Tips for Choosing Connectors and Wires for Safer Designs

Mistakes happen.  While you never can out-design evil-doers intent on destroying your designs, you should reasonably be expected to keep an undertrained, overworked, sleep-deprived assembly-line workers from turning your PCB into a fire or electrical shock hazard.

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Audience:

Choose the Correct Current Rating for Connectors

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Electrical wire gauge, like a water pipe, comes in a variety of diameters, and the diameter is chosen based on expected current flow.  There are charts that provide Ampacity ratings for wires at various temperatures.  Similar to the word “capacity”, “Ampacity” lists the maximum current through a wire or connector of a particular type.

This portion of NEC Table 110.114(C) shows the maximum current through a variety of insulated wire types (before derating) in typical household and industrial wiring.

This portion of NEC Table 110.114(C) shows the maximum current through a variety of insulated wire types (before derating) in typical household and industrial wiring.

Wires aren’t the only things with limits: Integrated circuits, copper traces, and connectors must also be chosen and sized based upon current demands.

This graphic comes from the    webpage    of a    screw terminal connector

This graphic comes from the webpage of a screw terminal connector

One mistake new designers often make is to ignore the current rating of their connectors entirely; this is a dangerous decision that can cause a fire.  The second mistake new designers often make is that they forget to “derate” the connector in high-temperature conditions. The hotter the environment, the lower the allowable current through the device.  Derating lowers the allowed current through the device.

Taken from the same    webpage    of the device shown above, this derating diagram shows that as ambient temperature increases, the allowable current through the device decreases.

Taken from the same webpage of the device shown above, this derating diagram shows that as ambient temperature increases, the allowable current through the device decreases.

The third mistake new designers often make is that they forget that while theory and datasheets are perfect, real-world conditions are not.  You should almost never plan to run exactly 12 A of current through a wire or connector with a 12 A limit. You never know when a wire will get a nick, a connector will be in the air-flow path of a hot-IC, someone installs a heater underneath your design, or something else happens.  Plan to either use multiple wires of the same gauge, or the next gauge up to give yourself a margin of error, because “stuff happens,” especially when you are a novice.

  • Choose wires and connectors that are more than thick enough to carry the expected current.

  • Derate the wires and connectors if necessary. You can do this by using larger wires, or multiple / connectors of the same gauge

  • Never trim multi-conductor wires before placing them in a connector.

  • Provide yourself a margin of safety.

Avoiding Connector Mishaps

The screw terminal connections shown above are wonderful devices used in factories and products all over the world.  The versatility of these types of connectors also results in an unavoidable design flaw: nothing prevents a technician from inserting the wrong wire into the wrong hole.

There are several options available to you to help prevent connection mistakes from happening.  Below are just a few.

Use Polarized Connectors

Polarized connectors can only be connected in one direction.  An example you are likely familiar with is already in your home.  3-prong electrical appliances and some 2-prong electrical appliances can only be inserted into the outlets in one direction.

Electrical connectors are typically polarized with protrusions, divots, or keyways to prevent them from being connected improperly.

These are two polarized three-position female connectors from two different product lines.

These are two polarized three-position female connectors from two different product lines.

Using different types of connectors unnecessarily can increase project cost, so another option is to use similar connects that are color-coded to mating wire-board or board-board connectors.

This example inside the BB8 charging station shows three connectors with different colors to help assembly line workers make the correct connections.

This example inside the BB8 charging station shows three connectors with different colors to help assembly line workers make the correct connections.

Yet another option is to use connectors from the same manufacturer and product line, but with different numbers of pins.  Most assembly line workers would know where to match the connectors without detailed instructions.

Shown above are 3-position, 4-position, and 5-position connectors.

Shown above are 3-position, 4-position, and 5-position connectors.

Finally -- it is possible to use identical connectors on a board, and physically prevent improper connection with the use of a key-pin.  In this configuration, one or more holes on a female receptacle are plugged, and male pins are removed to match. This ensures that the plug cannot be inserted into the receptacle incorrectly.  Additionally, multiple key-pins can be used in different positions to allow otherwise identical mating connectors to be used on a board.

This JTAG programming connector and mating receptacle have missing pins and plugged holes to ensure proper connection.

This JTAG programming connector and mating receptacle have missing pins and plugged holes to ensure proper connection.

Finding connectors

The easiest way to find connectors is with a visual selector guide at a manufacturer website, or at your favorite distributor.  Here are a few examples: Digikey , Mouser , AmphenolRF, Molex .  There are many more distribution channels and manufacturer websites out there.  If you have a preference, tell us about it in the links below! 

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