Issue 143 June 2002
Invisible Components

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Invisible Resistors

Circuitry in the analog domain has many components that don’t appear on normal schematics. In previous articles, I’ve discussed how parasitic inductances and stray capacitances can affect RF and switching circuits. Now, down at DC, you will meet, if not see, some invisible resistors.

Figure 2 seems to be about as simple a schematic as you can imagine: battery, fuse, switch, and bulb. Flip the switch and the light goes on. It looks like a beginning electricity project!

(Click here to enlarge)

Figure 2—Small resistances can add up to large trouble if you’re not careful. The resistor values are in milliohms.

But what about all those resistors? Their values, shown in milliohms, may seem trivial. After all, what effect can 33 mW possibly have on a circuit?

For low-speed logic signals, circuit board traces behave like idea conductors and digital folks may be forgiven for believing that schematics represent the real world. Unfortunately, there’s a tendency toward false generalization. Those power semiconductors that switch kiloamps and kilovolts at the flip of a bit are definitely not ideal!

Analog folks know that trouble comes in many shapes and sizes. In that simple bulb-and-battery circuit, those trivial resistors drop 140 mV at 1.6 A, a power loss of ~2%. Trivial, perhaps, but consider the power supply for a single Intel Pentium 4 CPU. The voltage regulator must supply about 1.5 V at up to 70 A, with a maximum voltage drop at CPU pins of only 130 mV.

When you work the numbers, this implies a resistance of 1.8 mW between the regulator and CPU. Actually, it’s worse because the power distribution specification restricts the voltage drop to £55 mV, a resistance of only 0.8 mW. Remote voltage sensing can compensate for the DC drop, but the power dissipated in the conductors represents a serious board-design problem.

If you’ve never measured the resistances of wires, connectors, and connections, you should invest a few hours in your own education. Unfortunately, most DMMs cannot measure resistances below 1 W, if only because the resistance of their test leads introduces a significant error. I have an old Fluke 8060A DMM that can resolve 10 mW with a Relative setting to cancel lead resistance, but cheaper meters are useless below a few ohms.

Photo 2 shows a useful DMM accessory: a LM317 voltage regulator wired to produce an accurate 1-A current. Figure 3 presents the schematic. You can use a 1-A wall wart or a bench supply to provide the input power.

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Photo 2—A trivial 1-A LM317 current source turns a DMM into a milliohm meter. The blue heat-shrink tubing insulates four parallel 5.1-W resistors that set the current.

A quartet of 5.1-W resistors in parallel sets the LM317 current to 1.0 A. If you have an accurate ammeter you can trim the resistors as needed, but for our purposes a few percent accuracy will suffice. At 5 V, a 1-A power supply produces an open-circuit output voltage of about 3.5 V, which may be a bit high for testing circuits with semiconductors already in place.

(Click here to enlarge)

Figure 3—LM317 voltage regulators make good current sources, too. Note that the LM317 output terminal does not connect directly to the source’s output jack. Set your DMM to read millivolts and think milliohms.

Because the source drives 1 A into what’s effectively a dead short, it’s useful only for brief tests. The LM317 will overheat and shut down in thermal overload after a minute or two, at which point the metal box will be toasty warm. You have been warned.

To measure the resistance of a conductor or connection, set up your voltmeter to display millivolts, typically around 200 mV, and attach it as close as possible to the device you’re measuring. Attach the current source leads beyond the voltage sensing leads. Turn on the juice and read the DMM as though it were displaying milliohms.

You must separate the current drive and voltage sense connections for this type of measurement to eliminate errors caused by resistance in the sense leads. To achieve more accuracy you must pay more attention to subtle effects, but a simple Kelvin connection will get you most of the way to the goal. Make a few measurements and see which invisible resistors lurk in your circuitry.