Identifying the Most Common Sneaky Resistances

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Most circuit designs look great on paper. The math checks out, you’ve selected the perfect component values, and your simulation runs like a dream. Sadly, when you build that first board, things never quite work the way they did in SPICE. What gives? Even with a perfect design, your system may be falling victim to sneaky resistances.

Think back to the earliest calculus class you took.  When you take the integral of a function, you end up with a generic “+C” at the end of your result to represent the unknowable offset of your function.  Resistance tags along with circuit components function in much the same way.  If you order a 1uF capacitor, you are really getting 1uF + C, where C is some amount of resistance.  This value is not unknowable and typically appears in the datasheet for the capacitor, but it can still give unexpected behavior if neglected. 

Capacitors

The inherent resistance in a capacitor is typically shown in a datasheet as ESR, or equivalent series resistance.  In small ceramic capacitors, the main source of ESR is just the resistance of the metal component leads, and the value tends to be very small, often less than 100mOhms. 

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Electrolytic capacitors are a whole different world of hurt.  Aluminum capacitors in particular can have ESR values in the 10s of ohms, enough to seriously disrupt a circuit.  This series resistance increases with temperature and frequency.  The heat generated by current flowing through that sort of resistance slowly damages the capacitor over time, which is why so many electrolytic capacitors are only rated to about 1000 hours of use.  This ESR actually decreases with increased capacitance ratings, so it may be worth a slight increase in cost to use a higher rated capacitor that will introduce less resistance and generate less heat. 

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As a capacitor degrades, the capacitance decreases which can affect timing and output voltages, and it can cause many other issues in a circuit.  If you are experiencing unusual behavior and have a large capacitor in the system, it is easy to test the ESR with a component tester

Inductors

All inductors are based on the principle of one loop of wire inducing a field in another loop of wire.  Wire has inherent resistance.  Small inductors typically use only wire and have resistances under an ohm.  Larger inductors that use a core experience other issues like eddy currents that further increase the resistance.  Though it is not necessarily incorrect to think of inductors having an ESR, the more commonly used parameter is the Quality Factor, Q.  Q is measured at a particular frequency and is a ratio of the inductive reactance compared to the resistance.  A higher Q factor is desirable and often increases as frequency increases.  It can be very difficult to get a good Q factor in a tiny inductor, so budget board real estate accordingly if this is likely to be an issue in your design.

Wires/Traces

Capacitors and inductors are not particularly interesting when driven by DC current.   Their capacitor or inductive properties disappear, and they simply become a small resistorWires and copper traces on boards are always small (or not so small) resistances that can develop reactive properties at high frequencies. Long, coiled wires introduce inductive effects, and neighboring copper traces on a board can generate unexpected capacitance in a system.  Even at DC, wire and copper need to be considered as sources of resistance.  The resistance per meter of 24AWG copper wire is about 80mOhm, similar to a ceramic capacitor’s ESR.  A 6mil wide copper trace in standard 2oz copper foil has a resistance of about 50mOhm per inch, also enough to cause unintended behaviors in a long or highly sensitive board.  

No design will ever behave quite like a simulation, but being aware of sneaky resistances ahead of time can help you get your design as close to perfection as possible before you ever print a board.  While a standard multimeter can help you find some resistance, a proper component measurement device may be a worthwhile investment for your home or business lab.  

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