Tom’s Circuits – Choosing Connectors

November 25, 2016 , in Blog, Tom's Circuits

“Which connectors and connector companies are best? In my last project, connector lead time hurt my schedule, and there are so many to choose from! How do I go about choosing connectors for my project? Which manufacturers are the best?” (reader question)

There are indeed a lot of connector part numbers out there. Last I checked, the in-stock RoHS part number counts are 200k from Digi-Key, 142k from Mouser, and 75k from Arrow.

Connectors have demanding requirements for both electrical and mechanical design. For connectors exposed to customers, there are more constraints for compatibility, ease of use, and aesthetics. These constraints will quickly whittle down the choices!

Connectors often come in part families, and there is a different part number for each possible number of pins or contacts. This inflates the part number count and makes connector standardization and availability a challenge.

Creating a printed circuit board design to be compatible with a connector is a mechanical challenge. Connectors often don’t have an electrical-component style datasheet. Mechanical drawings may be difficult to interpret, with incomplete tables of dimensions for many variations of a connector. Creating a working printed-circuit board footprint design requires measuring the part and making a few design iterations. If there is any doubt, it is best to verify connector fit and dimensions in a mechanical mockup before the full design is done.

My favorite connector companies provide 3D CAD models to help make sure that the connector fits. Even with these models, don’t forget about cables. Cables are difficult to model, and new designs will benefit from a mockup.

I tried the 3D STEP CAD models from Molex, CUI, and Neutrik. For testing I used Autodesk Fusion 360. I downloaded the Molex and CUI models directly from Digi-Key, and got the Neutrik models from their web site.

The Molex models are a single solid body.

Since there is just one body, the model is good for checking the fit of components. However, because it is one body, there can be only one material.

Molex connector rendered in Fusion360

Next I tried a model from CUI. This model is a component made from multiple 3D bodies.

CUI sj-43504-smt metal parts from the STEP file

This more detailed model shows the different parts of the connector, and makes it easier to see how the connector fits together and works.

For example, I can turn off the plastic parts to better see the metal parts and figure out what they do.

Neutrik NMJ6HCD2

I first saw this Neutrik connector in a high-end guitar effect. The rugged springs and the wiping action of the contacts make a solid and reliable electrical connection.

Neutrik has a reputation for high quality connectors. They also provide great component data.

The product photo shows the details.

This Neutrik model doesn’t have the multiple bodies like the CUI connector. Instead it has a minimalistic 3D model that seems intended to save resources in the CAD tool. The pins are round in the 3D model, and rectangular on the actual part. The panel mounting details are not all included in the 3D model.

The NMJ6HCD2 2D mechanical drawing is a thing of beauty.

Still, even with all the Neutrik online part data, it is difficult to understand just how well this connector works. In this case there is no substitute for hands-on experience with the connector.

Contact Resistance

In addition to the drawing, there should be a catalog page or online reference that describes the intended applications and specifications for the connector. For power connectors, check the maximum current handling capability. The maximum current numbers will typically correspond to a temperature rise such as 20degC. This temperature rise is due to the power dissipated by the contact resistance. I recommend staying well below this current. During short-circuits, a fuse or other protection circuit must trip before the connector or wiring catches fire.

To verify the quality of a connector, measure the contact resistance with a high-quality Ohm meter. This requires a meter that can accurately measure resistance in the low milliOhm region. (You will need this meter later for debugging EMI shielding problems. Just buy it!) A good connector will have repeatable resistance. Wiggling the connector shouldn’t cause intermittent connections or big changes in resistance.

If your power pin is not rated for enough current, use multiple pins in parallel, or choose a higher-current connector. To handle high currents, consider using connectors with blades instead of pins.

Most connectors are not rated for being connected and disconnected while power is applied. If such hot-plugging does need to happen, there are special connector features and electrical interface circuits to support this. For example, USB connectors have longer pins on the outside two contacts. These are the power supply pins. This design ensures that power is applied before the data is connected, and that data is disconnected before power is removed.

Connector DC ratings are usually much lower than the AC ratings. This is because DC current tends to arc-weld the connector terminals together. AC current causes less welding.

Some connectors require cleaning for maximum performance. Most connectors will get gummed-up from time to time. Check that the connector can tolerate cleaning with isopropyl alcohol.

Soldering

Surface mount solder processes can melt many of the plastics commonly used in connectors. The sad look of a melted connector is not quite captured by the phrase “deflection temperature.” Look for processing times as well as temperatures, and make sure that they are sufficient. Communicate with your board assembler about your board’s requirements for the solder temperature profile.

For high power, high temperature applications, choose a high-temperature plastic such as glass-filled Nylon / PA46.

Quality and Cost

I have an electrical outlet in my kitchen where I unplug my toaster and coffee grinder after each use. I decided to replace the outlet and went to Home Depot to get a new one. In the same form factor, the price of single a duplex straight-blade 15A 110VAC outlet ranged from about $0.50 to $5.00. I wondered, “what would make an outlet worth 10 times as much?” I bought it just to see. I have never had a better outlet. I confess to enjoying plugging things into it. The outlet is industrial grade, with solid Nylon and steel body parts. The outlet prongs glide in smoothly, and are captured by firm contacts. The outlet is rock-solid and does not deflect as the plug is inserted or removed.

Connector quality can make or break a product. After narrowing the choices, use the ‘buy and try one of each’ approach. Be demanding, and be prepared to pay a little (or a lot) more for quality. Find similar connectors in products that you like, and figure out where they came from. If you can’t afford good connectors, figure out how to eliminate them from your design.

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