Just about every system we look to design utilizes connectors of some sort. These connectors may be internal only, they might connect one board to another, add antennas for communication, or give us external inputs and outputs for power and signal. One thing that comes with every connector is additional resistance in our signal chain. The resistance, depending on the connector, can be a good or bad thing for what we are trying to achieve.
In a signal chain you have a number of different sources of resistance. The traces on the board, no matter how large or what material, add a measurable resistance to a circuits(unless you are putting superconductors on PCBs in which case I’d love to know how you accomplished that). While we love to talk about circuit elements as ‘ideal’ the truth is that they also contribute to the resistance, sometimes on purpose to help with the creation of filters or to match impedances for antennas. Wires between items are much like traces and add another source or resistance. Finally we get to the connector which may be something we just think of power or data passing through but we need to take it into account when thinking about resistances impacting our circuits.
See related product
Electrical Resistance Effects
So what happens as resistance shows up in our connection points? The first one that will be noticed is probably voltage drop. Like any resistor in a system you end up with a voltage differential across is as current is pushed through it, in some cases at high currents there can be drops of up to 2/10 of a volt which in the grand scheme of things isn’t a huge amount. If you are working on low voltage systems like FPGAs and microprocessor that utilize very high currents and happen to run at 3.3V or lower 2/10 of a volt is something you really have to pay attention to. In high voltage connectors, say 600V, 2/10 of a volt will probably get lost in your ripple voltage.
Power Loss Due to Resistance
The next element that gets noticed is heat or power loss, as you push current through a resistive element there will be some power lost to heat and the more resistive that element the more power you burn off. One downside of this is that the efficiency of the system goes down, the worst one I have seen is 1/10 of a percent power loss at the connector and when working on a system that is pushing for the highest efficiency possible a tenth of a percent is a decent chunk. Another impact of this power loss to heat is that the connector itself heats up and connectors tend to derate with increased temperature meaning your system stability may be compromised by a hot connector.
Electrical Resistance: Positive Effects
So I have said all these bad things about connector resistance but there are a few areas where controlled connector resistance is desirable, antenna matching and audio output being the first ones that spring to mind. In RF connectors you want your transmitter output impedance to be matched to the antenna impedance as closely as possible which allows for maximum power output. A lot of times the RF systems are based on a 50 ohm specification. In audio output the ideal impedance arrangement is for the amplifier impedance to be less than the speaker impedance so your connectors need to be low resistance not to impact this relationship.
See related product
An Electrical Resistance Experiment
Now that we have looked at a couple of the impacts of resistance in connectors we can line up a couple of them and see how they stack up. I grabbed data for a handful of Molex connectors mostly looking at their range of power connectors like the EXTreme, Sabre, and Mini-Fit series. These connectors have resistances ranging from the extremely low 0.15 milliohms, like in the Zpower series, to 20 milliohms, in the PowerPlus signal contacts, and support voltages and currents up to 600V and 50A respectively. I expected to see a simple pattern of increasing power and decreasing resistance but this wasn’t the case. Molex has gone further than just decreasing the resistance but looked more holistically at the applications that the connectors are meant for and tuned the connectors to meet specific needs.
Depending on the applications just connector resistance is not the deciding factor but you also have to take into account durability, cycle count, materials, use case, and costs. On the whole as power levels increase the resistance does decrease but in power connectors there is a concerted effort to keep resistances low, there is more variance in resistance in signal lines. Resistance does impact your circuit design, affects your impedance matching, and should be looked at closely when you make component decisions.