets stored in L1 while the reverse bias on D1 prevents the C2 output capacitor from discharging for the duration of switching.
 
When IC1 opens (disconnects pin 4 from 3), charging current from ground to L1 suddenly ceases, its magnetic field collapses inducing a voltage in series with the +12VDC supply that adds to the power input voltage. D1 becomes forward biased and charging current flows into C2 building up its voltage to the +18VDC level. C2 also filters out the DC ripple on top of the +18VDC providing clean power to the Meade LX200 series telescopes.
 
Tight voltage regulation is maintained by feedback provided by the R1, R2 and Rt series connected voltage divider resistors. Rt is a 33 ohm fine trim output voltage adjustment resistor wired in series with R2 in this prototype DC-to-DC converter. Rt is a selected value that achieves the desired +18VDC output voltage level. The regulator has an internal 1.24VDC reference voltage level fed to one input of an internal op amp voltage comparator and the voltage feed back line is connected to pin 2 is fed to the other input of the regulator’s internal voltage comparator. As the feedback voltage approaches 1.24VDC, the regulator reduces the time (aka, dwell) that pins 3 and 4 are shorted, less magnetic energy is stored in L1 and a smaller pulse of current charges C2. If there is no output load, the dwell time of the regulator is very short since the only drain on C2 is the one milliamp drawn by the series feedback voltage divider resistors. Higher current loads try to discharge C2, but when the feedback voltage drops below 1.24VDC, dwell time of the regulator increases as necessary charging L1 with more magnetic energy which in turn “pumps” more current into C2 to keep its voltage at 18VDC. With its switching rate of 100,000 repetitions per second, the output regulation occurs extremely fast in less than the blink of an eye.

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.
 
The C1 electrolytic capacitor smoothes out 12VDC input ripple current caused by the switching regulator and C2, also an electrolytic capacitor, smoothes out any DC ripple on the 18VDC output. C3 and C4 are 0.1 microfarad non-inductive bypass capacitors that remove high frequency and RF noise components from the DC voltages keeping RFI noise contained within the die cast aluminum enclosure that also serves as an excellent RFI shield. D2 is there for reverse polarity protection in which case it will become forward biased and conduct the large reverse current flow causing the inline fuse (F2) to blow immediately protecting all circuitry. The DS1 LED indicator is in series with Rb (ballast resistor) and lights up when 18VDC is present. Rb drops the voltage so that this LED does not shine too bright when observing in the dark.
 
Construction
 
I’m just going to show the photograph with the cover removed (figure 1) rather than provide detailed hole templates for drilling. Hole sizes and locations can vary depending on the parts available to you that will do the same functions. The prototype vector board components are laid out much the same way as illustrated on the schematic. Here are the basic construction steps:
 
Cut the vector board to fit the inside of the enclosure and leave ¾” end clearances for the power cords & their strain reliefs.

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 12VDC power input cable, use 14/2 gauge low voltage landscape wire. Be sure to mount an inline 5 amp fuse holder (XF1) in the positive (+) side of the power cable. I used red/black battery clamps and used heat shrink tubing over each connection (splice) to dress it up. You can use any type of connection or disconnect plug to the battery that you wish. Just make sure the connection is good, solid and reliable. Choose a cable length that is long enough to be practical but not longer than necessary.

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.



           

Parts List
 
ID          Type         Value     Qty     U/M   Vendor                  Part #                  Unit $   Total $
IC1           IC         T220      1       Ea.   Digi-Key                LT1170CT#PBF             10.63     10.63

 DC Switching Regulator

---          Terminal             20       Ea.   Digi-Key                1562-2K-ND                0.27      5.40
   Solder Turret Post / 0.062” Swage Mount

L1          Inductor     330uH     1       Ea.   Jameco                  371360                    1.65      1.65
   Horiz. Mounted Toroid Ferrite Core

Cc         Capacitor     1uF       1       Ea.   Jameco                  528460                    0.85      0.85
   50V Metal Film 5%

C1         Capacitor     100uF     1       Ea.   Circuit Specialists     CET50-100                 0.50      0.50
   50V Electrolytic

C2         Capacitor     470uF     1       Ea.   Circuit Specialists     CET50-470                 1.00      1.00
   50V Electrolytic

C3 & C4 Capacitor        0.1uF     2       Ea.   Circuit Specialists     23PP410                   0.55      1.10
   250V Poly Non Inductive

D1 & D2 Diode            1N5821    2       Ea.   Circuit Specialists     1N5821                    0.41      0.82
   Schottky or Hi Speed Equiv.

R1         Resistor      16.9K     1       Ea.   Circuit Specialists     RC16.9K                   0.15      0.15
   1/4 Watt Precision Metal Film 1%

R2         Resistor      1.21K     1       Ea.   Circuit Specialists     RC1210                    0.15      0.15
   1/4 Watt Precision Metal Film 1%

Rt          Resistor     33        1       Ea.   Circuit Specialists     RA33                      0.07      0.07
   1/4 Watt Carbon Film 5%

Rc         Resistor      1K        1       Ea.   Circuit Specialists     RC1000                    0.15      0.15
   1/4 Watt Precision Metal Film 1%

Rb         Resistor      330       1       Ea.   Circuit Specialists     RA330                     0.07      0.07
   1/4 Watt, Carbon Film, 5%

CBI        Vector Board            1       Ea.   Circuit Specialists     64P44WE                   7.95      7.95
   5.4" X 6.5" X 0.62" thick unclad punchboard / 0.042" dia. holes on 0.1" centers

Grease   Thermal                   1       Pk.   Circuit Specialists     864-4G                    0.80      0.80
   1/8 Oz. Packet of Silicone Heat Transfer Compound for Heat Sink

HS1       Heat Sink*               1       Ea.   Circuit Specialists     530613                    0.35      0.35
   Heat Sink For TO220 Type Integrated Circuit

Case      Enclosure                1       Ea.   Circuit Specialists     03-125BR                  6.99      6.99
   4.7" X 1.6" X 1.4" Die Cast Aluminum Enclosure/RFI Shield

S1         Switch                  1       Ea.   Circuit Specialists     8013                      0.98      0.98
   Toggle SPST, 5 amps/250 V rated

E1 - E4  Terminal                  4       Ea.   SMC Electronics         MCONN01                   0.25      1.00
   Insulated Solder Post/4-40 Thread

XF2        Fuse Holder             1       Ea.   Radio Shack             2701217                   2.49      2.49
   In Line

XF1        Fuse Holder             1       Ea.   Radio Shack             2700364                   1.99      1.99
   Panel Mounted

F2          Fuse           5 amp   1       Pkg.  Radio Shack             2701027                   2.99      2.99
   1/4" X 1 1/4" Slow Blow, pkg of 4

F1          Fuse         1.5 amp   1       Pkg.  Radio Shack             2701022                   2.99      2.99
   1/4" X 1 1/4" Slow Blow, pkg of 4

BC         Bat. Clamps    30 amp   1       Pair  Radio Shack             2700343                   3.69      3.69
   One Set of Red and Black

DS1       Lamp                     1       Ea.   Radio Shack             2720345                   1.49      1.49
   12V Red LED Pilot Lamp

P1         Plug                    1       Ea.   Radio Shack             2731717                   Free      Free
   "Adapt-a-Plug" type N DC Coaxial*

             Power Cable           1       Ea.   Radio Shack             2701558                   8.99      8.99
   5 amp. DC Power Cable, 18 Gauge/Adapt-A-Plug Outlet

             Tubing
   Various Sized Heat Shrink Tubing

Misc.     Hardware
   Solder, Hookup Wire, etc.

Cord       Power          #14     16’    Ace Hardware                    31938                     0.59      9.44
   14/2 Low Voltage Landscape Cord

Clamp                       4     Ea.    Ace Hardware                    3093358                   0.16      0.48
   Wire Clamp, Poly, for 1/8" wire

Screw    4-40X1/4"          4     Ea.    Ace Hardware                                              0.08      0.24
   Stainless Steel Machine Screw

Screw    6-32X1/2"          4     Ea.    Ace Hardware                    59254                     0.10      0.40
   Stainless Steel Machine Screw

Nut        6-32             4     Ea.    Ace Hardware                    59556                     0.23      0.92
   Stainless Steel Machine Nut

Washer  #6                 12     Ea.    Ace Hardware                                              0.09      1.08
   Stainless Steel Flat


Total Parts Cost Not Including Shipping Charges and State Sales Taxes: $77.13

 
  
 
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