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 <[email protected]> To: [email protected] 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 -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
