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28 Volts from 14 Volts

Brian Lloyd, Bay Area RVators
[Brian is an engineer at Lucent, and a regular contributor to the RV List—Webmaster.]

I received a sufficient number of requests for info on running 28V gyros off a 14V electrical system that I felt it warranted a reply here.  First, I want to amplify on my comments about 28V gyros being cheaper.  I found that older 28V gyros are cheaper than new ones and they appear to be more robust.  (I like to do aerobatics which tends to bash gyros.)  I was able to purchase an overhauled and yellow-tagged AIM attitude gyro for $700 from the Gyro House in Auburn, CA.  (No, I don’t know if they have anymore.) This is the older style gyro with the black background and a single white line for the horizon.  None of this “the sky is blue and the ground is brown” sissy stuff.  It is a standard 3.125” instrument.  This gyro handles 360 degree pitch and roll.  (It does incorrectly show a roll if you hold it at exactly +/- 90 degrees of pitch for any length of time but I am not too worried as I do not keep my RV-4 pointed straight up or straight down for any significant length of time.

OK, I have just saved $700 on the attitude gyro, but how do I make it run on my 14V electrical system?  (No, it won’t do anything if you try to power it off of 14V—I tried just for grins.)  I turned to one of my electrical engineer friends who builds power supplies for a living.  He pointed me at the Maxim Max 1771 adjustable, high-efficiency, step-up DC-DC controller chip.  You can pick up the data sheet at http://209.1.238.250/arpdf/1030.pdf.

One of their examples is a “2V to 12V input to adjustable 24V output” converter, which is almost exactly what you need to build.  The kicker is that Maxim makes an evaluation kit for this chip which is a complete, ready-to-use converter on a 1” square circuit board.  This board is designed to take 5V in and output 12V but you can make it do 14V in and 28V out just by adding two resistors, for which they have included space on the board, and changing the output capacitors from 20V units to ones that will handle the output voltage.  (I used output caps with a 36V rating.)  You can get the data sheet for the evaluation kit at http:/209.1.238.250/arpdf/1260.pdf .

If you are comfortable scratch-building circuits like this (the parts count is pretty low—about 10 components) you can talk Maxim out of free samples of the chip.  I am lazy so I paid the approximately $30 for the evaluation kit since it had the somewhat-hard-to-find components already there.  (No, you can’t get all the components you need at Radio Shack.)   The hardest part of building the converter is solving the formula that selects the two resistors that set the output voltage.  As I recall the two nearest standard resistor values resulted in an output voltage of just short of 27V (something like 26.8V).  The values I used are 10K for R1 and I think 168K for R2 (these will make sense when you read the data sheet which describes the circuit).  The resistors are chip resistors which require somewhat of a delicate touch to solder to the board.  This was my first experience soldering surface-mount components (no leads) but it wasn’t too difficult and I didn’t destroy anything in the process.  If you are concerned about soldering just remember that soldering is a skill like driving rivets and you managed to learn how to do that, right?

The finished converter seems to be able to source about 1.5A at 27V.  Since the gyro only draws about .8A at that voltage, it looks like there is ample margin.  The converter gets only mildly warm (maybe 20 degree F temperature rise) in operation.  The gyro seems to like it just fine and the whole mess has run for 24 hours on my work bench with no sign of trouble.  If you are planning to power two gyros, I would build one converter for each gyro.  That way not only is the converter running at only 50% of its rating, but a failure of a converter would only disable one gyro.

One possible concern is interference with the radios.  Since the converter runs at about 200 kHz, I suspect that it would interfere rather badly with an ADF.  If you have an ADF you will probably need to build the converter into a shielded box with filtering at the input and output.  I did not shield mine since I did not plan to have an ADF in my RV-4.  I have run tests and have not detected any interference with my comm, nav, or GPS receivers.

I hope this helps someone.  It saved me about $700 on the price of a gyro and was a relatively quick and rewarding project.  If you decide to go this route feel free to drop me a note if you have questions or need assistance.


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