Basic Interconnection- Pin headers, MTA connectors, Banana Jacks
This article deals with a problem nearly all inventors will have at some point: Plugging something into something else in a way that is reasonably robust, yet simple. The easiest solution to plug circuit boards together, or to plug things into circuit boards, is to use plain old Pin Headers. The next step up in my mind is using crimp-style MTA-xxx connectors, shown in the center photo. Many production devices use MTA-xxx style connectors internally, and they are very robust when used correctly. The photo below shows my plastic organizer box with my standard-use connectors.
My standard stock of pin headers and banana connectors
The Basic and Versatile: 0.1″ Pin Headers
0.1″ pin headers are almost universal on dev kits and sample boards, so it would serve you good to have some around the shop. A standard old-school 40-pin IDE is a dual row 0.1″ pin header, and things like the Make Controller and many Arduino kits use this same style of connector.
Pin header assortment, male and female with example
Above is a hand-soldered level converter board I put together in a pinch, featuring 2x 5-pin male pin headers with heat-shrink on the female connector. The male connectors I used are Digi-KeyPart Number SAM1059-50-ND “CONN HEADER 50POS .100″ SNGL TIN. I buy them in the 50-pin length, and then just cut off the number of pins I need. Pin headers are available with a number of different lengths of pin, but I tend to buy longer ones, as you can always cut them shorter if need be. Also, having longer pins means you can easily bend them to make odd-angled connectors if you need to. The photo shows examples of the raw pin header, as well as the female header to which it mates.
When using the female headers, Digi-Key # S7034-ND “CONN HEADER FEMALE 36POS.1″ TIN”, you will always lose a pin when cutting them to length, as the female headers do not have the space between them that the male pin headers do. You can get these in various lengths as well, with the above being a 36-position. It is worth noting that these female headers can mate with various types of male connectors, including MTA-100′s (as discussed below). You can also stack two of these together to mate with dual-row headers, if you need to jury-rig up a JTAG cable harness or what have you. They also can be used instead of jumper blocks for equipment which uses 0.1″ jumpers for configuration. You can also stack circuit boards using these connectors.
I wouldn’t recommend these connectors for anything above a few 100 mA. Small stepper motors are okay, but for higher currents use MTA-xxx or screw terminals. Pin headers have very small actual contact areas, when compared to other styles of connector. Don’t use them for applications which need many reconnection cycles, as the pins eventually will wear out. They will also vibrate loose. I use these a lot, but never in a production environment unless used to stack boards where other physical locking systems can be used.
When soldering either of these, make sure you tin the pin and the wire before you solder. People tend to keep heat on these plastic solder-on connectors for too long, which results in the pins coming lose of the male headers, or the plastic enclosure melting on the rectangular header. Even when done properly, the resulting joint will be quite weak, as there is no strain relief on the wire. This is fine for prototyping, but if you want something that will last longer, use some heat shrink, or upgrade to MTA style connectors,as discussed below.
More Strength: MTA-XXX connectors
For those applications that need:
- Locking Connectors
- Higher current
- Basic strain relief
- Vibration resistance
I suggest the use of MTA-xxx style connectors. I keep two styles on hand, MTA-100 and MTA-156. The MTA-100′s have the same pin spacing as the basic pin headers above (indeed, female MTA-100′s can mate with male pin headers, and female pin headers can mate with male MTA-100′s). MTA-100′s feature a strain relief on the wire, and they can therefore be used in high-temp and vibration prone environments. MTA connectors are very commonly used inside equipment to bring wires to chassis connectors and front panels, or internal wired interconnection. Their locking mechanism is quite robust, and I have had excellent experience with them. You can not easily slice MTA-100′s to the appropriate number of positions due to the presence of the locking mechanism. The following photo shows an application of MTA-100 connectors on the prototype FEC motor drive “cube” I built in college.
Example of MTA-100 connectors used on prototype.
The MTA-156 connectors have a 0.156″ pin spacing, instead of the .1″ as in the MTA100, which means MTA156′s will not fit in standard PCB protoboard without drilling and mucking about. However, MTA-156′s are rated to 7 Amps, while MTA-100′s are rated to around 3 Amps.
MTA-XXX style connectors are IDC (Insulation Displacement Connectors), which means they have little teeth that rip through the insulation when the wire is inserted. Wire size with IDC connectors is extremely important. If you cram an over-sized wire into any IDC connector, the connector will be damaged and the crimp will fail. Same thing with putting small wire in an over-sized crimp. They also need a special tool to easily assemble, as discussed below.
The different sized MTA connectors are color coded (RED = 22 AWG, GREEN = 28 AWG, WHITE=24 AWG, etc) for ease of use. 22 AWG is the largest wire size supported by MTA-100 connectors, so I keep a stock of those around in the interest of versatility. If you want to use crimp ends, buy wire of the proper size, or they will give you nothing but trouble. You can’t use old Ethernet or Serial cables in a 22 AWG Red MTA-100 crimp, as the wires will fall out. If you try and solder these, chances are good they will fail. You’re better off using the 0.1″ pin header for found wire and cable.
If you are installing hundreds of these connectors, you should drop the several hundred dollars on the neato ratcheting MTA-crimp tool- Digi-Key #58579-1-ND, “TOOL HAND MTA 100″ This is a really nice ratcheting tool that lets you put these together very quickly and very reliably. However, it’s tremendous overkill for most hobbyists. A significant benefit of using MTA type connectors is that the kind people at Digi-Key sell $20.00 t-handle tools for both the MTA-100 and MTA-156 style connectors. This produces a crimp on par with the ratcheting tool, but it is slower and more tricky. The MTA-100 T-handle tool is Digi-Key #A9982-ND and the MTA-156 is #A9981-ND. The availability of this silly tool is why I highly recommend MTA style connectors over other crimp-terminal style connectors.
Some people prefer individual crimp-pins that slide into plastic housings. I find these to be terribly annoying, as properly crimping on the pin for most of these types of connectors is very difficult without the proper tool. The MTA type connectors from Tyco are a very good compromise of robustness vs. cost of installation tool. In a pinch, you can use a flat-head screwdriver and get a high quality crimp, if you do it very carefully.
MTA connectors with low-cost insertion tool
The photo above shows (from left to right), an MTA-156 pair side view, an MTA-100 front view, a wire assembly made with the T-handle tool, and the T-handle MTA-100 tool.
Banana Jacks: Whoooo boy!
Banana jacks are used on most bench power supplies, and DVMs. The far right photo shows my assortment of connectors and things I keep on hand, and one bin is full of screw-on banana jacks. I have sets of banana-jack jumper cables, but for interfacing my power supplies and other banana-jack equipped test equipment to my prototypes, I make extensive use of screw-on stackable banana jack ends. I find this much cleaner and easier than using binding posts and raw wires, as it means I can rapidly string things together or reconfigure my power supply equipment to measure currents.
The ones I really like are set-screw based Banana jacks: Red (Digi-Key #501-1077-ND) , Black (Digi-Key #501-1076-ND) and Green (Digi-Key #501-1247-ND). Get a bag, and then you can make banana-jack jumpers to your heart’s content.
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