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January 2017

How Much Tolerance Is Enough?

When designing circuits, is there a rule of thumb for picking voltages and tolerances of components? For example, if my power source is 12 volts, is an electrolytic capacitor with a 24V rating  “better” than one with a 16V rating? What do good designers use as a margin?

#1174
Enrico Gutiérrez
Panama City, FL



Answers

I used to design circuits for military applications and in that environment, a voltage rating double the expected maximum was considered adequate. In an automotive application, the spikes from the starter can exceed 60 volts, so you need to keep that in mind and provide isolation.

Ripple current is another capacitor parameter that needs to be addressed. It turns out that ripple current rating increases with voltage rating, so you might use an electrolytic cap with 10 times the needed voltage rating just to get the ripple current rating.

In general, the MTBF (mean time between failures) is calculated based on the stress on the component.  A component (resistor, transistor, transformer, etc.) that is rated 70 degrees C but is running at 150 degrees C will have a short life, but if it’s running at 30 degrees C the life will be normally long.

Russ Kincaid
Milford, NH

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