I've been given, for repair, a PSU for 24V theatrical DMX lighting accessories, which contains an isolating DC-DC converter. I'm trying to find specs on this part, and it's proving to be elusive.
The part number on the package is M1WA0505S, and it looks like this:
MEGA's website doesn't seem to have any detailed specs or datasheets. Only a catalogue:
Those general specs seem to match what I'm seeing. It is a 5-terminal device in a SIP package with a group of two terminals and a group of three terminals. The two terminals are being fed with 5V, and the three terminals seem to be a dual output of -5V/0V/+5V. All output terminals do indeed seem to be isolated from the input terminals.
This -5V/0V/+5V arrangement would seem to be supported by the part number (M1WA0505S) and the catalogue entry, although it isn't an exact match. Maybe an obsoleted part?
In any event, I'd like to be certain of the specs for this part, because my analysis of the PSU in which it is found shows something very strange. The isolated output powers an SN75176B, which is a 5V part. The strange part is that it is powered by the M1WA0505S's -5/+5V lines. The 0V line is unconnected. That's 10V, confirmed with a DMM and an o'scope.
So I've got two possible conclusions:
- The M1WA0505S is not a -5V/+5V part and/or is damaged.
- The designers of the PSU should get an angry letter.
I can fix this either way. If it's 1), I'll replace the converter. If it's 2), I'll correct the wiring between the converter and the SN75176B. I actually have a hunch it is 2). Either way, I'll also replace the SN75176B.
Incidentally, this mystery was not actually related to the initial repair which the unit needed. That repair is complete. The mystery was uncovered during the diagnosis of the problem. Believe it or not, the unit still works fine even with the transceiver powered by 10V! Although the transceiver gets pretty hot, of course. I have no idea how long it has been operating this way, but as I say I suspect it is a design flaw.
However, there is a remaining issue which is non-critical, but I believe it may be related to the over-volting of the SN75176B. There is a 'DATA' indicator which lights when DMX is present. It doesn't actually validate proper DMX framing, it merely lights when the 'R' output of the transceiver is high. Under normal circumstances, without a DMX signal entering the unit, this LED would remain off. Now, however, its behaviour is erratic. I expect progressive damage to the transceiver is responsible.
Now the questions:
- Any ideas where I might find a datasheet for this DC-DC converter?
- Anyone used these converters from MEGA? Any advice? Issues? War stories?
- Can a sparky genius provide a 3rd, possibly more sane, conclusion? Or poke any holes in my analysis?