TO-3 case terminal-to-terminal bonding  - Page 1
Forum section: electronics / general

author: codeboy2k
author_rank: Super Contributor
timestamp: March 13, 2015, 02:35:34 pm
content: I came across this ebay listing :http://www.ebay.com/itm/Transco-DC-Motor-amp-Tachometer-22-VDC-3-0-Amp-28000-RPM-31510144-/261808234695?&_trksid=p2056016.m2518.l4276There's a picture of a TO-3 transistor that is isolated from the mount, using nylon or PTFE insulated bushings.(click the picture for fullsize image)The black wire at the bottom right is the collector connection back to the inside circuitry, and it is under the bushing, so it is in contact with the case.But what caught my eye was: why the need to bond it to the other side? Is there an engineering reason for bonding the two mounting points like that, or is someone just "gildingpainting the lily" and being extra cautious?I was also thinking it was some attempt bring the collector current outside the case as soon as possible (i.e. the die could be closer to either screw terminal, so they bond a wire to both to be sure).. but I was just grasping at straws, I didn't see any real need for that, the case can handle the current just as well.The only other thing I can think of is perhaps they don't want the collector current flowing under the die for some reason ? base node is sensitive?I'm asking because I would not do it this way, and I would just use the single black lug. But when I see someone else engineer it like this, I wonder why and under what conditions might I need to the same.Comments? Does anyone else have an idea?EDIT: painting the lily.. thx TimFox

author: nitro2k01
author_rank: Frequent Contributor
timestamp: March 13, 2015, 02:58:29 pm
content: I couldn't tell for sure, but my guess would be distributing the contact resistance. Maybe the bushing is fragile, and they could't apply as much torque to the screw as they would have wanted, and for this reason added a second lug to get more parallel conductance, maybe for a worst case scenario with corroded contacts over time. So yes, gilding the lily.

author: German_EE
author_rank: Super Contributor
timestamp: March 13, 2015, 03:09:42 pm
content: Use two lugs and halve the contact resistance. Plus, if one bolt comes loose you still have the other contact.

author: TimFox
author_rank: Super Contributor
timestamp: March 13, 2015, 04:03:00 pm
content: "Gilding the lily" is a common error for "Painting the lily".Gold plating a floral substrate is very difficult.Original version (Shakespeare, King John, Act IV, Scene 2):"Therefore, to be possess'd with double pomp,To guard a title that was rich before,To gild refined gold, to paint the lily,To throw a perfume on the violet,To smooth the ice, or add another hueUnto the rainbow, or with taper-lightTo seek the beauteous eye of heaven to garnish,Is wasteful and ridiculous excess."

author: codeboy2k
author_rank: Super Contributor
timestamp: March 13, 2015, 08:22:54 pm
content: reducing contact resistance sounds plausible.@nitro2k01 , re: fragile bushings, that could be it too. Perhaps they wanted to ensure a good connection even if the screws are torqued to lower specs to save the bushings.I just thought, there's also vibration because of the motor, so perhaps it's just a second safety net to not lose the contact too early in its serviceable lifetime.i.e. what German_EE says, if one bolt comes loose..

author: amyk
author_rank: Super Contributor
timestamp: March 13, 2015, 09:18:36 pm
content: I think it is so that a test clip can be easily attached.In fact, near the left terminal you can see a scratch in the wire so it may have been used for this purpose before.