Trying to work out power rail regulation in undocumented hardware - Page 1
Forum section: electronics / projects

author: CambridgeMart
author_rank: Regular Contributor
timestamp: July 12, 2025, 12:38:56 am
content: I am trying to get a laser engraving system working after some time in storage; the control unit is not communicating with the laser / scanner assembly. After some digging, I found that the only logic is in the XY scanhead unit, which is a GSI HB-X10 unit. Within the head is a scan control board which has linear regulators for some of the supply rails. The incoming supply is specified as +/- 15V to 24V, as far as I can determine. The incoming supply goes to a 78M12 regulator, a 7805 regulator, and a 79M15 regulator. The incoming +ve supply reads +14.9V, which is sufficient to power the +12V rail via the 78M12 regulator. On the other hand, the measured -15.0V supply to the 79M15 regulator is insufficient to generate the -!5V supply.How do the 79 series regulators perform when Vin is just marginally higher than the rated output, given that the datasheet specifies a 1.1V dropout voltage?

author: moffy
author_rank: Super Contributor
timestamp: July 12, 2025, 04:02:45 am
content: While at or below the nominal output voltage at the input, the output should follow the input voltage minus a voltage drop, which is probably current dependent. If it exceeds max current it should probably current limit but I am just guessing its behaviour, you could measure it. Are you sure that the 79M15 shouldn't be a 79M12 instead?

author: Whales
author_rank: Super Contributor
timestamp: July 12, 2025, 04:23:38 am
content: Also when below dropout voltage the regulation (PSRR) will likely suffer.

author: CambridgeMart
author_rank: Regular Contributor
timestamp: July 12, 2025, 09:38:31 am
content: Thanks. The unit is powered from an SMPS in the main unit, so the only thing affecting regulation would likely be the resistance of the supply cables.I have no way of determining if the -ve regulator should be 15V or 12V, but if it tracks the input, less the dropout voltage, it probably doesn't make a great deal of difference.Documentation for these units is very hard to come by!

author: Whales
author_rank: Super Contributor
timestamp: July 12, 2025, 11:59:04 am
content: It might matter if the extra few volts takes a chip outside of its limits or puts too much thermal stress on a resistor.

author: CambridgeMart
author_rank: Regular Contributor
timestamp: July 12, 2025, 07:47:07 pm
content: I was specifically referring to this case, where the output of the -15V regulator is hovering around -13V. I have no particular reason to believe it should be a -12V regulator, in that the unit worked for 10 years before I got it; that said, it would make more sense if it were a 12V version in that position.

author: MathWizard
author_rank: Super Contributor
timestamp: July 15, 2025, 02:52:18 pm
content: Where does the -15V input come from ? Is it from a rectifier ? Maybe there's a lot of ripple and some large cap after the rectifier is wearing out and can't charge up enough ?

author: coromonadalix
author_rank: Super Contributor
timestamp: July 15, 2025, 03:57:08 pm
content: any 3 pins classic regulators need at least 2-3 volts more to do regulationif they are LDO, even 1v more in some cases is enough to make them worktry to understand how the supply line(s) are made / created floating or not pre-regulation or direct after rectifier bridges, or one can supply many others ?sometimes you need to figure out the real ground versus floating ones if any, you could miss-understand things ... and get wrong ideassome smps blocs may have compensation pins, or the basic ones having a plus and ground and or a negative pin ...show us some photos ? it would help better than do guessing ?