[time-nuts] powering Trimble Thunderbolt with -5V rather than -12V

Stewart Cobb stewart.cobb at gmail.com
Thu Oct 31 02:11:44 EDT 2013


Executive summary:  you can power a surplus gold Thunderbolt using a
-5V supply in place of a -12V supply, and it will probably work just
fine.

Details:  The manual for Trimble Thunderbolts specifies power supplies
of +5V, +12V, and -12V.  It turns out that power supplies that provide
+5V, +12V, and -5V are easier to obtain locally.  I began to wonder
what circuitry in the Thunderbolt required -12V, and whether it would
run just as well on -5V.  So I took one apart and started probing.

As far as I can tell, the -12V supply goes to only two places.  One is
the negative supply pin for the quad op-amp (LT1014) in the DAC
circuit for the OCXO.  The other is a strange little circuit involving
a 2N3904 (SOT-23 marked 1A) near the "232" driver chip, right next to
the serial port.  This circuit seems to be comparing the -12V input
with one of the charge-pump pins on the 232 chip.  Its output (?)
connects to a test point labeled "MON".  I assumed this was
non-critical and decided to ignore it.

The LT1014 op-amp is rated for operation on supply voltage as little
as 5V and as much as 30V (+/- 15V).  The spec sheet says the output
saturates about (1V typical / 3.5V max over temperature) above the
negative supply.  Presumably, if the op-amp is not asked to generate
output voltages lower than -1.5V, it should run fine with a -5V
negative supply.
The only negative voltages I could find, probing around the op-amp
circuit, were generated by AC-coupling digital square waves.  None of
the op-amp outputs were negative.  (My DAC steady-state value was
around +300mV, which appeared many places in the circuit.  Presumably
a slightly negative DAC value would also appear in many places, but as
long as it's greater than -1500mV, it won't matter.)

Armed with theoretical and practical confirmation that this should
work, I tried it.  And, oddly enough, it appears to be working.  Two
different Thunderbolts have been powered by +5/+12/-5 supplies, and
both have settled down and started tracking exactly as one would
expect.  For one, the "settled" DAC voltage was within a few
millivolts of the value it had on the specified power supplies,
shortly before the change.  The other had not been powered on for a
while and is still settling, but it seems happy.

There is a subtle possibility for concern, in that the sensitive DAC
signals near ground are now about 3.5V away from the center of the
op-amp supply range.  This could theoretically cause increased
distortion, offset, or offset drift due to the larger common-mode
voltage on the op-amp inputs.  In practice, it does not appear to be
an issue.

This note applies to the common surplus Thunderbolts in the
gold-anodized box, with the Trimble-branded OCXO.  All of those I've
tried seem to settle with DAC voltages near zero.  If you try this
with another style of Thunderbolt, you're on your own.

Cheers!
--Stu


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