Hi,
Interesting investigation, thanks for sharing. Another option I have used was
+12V +5V with the -12V rail powered by a 9V output isolated DC-DC converter
from an old thin ethernet network card. These come in either 5V or 12V input
and are effecitvely free (you also get 10MHz isolaton transformers filters and
Balun on most cards). They are the fat 24pin DIL module. A little additional
filtering may be desirable. So they work on -9V to0. this then just needs a
+12V +5V disk drive type supply (linear) and the DC-DC converter. I still think
the Condor / PowerOne HTTA 16W linear open frame supply (e.g. ebay item
221307930971 no connection) is the best solution though.
Robert G8RPI.
From: Stewart Cobb stewart.c...@gmail.com
To: time-nuts@febo.com
Sent: Thursday, 31 October 2013, 6:11
Subject: [time-nuts] powering Trimble Thunderbolt with -5V rather than -12V
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
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
