Meade Portable 12v to 18v Converter

Description

Meade Instruments produces the #1812 Electronic DC Adapter for converting 12vDC to 18vDC for use with the LX200 series telescopes. Unfortunately it is in-efficient and can lead to overloading to the point of failure.

The circuit is very similar to the one provided in the Linear LT1170 data sheet. The component values shown are as the Meade component schedule and not as calculated by Linear Tech's Switcher CAD program.

The main problem encountered with this unit has always been the failure of the inductor due to an internal shorted turn. This is caused by the heat dissipation of the component breaking down the wire insulation on the inductor bobbin. Remake the component to cure the problem. The details are below, in the component list, add heat shrink sleeving for a professional finish. Also add the disc decoupling capacitors and the additional heatsinking to the IC while the unit is dismantled.

Schematic

Parts list

Id

Type

Value

Part #

Notes

L1

Inductor

330 µH

 

Must handle at least 3 amps (Meade = 94 turns of 24SWG Enamel wire on original bobbin for 330uH, saturated in Polyurethane – original fault showed low conversion once the chip had been changed – due to inductor turn short circuit)

C1

Capacitor

56 µF

Nichicon x1 UPL1E330MAH

25V ESR=.800 Irms=.155A (1 x 100nF multiplayer was added in parallel for EMC precautions)

C2

Capacitor

390uF

Nichicon x2 UPL1V391MPH

35V ESR=.028 Irms=2.440A (1 x 100nF multiplayer was added in parallel for EMC precautions)

Cc

Capacitor

1 µf

 

 

LT

IC

Switching Regulator

Linear Technologies LT1170CT

data sheet (additional heatsinking was added)

D1 and D2

Schottky

5A

Motorola 1N5824
1N5821

Any high speed diode that can handle the power would do. Schottky type is best but not absolutely required.

R1

Resistor

16.8k; 1%

 

R1 and R2 set the output voltage; The ratio of R1 to R2 should be 13.5 to 1.

R2

Resistor

1.24k; 1%

 

Rc

Resistor

1k

 

 

 

How it works

The supply works by shorting the inductor across the 12 volts through the regulator. This stores energy in the inductor. When the internal control in the LT1170 switches off, the inductor is placed in series with the 12V supply, adding power to it. This voltage pulse is stored in the output capacitor and smoothes the output. The diode is used to keep the output capacitor from discharging during switching.

The IC contains a 100 KHz current based oscillator whose output is controlled by feedback provided by R1 and R2. These make a voltage divider such that at the wanted output voltage there is 1.24V at the junction of these two resistors. They carry only 1 ma of current and are there to provide a reference voltage to an op amp in the IC. As you draw more current from the output the total voltage would drop as would the 1.24v reference signal. The op amp within the IC feeds this to the oscillator telling it to provide more current to fill the demand. This brings the voltage up to 1.24V reference or 18V output. Reduce the load and the opposite happens.

The modified unit, with additional heatsinking and re-wound inductor, was loaded to 2Amp at 18V for 24 Hours. Heat dissipation in the inductor was high and coloured the Polyurethane potting compound, but did not cause a breakdown within the inductor turns, which occurred in the original. Efficiency was still high.

 

The main component of this circuit is the LT1170, and it's datasheet is available at Linear Technology's Web site, but the PDF version of the sheet has much more information. Here is a block diagram of the LT1170; the pin configuration is at right.

1. The criptic output from the linear program claims that both the LT1170 chip and the Schottky diode REQUIRE a heat sink! Basically a large chunk of metal with fins that the part is screwed/clamped to that will carry away the heat produced by the parts. The IC heat sink is specified at less than 8 degrees C/Watt. Since the IC package has its own thermal resistance which is only 2 DegC/W in the LT1170CT, the TO-220 package, if you get good coupling from the package to the heatsink (not real likely unless you use heat coupling grease) you'll end up with about 10 DegC/W and since the IC will be dissipating about 3 Watts that will raise the temperature inside by 30 degrees over ambient. -The good news is that ambient is usually not all that high while doing field work with a telescope- even 104 F is only 40 degrees C which should keep the temperature low enough! Note that with no heat sink, the thermal rating on the IC is 75 DegC/W which will raise the temp by 225 degrees, easily exceeding the 100 degree C maximum rating on the part. Be aware that if you enclose the heat sinks inside a box, the ambient temperature inside the box will rise. A metal project box, or one that has at least a metal lid might provide enough cooling, if the parts are well connected to the metal, but I don't have the numbers handy. Be aware that the heat tab on the IC is connected to its ground pin, and that the diode terminals must not be accidentally connected to the IC through the heatsink.

2. The data sheet specifically states that the IC cannot provide protection from short-circuits on the output because current can pass from the battery straight through the inductor and the diode to the output. The implication here is that a Fuse would a real good idea to prevent a total melt down. Most batteries are capable of Huge currents, and with nothing to limit them, you're likely to melt the insulation right off the wires. The output of the converter is a better place to put a fuse in this step-up type of converter, although either place is better than none. I would recommend a 3 amp fuse. A fuse on the input should be rated for higher current, say 5 amps. Any fuse will cause a slight decrease in overall efficiency.

3. Reverse voltage protection can be added with the addition of another Schottky diode in series with the +12V input rail which would prevent any damage if the input polarity was accidently reversed. This diode must be a Schotty type to minimise forward voltage drop and may also require heat sinking. It might lower the overall efficiency by a couple of percent- but would prevent an accident from destroying the converter.

Alternatively, the protection diode could be added in parallel to the 12V supply with an external fast acting fuse to provide protection. This modification has not been tested to date.

 

The photograph to the right shows the re-worked inductor as described in the component schedule above. At this stage the polyurethane varnish had not been applied and is a guide to the layer winding of the original ferrite bobbin.

This component has always been the problem with this unit. My first attempt at a repair was to replace the IC with no avail, then head scratching, then finally the inductor. The unit worked faultlessly after that.

A number of other modifications followed to increase the efficiency and decrease the EMC noise produced by the unit

 

 

 

This view is the rear of the PCB showing the beefed up PCB tracking and the extra EMC capacitors. These mods increased the efficiency of the original by approx.10%

 

 

 

 

 

This picture shows the test rig used in final test. The current being drawn by the electronic load was 3.5A at 18V and was maintained for a period of an hour.

The original IC heatsink had been replaced for a higher thermal spec unit at this stage. Also the reverse polarity diode had been removed for this test as the component got so hot it melted the solder between the cathode and the supply cabling. Efficiency was now about as high as it could get. The inductor still got very hot, but as this component supplies the flyback power to the output, it wasn't supprising.

 

 

The final version in it's original case. Note the heatsink fins have been pressed out to allow for mounting, and the setting of the polyurethane varnish on the inductor. (Higher powers could be obtained by mounting in a cast aluminum box allowing greater heat dissipation to occur).

The unit in actual use with a Meade LX200 was never expected to provide the continuous power this unit was subjected to during the modification process. The peak power consumed by the telescope is the pulse width control to the motors and averages about 9 -10 watts increasing with slew rates.

(A Note:- Best repeatable high level slew rates on my telescope have always been around the '5' setting and not the default '8')

 

 

Disclaimer

This page was put together by reverse engineering a few faulty Meade units returned for service.

I have tried to be as accurate as possible. My unit is >85% efficient at delivering a continuous 36Watts of output power for an 18Volt output. Note as above, that additional decoupling and heatsinking was added to make the unit stable and more efficient than the original.

 

Return to Home page.