A while ago I needed some small value resistors as current shunts and I started wondering about the feasibility of using traces as resistors.
There are many theoretical reasons as to why copper traces make bad shunt resistors, but I found surprisingly little practical data.
The traces are (from left to right):
- 20 mil wide, 1221 mil long, should be ~30mOhms
- 6 mil wide, 611 mil long, ~50mOhms
- 12 mil wide 1221 mil long, 50 mOhms
- 6 mil wide 1221 mil long, 100 mOhms
Using traces as resistors - [Link]
Clayton Grantham writes:
Some years ago, one of the fundamental electronic instruments was the laboratory curve tracer. A CRT display would sweep out terminal behavior (current versus voltage) from which you could derive mathematical models. Classic presentations of diodes, transistors, and other devices enlightened designers about linear and nonlinear operation. From the displays, you could determine the bias points for optimum design performance. Today, however, you rarely find the classic curve tracers in the lab. Instead, you find design-simulation software, such as Spice, that’s removed from hands-on, empirical analysis. Spice models now exist for almost all electronic components. Characterization analyzers still make the voltage-current measurements but not at the design-engineer level. Rather, departments are dedicated to characterizing processes and components and incorporating these characteristics into the simulated models. The low-cost circuit in Figure 1 allows you to return to the hands-on approach by using your PC as a limited curve tracer.
Trace voltage-current curves on your PC - [Link]
If you need to know the exact length of a trace in Eagle — for differential signals, for example, or if you have a high-speed data bus like on SDRAM and you’re not using a buffer so you need to make sure all the traces are the same length, and make sure they’re no too long for the strict timing constraints, etc. — Eagle has a nice little ULP script that will display some basic information on every trace on your PCB, including the maximum theoretical current you can draw through the traces, etc. Just type “run length-freq-ri” in the command window of the PCB view and you should get the displayed info above! (Note that by default this ULP assumes 1oz/0.035mm copper, though this is the most common anyway.)
Eagle QuickTip: Measure Trace Length (and more) - [Link]