I’m planning to use some Lundahl transformers (LL1555, LL1544a, or LL1545a) for a project, and am planning the PCBs now, but I do not have any samples in hand, and the datasheet gives only minimal information on the terminal pin dimensions. These three parts have the same mounting pin arrangement, and I suspect the dimensions of these pins would be the same for any other Lundahl whose pins are placed on 0.1” centers.
The datasheets say that the PCB should use 1.5 mm diameter holes for the PCB terminal pads, but with 0.1” / 2.54 mm pin spacing, a 1.5 mm hole leaves little room for a annulus as well as spacing between the pins. Bob Starr published an old Eagle library for Lundahl parts that can be found online, and in those footprints, he used a 1.27 mm hole for all parts with an oval pad shape. This should probably work fine, but still, it would be nice to know what the pin dimensions actually are, since a 1.27 mm hole still leaves only minimal space for clearance and an annulus.
If anyone has any Lundahl parts out there and a micrometer, can you measure a mounting pin for me? I assume they’re square pins, but again, I have no idea. The length of the pin is unimportant, only the diameter or width dimension is interesting. I’d imagine they would be square pins with a width of anywhere from 0.025” to 0.040”, but the details would be of great use.
Thanks!
The datasheets say that the PCB should use 1.5 mm diameter holes for the PCB terminal pads, but with 0.1” / 2.54 mm pin spacing, a 1.5 mm hole leaves little room for a annulus as well as spacing between the pins. Bob Starr published an old Eagle library for Lundahl parts that can be found online, and in those footprints, he used a 1.27 mm hole for all parts with an oval pad shape. This should probably work fine, but still, it would be nice to know what the pin dimensions actually are, since a 1.27 mm hole still leaves only minimal space for clearance and an annulus.
If anyone has any Lundahl parts out there and a micrometer, can you measure a mounting pin for me? I assume they’re square pins, but again, I have no idea. The length of the pin is unimportant, only the diameter or width dimension is interesting. I’d imagine they would be square pins with a width of anywhere from 0.025” to 0.040”, but the details would be of great use.
Thanks!
I have different transformer with less number of pins, but the diameter is 1.5 mm round for sure. I use point-to-point wiring. Think about future desoldering, the internal hair-thin wire is easily get broken by mechanical and soldering heat stress.
Thanks for the data point!
FWIW, I wrote Lundahl and Per Lundahl was nice enough to reply:
"In our production the transformer pins are dip soldered. Thus the outer dimension of the pins are not super well defined, as the tin buildup may vary. If the PCB hole is 1.5mm the pins will always fit.
With 1.27 mm holes we believe that pins normally will fit, but we can not guarantee."
So, for now, I'm thinking that 1.27 mm PCB holes will probably be OK unless a random pin has too much solder remaining, in which case I can use my Hakko FR-300 to remove the excess quickly and safely. I'll report back once I get some parts in hand. FWIW, 1.5 mm is really brutal if you work out all of the PCB clearances and solder mask requirements, so I'm gonna try 1.27 mm for now. At worst, if the pins won't go into the PCB, I can rest the transformer pins on top of each pad for this batch of prototype boards and alter the geometry next time.
It seems paradoxical, but a 1.5 mm hole forces me to generate an extremely small annulus, which will make soldering much more difficult. Too small of an annulus means it takes longer to force heat into a 4 layer PCB, probably requiring a higher soldering iron temperature or some form of pre-heating, and that is actually more stressful than a smaller hole with a larger annulus.
It's always a tradeoff...
Thanks again!