Circuit Protection without Circuit Protection Components

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Have you ever thought about circuit protection as analogous to fire protection? There are lots of ways to prevent fires. There are active systems (fire sprinklers, automated extinguisher systems) and passive systems (self-extinguishing materials, self-closing fire doors) and safety-by-design systems where careful selection of non-flammable and insulating materials as well as physical layout make fire propagation nearly impossible. As it turns out, circuit protection has similar options. Here is an example of circuit protection via a safety-by-design technique: 

When you claim to have expertise in a subject, you tend to want to measure up, so when a colleague called to ask me what I would do to protect a digital multi meter, I dove right in with my usual alacrity. But after spending at least half an hour happily conversing with myself on the dangers of under-protecting, I realized I had a conundrum on my hands. A digital multimeter (DMM)….any test equipment, for that matter, has the responsibility of accurately measuring a signal, any signal, without complaint. 0.05mV? No problem. 120V? No problem. 1000V? …..wait a minute. How am I supposed to dispense pearls of wisdom on protecting input circuitry that has to discern truths at 0.5mV and at 1000V with the same circuit protection? Devices for circuit protection have limits, you know.  

So the multimeter presents a tricky design issue: with this being a meter that is designed to handle such a huge range and is subject to application errors (like trying to measure the resistance of 120VAC!), it has to be sensitive enough to make it useful as a measurement device and be robust enough to withstand operator-errors and other stupid mistakes. 

Test equipment is not your typical embedded product that has fairly predictable inputs in its lifetime, and any circuit protection needs to retain the precision aspect. The use of traditional circuit protection components is going to be problematic, as they are highly non-linear when driven to their operational endpoint. The input to a DMM must be extremely linear for the instrument to have any hope of useful accuracy. 

There’s no real magic to engineering anything. Engineering is just a combination of applied training, common sense, and attention to detail. The most difficult part is trying to think up all the “corner cases” and stuff that could happen to break whatever it is you’re designing. Neil Armstrong said that “Science is the study of what is. Engineering is the study of what can be.” I might be paraphrasing, but I like it. And I could add that engineering is also the study of how to prevent things from breaking, which usually means assuming that at some point someone is going to connect a multimeter set on “.00mV” range to a 1000V source. At this point, I am really respecting the idiot-proofing challenges that the folks at Fluke, B&K, and LeCroy have to deal with.  

Even the cheapie “give-away” DMMs usually have some regulatory approval. I own one that is marked with warnings not to exceed 750VAC or 1000VDC. That means that the input circuits are designed to withstand and operate normally at those voltages. So common sense says that there is no need for overvoltage protection after the test leads (inputs) of the multimeter, because the components themselves are supposed to handle these high voltages and anything higher would violate the safety warning. It does come equipped with a fuse for fire safety. 

In other words, what ordinary electronics would view as hazardous voltages, a DMM considers them just another input. The safety of the input is largely inherent in the design of the switching and scaling circuits that knock those high voltages down to levels that are in range of D/A and A/D converters. Designers must carefully select these components based on worst-case scenarios. Beyond the fuse, I doubt there is much CP content - perhaps some ESD protection on the display… 

In my fire protection analogy, the DMM represents an inherently non-flammable design, owing to careful selection of interface components that have the inherent ability to withstand the unpredictable abuse of the DMM.