Does your circuit require a special connection or disconnection sequence? There are connectors that can help.

Recently I was reading Adafruit’s information on proper support circuitry for Neopixels, and I noticed some information about how to connect and disconnect the Neopixels to a live circuit: ground first, then power, then data (connect); data, then power, then ground (disconnect). I know how easy it is to forget about connection sequence, and I can’t help but wonder if there are actually very few people who observe (or are even aware of) this recommendation.

So anyways, this Neopixel thing was fresh in my mind when I came across the FMLB (first mate last break) connector series from TE Connectivity, and consequently it caught my eye. The idea here is that the connector establishes a ground connection before all the other signals come into electrical contact, and the ground connection remains after the other terminals have separated. Hence the name “first mate last break”. Credit to the product-titling folks at TE, because this name (which refers to the ground connection) is not only catchy but also informative.

I have never seen a connector that provides this type of functionality, but maybe that’s simply because I’m out of the loop. Also, I should point out that the FMLB series has been around for at least a few months, but TE recently introduced a subfamily with 5.08 mm pitch. They refer to this group of connectors as D-3000; the other subfamilies are D-2000 (2.5 mm pitch), D-4000 (6.35 mm), D-5000 (10.16 mm), and D-7000 (16 mm).

 

Visual Aids

So how exactly does the connector manage to establish one connection before all the others, and then maintain that connection after all the others have been released? Let’s take a look:

The first technique is very straightforward: make one contact longer than all the others.

 

Image taken from this product flyer.

 

Obviously, if you want to achieve the first-mate-last-break grounding performance, you need to have your ground signal connected to the right pin. I would consider this a very minor inconvenience, but maybe it’s more of a problem for some applications, because TE specifically mentions that you can specify the position of the grounding contact.

There’s another aspect to the FMLB thing that I’m not quite sure about. I think the issue is this: The actual electrical connection occurs when the plug contact (i.e., the pin) reaches the springy portion of the receptacle contact. If the springy portion is at the back of the receptacle contact, one of the other pins might bump against the exterior portion of the receptacle contact before the lengthened pin reaches its corresponding springy portion. Maybe this will make a little more sense if you look at this comparison:

 

Image taken from this product flyer.

 

So there is metal before you get to the springy thing, and of course you can’t guarantee that the pin will slide perfectly in and reach the springy thing without brushing up against the exterior metal. Apparently the solution is to move the springy thing to the front of the receptacle contact. This ensures that the lengthened pin will make a solid electrical connection before the other pins have a chance to make a spurious connection.

 

Features

The FMLB series has other good features. The product flyer specifically mentions “positive audible locking”—it clicks or some such when the connectors are fully mated. I can imagine that some people would consider this a rather unimportant detail, but I would disagree. A difficult testing/debugging phase can result in frequent removal and insertion of connectors, and furthermore the engineer might not be in the best mood at this time. The little click at least allows me to be confident that yes, the connectors are fully mated, and therefore if the confounded board still doesn’t work, it’s not because I didn’t push in the connector all the way.

 

The FMLB series comes with a strain-relief option. This is a good thing: though system-integration-related frustration has never caused me to apply excessive force to a cable, I know that other engineers might do this. Image taken from this product flyer.

 

TE also claims that the FMLB series is rugged. I don’t think they’re suggesting that you use it in a jet fighter, but the new D-3000 series is specifically described as “resistant to vibration and mechanical shocks” and as “intended for” industrial applications.

 


 

Are you familiar with any other connectors that provide this specialized grounding functionality? Let us know in the comments.

 

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